WINDOWS & DOORS

 

Visual Comfort

 

Visual comfort is a key indicator of the performance of our indoor environment. It is a combination of the amount of daylight, the lighting quality from both natural and artificial lighting sources, glare as well as our access to outdoor views. It plays a key role on our well-being, physical capacities, memory, focus and health.

 

Daylight and glare

 

Sunlight is a multisensory phenomenon that can enhance an occupant’s comfort, health, and connection to the outside environment through its dynamic luminous and thermal attributes. We often refer to this as daylight. Daylight pertains to the incoming natural light entering our visual space. The necessary ratio of this natural light to artificial light can be designed for. Meanwhile, glare is the result of discomfort caused by excessive brightness. A good daylighting design therefore relies as much as possible on natural light integration while avoiding glare issues.

 

Recommended lighting levels are1:

 

● 150 – 200 lux for general household activity – for example, vacuuming or washing.

 

● 300 – 500 lux for focused activity – for example, reading or studying, working on a car.

 

● 800 – 1000 lux or more for concentrated activity – for example, fine detail sewing.

 

Views

 

A window is so much more than just something to let in light and air. When early humans could observe their natural surroundings from a secure vantage point, with a clear view of any approaching danger – like lions – they could relax, feel at peace, reflect on life and make a plan for it. The same theory still holds true today.

 

Most of this desire to be connected to nature happens unbeknownst to the homeowner. They might realise they like looking at nature and find it calming, but still have difficulty explaining why. According to psychologists2 the ideal view for restoring our minds is rolling countryside, as green landscape is pleasing to the eye and subconsciously, your mind can see danger approaching, allowing your mind to relax, drift and replenish. Accordingly, you don’t want to be deep in a jungle setting, that’s because danger lurks in the jungle, and you can’t see very far ahead. The phrase ‘it’s a jungle out there’ shows this.

 

Therefore, for the visual comfort you want to optimise the ability of building occupants to see wider nature. This can be as simple as placing high clerestory windows in rooms that shows just the sky, rather than bigger and lower windows that just show the walls and roof of the building next door.

 

Visual comfort increases the level of vitamin B and D in the body as well as synchronizes the body clock, improving the quality of sleep and concentration while reducing your energy need. Thus, this performance indicator plays a role on your health, productivity, and energy consumption3.

 

Introduction to Designing Windows

 

There is a lot to think about when it comes to designing and building an environmentally sound, energy efficient home. Windows and doors are hugely important, as they are usually the weakest link in the thermal envelope of the building, at typically around 13% of the insulation value of walls (see Fig. 3 below).

 

The location and proportion of windows and doors in relation to orientation to the sun is critical. Refer to sections on design.

 

If you do a great job of designing the other elements of the structure (such as the roofs, walls and floors), then fail to give windows careful consideration, all the gains literally disappear “out the window”. With up to 31% of a home’s heat escaping through windows (see illustration below), doing this well makes a big difference to how comfortable, healthy and easy it is to heat a home.

 

1http://www.level.org.nz/energy/lighting-design/appropriate-lighting-levels/

2https://designwithscience.com/

3https://www.greendesignconsulting.com/single-post/2018/04/04/How-can-green-design-and-smart-technology-improve-visual-comfort

Typical sources of heat loss from a home, courtesy of EECA.

 

In New Zealand our window standards are too low, and we traditionally have not done windows well. In the United Kingdom for example, where climatic conditions are similar to parts of the South Island (see table below), the Building Regulations define the minimum requirement R-value for a window as 0.7m2 deg C/W, almost 3 times what they are in New Zealand.

From Build, Issue 179, August/September 2020.

From Build (published by BRANZ), Issue 179, August/September 2020.

 

In this section of the Healthy Home Design Guide we will look at what makes a super window or door, the key frame and hardware options, and the importance of the glass and installation methods. At the end of this section we will look at some design considerations and the supply options that are available for New Zealand home owners.

 

New Zealand Building Code

 

Regulatory Requirements

 

The Building Code Requirement that applies to windows is G7. It provides for sufficient natural light for occupied spaces and appropriate visual awareness of the outside for occupants.

 

This clause requires habitable spaces to have adequate windows for natural light and visual awareness of the outside environment to safeguard against illness, and loss of amenity due to isolation.

 

It requires natural light of no less than 30 lux at floor level for 75% of the standard year, and for transparent openings in certain buildings.

 

For further information https://www.building.govt.nz/building-code-compliance/g-services-and-facilities/g7-natural-light/

 

It is important to remember that the New Zealand Building Code is performance based. Whilst there are a number of ‘Acceptable Solutions’, this is just one pathway to compliance. Alternative solutions can be accepted provided there is sufficient theory and proof that the solution will work. Typical examples of this in everyday practice include:

 

● Raked window heads.

 

● Door units wider than 6m.

 

● Window units wider than 5m.

 

These examples are not covered by ‘Acceptable Solutions’ but are relatively common in architecturally designed homes. The wide units for example are only non-compliant in that lintels at those spans are not covered within NZS3604. A steel lintel or LVL timber beam can of course be engineered to span these distances.

 

The ever-growing application of glass in buildings is recognised by the Building Code, which details the minimal requirements and standards in the use of glass in buildings based on the targeted application of glass.

 

In respect of energy management and control, thermal efficiency is addressed, where double glazing is not mandatory in New Zealand except in climate zone 3. This sets minimal standards to be achieved from a “heat loss” perspective, without distinguishing the need for differences in performance necessitated by climatic factors that vary considerably across the geographically diverse climatic zones in New Zealand – currently designated 1, 2, 3 and 3a in the Acceptable Solutions and verification methods.

 

Also, the Regulatory System is silent on minimal energy performance requirements beyond thermal efficiency (retaining heat internally) – for example it does not address “solar control” which is of importance for human comfort in warmer regions/seasons to address direct heat gain from the sun (potentially causing over heating) and consequent costs in cooling the interiors. 7 Windows & Doors/E2 AS1 window installation – sill – aluminium frame.pdf

Thermal Model showing thermal bridging in a ES/AS1 Window Frame.

The Way Forward

 

Advancements in glass technologies already enable step changes (improvements) to glass performance relative to what is mandated by existing codes and current practices in New Zealand. With the construction/building sector being a relatively large nett energy consumer, it is vital to exploit current technologies to close the gap between what is feasible and what is necessary to becoming a Net Zero Carbon or Near Zero Carbon – economy (i.e. carbon neutral) by 2050.

 

The cardinal glass performance indicators that address energy efficiency in cold regions/winter and warm regions/summer are respectively:

 

(1) U-Value: air-to-air heat transmission due to thermal conductance.

 

(2) Solar Factor (or Solar Heat Gain Coefficient): direct solar heat gain.

 

Minimal values for each indicator vary based on climate / region. For instance, U-Values carry a greater importance in the relatively colder European regions requiring much lower values than (say) in the Middle-East. In contrast, the lower Solar Factor (or Solar Heat Gain Coefficient) is more important in warmer regions such as the Middle-East.

 

It is necessary to map New Zealand regionally – based on applicable climatic factors – to relevant performance indicators along with compliance standards and a feedback loop. Ideally, the framework should also stipulate adjustments necessary with variable glass coverage in buildings (ratio of glass area to overall building envelope size).

 

Glass processing facilities worldwide can provide insulated glass solutions customised to meet performance and aesthetics solutions as well as regulatory requirements and personal preferences. The resulting energy-efficient glass delivers a more comfortable building interior and helps reduce energy costs across the service life of the glass.

 

The Anatomy of a Window

 

The word “window” comes originally from the Old Norse “vindauga” meaning “wind eye” and refers to an unglazed hole in a roof! Thankfully times have changed somewhat, and these days we expect our windows to manage:

 

● Light – natural light is known to boost Vitamin D, ward off seasonal depression, improve sleep and counter the effects of artificial light. Window sizes and glass treatments will have a big impact on this.

 

● Ventilation – good ventilation in your home will greatly reduce the risk of health issues such as asthma, allergies, and headaches. It also helps reduce dampness, which will reduce the triggers of many common respiratory problems, aside from damage to walls and furnishings. The number of opening windows, location and hardware choices will be important considerations here.

 

● Temperature – the World Health Organisation in 1987 found that comfortable indoor temperatures between 18 - 24°C were not associated with health risks for healthy adults. For infants, the very elderly, and those with significant health problems, a minimum 20°C was recommended. Temperatures lower than 16°C with humidity above 65% were associated with respiratory hazards including allergies. The thermal properties of the window frame, the glass selection and the installation method are important design decisions to help manage internal temperature and comfort.

 

● Acoustics – Sounds at or below 70dBA are generally considered safe. At 70dBA you can hold a conversation at normal volume. Any sound at or above 85dBA is more likely to damage your hearing over time. Again, window frame material and glass selection are important, as well as the quality of seals and hardware.

 

● Security – windows are the most common point of entry for burglaries. Making these as difficult to enter as possible is a good start to reducing the risk of unwanted intrusion, and keeping homeowners and their families safe. Window hardware and glass selection have a big impact on how safe the windows are.

Typical uPVC bungalow window – 2 casement sashes and a fixed pane. Photo: NK Windows.

 

● Inwards opening “tilt & turn” or “tilt (hopper) only” – these are a more “European style” window, offering very good secure ventilation, and were originally designed to provide ease of cleaning on multi-storey homes .

 

● Sliding – these are a great option in bathrooms and kitchens when you want a larger unobstructed opening. They are easy to operate and can be constructed in multiple panels.

 

Typical Door Types

 

● Single (Entrance) Doors – these come in a wide range of styles and are often used to make a real statement as you enter the home. They can be inwards or outwards opening and be constructed with sidelights, top lights and a range of inserts. Inserts can include glass and solid panels, constructed of a wide range of materials – including aluminium, timber, PVC and composite construction. They can also be complimented with a range of security and entry options, including spy holes, electronic key pads etc.

● Double (French) Doors – again, these can be configured as either inwards or outwards opening. They are typically the simplest and most cost-effective way to fill narrower openings with good airtight doors, and still achieve a 100% opening area.

 

● Sliding Doors – these are a Kiwi favourite and allow us to enjoy both unrestricted views, and open up large areas of the home to the outdoors – typically 50-65% of the opening. They start with basic brush seal, “2 panel”, “ranch sliders” and go right up to large 10 meter plus openings with more sophisticated sealing systems.

 

● Bi-folding Doors – bi-fold doors are another very flexible opening style, offering the choice of opening part or all of the space up. They are constructed with multiple panels (typically 2-6) and open in a “concertina” style which stops the sashes swinging in the wind. They tend to be a high maintenance door, and more difficult to make airtight due to the number of seals.

Altus’ patented Foldback® Bi-fold doors, 3 bi-fold panels folding each way then 180° back against the cladding.

 

It is perhaps useful at this point to start looking at the make-up of a window in terms of three separate elements:

 

● The frame material and associated hardware.

 

● The glass.

 

● The installation method.

 

Frames

 

In the New Zealand market currently, the homeowner has the choice of three main window materials that are readily available – aluminium, uPVC and wood. Each of these materials has different properties which require careful consideration when designing and building a warmer and more comfortable home.

 

Aluminium

 

Aluminium is one of the most popular and versatile materials for framing glazed windows and doors in New Zealand. Aluminium makes up 80-90% of the residential market for windows and doors. Light but strong it can withstand a wide range of conditions and being extra durable it stands the test of time.

 

Aluminium is also a high-value recyclable that can be melted down and used again indefinitely. Aluminium frames are made using an extrusion process, whereby logs of aluminium are heated and pushed through a die. Given the highly customizable nature of aluminium extrusion, the possibilities for design are almost endless, hence why we see such a large range of options and configurations for windows, doors and curtain wall façades. Unlike other materials, aluminium will not swell, split or warp over time.

 

Surface Finishing

 

When aluminium frames were first introduced to the New Zealand in the 1960’s, there were limited options for surface finishing. Silver (natural) anodised was the standard colour. Now we have a wide range of finishes and colours available. The most common types of finishes are powder coating and anodised.

 

Anodising

 

Anodising is a method of surface-finishing aluminium, in which colour is electrochemically etched into the surface of the material. The process creates an oxidised layer on aluminium in a controlled manner. As such the finish is a part of the aluminium and is therefore extremely corrosion resistant. For exterior applications there is a limited range of colours and the raw aluminium extrusion lines will remain visible.

 

Powder coating

 

Powder coating is the process of thermostatically bonding a polyester powder to aluminium. The main advantage of using powder coating is the consistent finish and wide range of colours available. This is due to the powder being a coating rather than a part of the aluminium.

 

Energy Efficiency

 

Traditional aluminium frames are singular extrusions. This allows the free passage of heat from inside to out because among its other characteristics, aluminium happens to be an extremely good thermal conductor. This is not a desirable feature of a building material. Even with low-e double glazing there will still be a likelihood of condensation occurring on the frame. With modern manufacturing technology we can now improve this performance by thermally breaking the frames to increase energy-efficiency.

 

When using thermal break technology, there are two main formats:

 

1. Pour-and-debridge – a channel in the extrusion is filled resin, with the bottom of the channel then cut out to create the thermal break. Most popular in North America.

 

2. Polyamide strip – reinforced polyamide plastic profiles that are inserted between the inside and outside aluminium extrusions. Most popular in Europe.

 

By thermally breaking the frame, the chance of condensation dramatically reduces. Depending on the glazing pocket size either double or triple glazing can be accommodated.

 

Aluminium Window Frames

 

As mentioned there are wide number of aluminium frames and internal profiles available to suit any situation. The selection of any frame will ultimately depend on the size, configuration and environment they are going to be used in.

 

New Zealand typically uses what is called a ‘facing-reveal’ frame as shown below. This allows for several key elements of a window installation:

 

1. Structural fixing – typically nails or screws.

 

2. Air seal to trim gap – PEF backing rod with expandable foam or silicone seal over.

 

3. Cladding cover.

Thermal break window technology.

Architectural Residential Thermally Broken frames

 

These frames have better thermally broken designs. They can typically take double or triple glazing and are suitable for more window and door sizes and configurations. Typical RWindow Value between 0.41 and 0.46.

 

Use in Zone 1

 

● Upgrade for larger window and door units.

 

Use in Zone 2

 

● Standard window and door specification.

 

● Further low-e upgrades recommended for improved R-Values.

 

Use in Zone 3

 

● Standard window and door specification.

 

● Consider use of triple glazing with low-e upgrades for improved R-Values.

 

Poly Vinyl Chloride (PVC)

 

PVC (polyvinyl chloride) is a strong and lightweight plastic with an extremely wide range of uses. It is made softer and more flexible by the addition of plasticisers. If no plasticisers are added, it is known as uPVC (unplasticised polyvinyl chloride), PVC-U, rigid PVC, or vinyl siding in the U.S.

 

First produced commercially in the late 1920s, PVC has become one of the most widely used polymers in the world and represents a highly efficient conversion of raw materials. Due to its versatility, PVC is used across a broad range of industrial, technical and everyday applications from window profiles and pipes to credit cards, water bottles and blood bags.

 

The first commercially available windows were installed in Germany in 1959. While the technology for producing these windows has naturally advanced over the years with, for example, the introduction of better performing acrylic-based impact modifiers and lead free additives, some of these earlier uPVC windows are actually still in use. Over 60% of European homes now have uPVC windows and doors.

Combined with different glazing options we can now compare R and U window values for the different uPVC frame sizes:

Comparison of R and U values for a “standard” window – 1800mm W x 1500mm H, 2/3 fixed 1/3 opening.

 

If these frames are sourced from one of the large reputable suppliers out of Europe they will come with:

 

● Either 24, 40 or 55mm glazing pockets – able to support double, triple or quadruple glazed glass units.

 

● At least dual seals, often co-extruded – this improves water tightness, air sealing and acoustics.

 

● Steel reinforcing – to fasten hardware and provide frame “stiffness”. The correct sizing of this is critical to maintain the structural integrity of the window frame, and is based on the opening size, construction and wind pressures. Options also exist to remove the steel reinforcing to further improve thermal performance (see table above).

 

● A range of hardware options – standard hardware locations in the extrusions allow a choice of hardware suppliers and opening styles.

 

● Internal glazing – this is a European standards requirement, to reduce unwanted intrusion.

 

● External moisture drainage – moisture remains on the exterior of the house.

 

● UV resistance – special “tropical” compounds for high UV regions of the world ensure the profiles do not degrade, deform or discolour.

 

● Recycled material – again this is a European standards requirement.

 

● Fire resistance – uPVC formulations for windows are designed to self-extinguish.

 

● Colour range – colour can be added either by applying a laminated foil (to either the interior or exterior surfaces) or by applying an exterior aluminium “skin”, which can be powder coated in a wide range of colours. This offers the homeowner the option to vary the colour and finish on different surfaces of the frame.

 

● Welded mitre joints – these provide extra strength and weathertightness.

 

Wood

 

Timber (wooden) window frames have been around for a long time. The oldest known working window dates from pre-1066AD. Builders in the UK found a small wooden window in a Saxon Church that had been covered with dirt for over thousand years. Surprisingly it still worked and shows just how wonderful wood is if it is used in the right conditions. The oldest surviving sash windows can also be found in the UK. Installed in the 1670’s in Ham House the double hung sash window is a prime example of craftsmanship and care.

The tiny wooden frame measures 2ft tall by 1ft wide and had been buried in the church's wall https://www.dailymail.co.uk/sciencetech/article-1314476/Britains-oldest-working-window-built-1-000-years-ago-buried-wall-Saxon-church.html

https://upload.wikimedia.org/wikipedia/commons/0/04/Ham_House_04.jpg

 

In New Zealand windows have traditionally been made from locally grown softwoods such as rimu (frames), matai (sills), totara and kauri (sashes), and Californian redwood. As native timbers have become increasingly scarce, other timbers have come into common usage such as:

 

● Western red cedar (imported).

 

● Pinus radiata (both clears and finger-jointed, the latter for paint finish only).

 

● Cypress species such as macrocarpa, Mexican cypress, and lawson’s cypress.

 

● Kwila (imported).

 

● Heat modified softwoods (Abodo and others).

 

Ref: www.nzwood.co.nz  – how-to-build timber windows.

 

The correct timber is strong, durable, decorative and has better insulating properties than some other window materials. It can be prepared in a variety of ways which allow it to withstand the elements while remaining aesthetically pleasing.

 

Environmental Concerns

 

Wood used in its natural or engineered state is an environmentally friendly product, especially when it comes from sustainably managed forests. The treatment of timber is important and this has environmental implications. The Living Building Challenge has a RED List of materials to avoid and the treatments of wood are itemised among this.

 

Broadly speaking, timbers that come from sustainably managed forests that have not been treated with RED LIST materials are the best environmental choice we can make. As long as they are durable and still fit for purpose and not travelled too far.

 

The first graph shows the embodied energy comparison of different window designs.

Embodied energy comparison of different window designs.

 

As can be seen, timber windows use far fewer Megajoules (MJ) to manufacture, maintain and dispose. Timber is closely followed by aluminum clad timber. The following graph shows the Global Warming Potential (GWP), it is measured in Kg of CO2 per m2, for different window frames. The production of timber windows (growth) actually balances the post production, or nearly. What is not included here is the carbon used for transport or painting. The aluminum clad timber windows do not require painting and have a very small “footprint” in terms of CO2. This does not take into account the use of Hydro Electric power to produce aluminum as we have traditionally done here in New Zealand.

By cross graining the timbers the forces present oppose each other and keep the window straight and true. The gaps between the growth rings are also important. The window above shows pine with gaps of around 1mm, which means the tree has grown 1mm in diameter over one year. Faster growing timbers such as Pinus Radiata from New Zealand will grow 10 to 15mm each year. This leaves large air pockets and very little resin. The air pockets can fill with water and the timber begins to rot and change shape.

 

Durability

 

Timber will last for hundreds of years in the right conditions. These conditions are not the ones we typically find in the built environment. Exposure to the elements, ultraviolet light, rain, cold, heat and humidity ensure the slow and steady decline of all products. In order to slow that decline as much as possible we can treat the surface of the timber exposed to the elements with a range of materials. Or we can choose a timber with a high resin content (Pine, Fir, Larch, Cedar) or closed cell structure (Oak, Meranti). Arguably the best protection for timber is aluminum. A thin covering of aluminum off-set from the timber by >5mm (to prevent capillary action) will reduce the need for external treatment of the windows, provide a decorative finish and extend the life of the product.

 

Windows that do not have an aluminum external cover will need some maintenance but this will depend on the environment in which they are located. A light sand and a recoat of polyurethane or paint every 5 years should be enough to keep them looking good and performing well.

 

Thermal performance

 

Wood is a good insulator; it has a relatively low Thermal Transmittance (U-value). The thermal conductivity of timber is divided into softwoods and hardwoods. Softwoods are gymnosperm trees, those which are typically, but not exclusively from conifers. They don’t lose their leaves and tend to grow faster than hardwoods. The term “Softwood” has little to do with actual hardness or softness of the timber. For example, balsa is a hardwood. Hardwoods are from angiosperm, flowering trees that generally lose their leaves. The image below shows the cellular structure of the two. The first picture shows Oak, which has a very closed cell structure with pores or holes. The Pine (second picture) has no pores but instead a sponge-like quality that traps air.

The table below shows the U-value in W/m²K Watt / metre squared Kelvin. This is the rate of transfer of heat through a structure. The lower the number the slower the transfer of heat, or better the insulation.

From a thermal conductivity perspective, timber and uPVC are virtually interchangeable. As mentioned above, timber windows have excellent thermal properties and perform as well as uPVC windows provided they are designed to be airtight. European windows have to go through rigorous testing in order to gain their CE marks. New Zealand Standards allows for 8 litres per second per meter squared of opening tested to 150pa of pressure. European Standard is 1.01 l/s. So, not all windows are created equal. Ensure the windows you are buying or specifying have been testing to meet the best possible standard for performance, not just appearance.

 

Aesthetics

 

Timber windows can be produced in a range of finishes from raw cedar to painted or aluminium clad. Timber windows have a huge range of finishes that are both durable and beautiful.

Timber windows will always have a place in the construction industry as long as they are sourced from Sustainably Managed Forests.

 

Hardware

 

The other key component of a window frame is the hardware. This comes in a wide range of styles, quality and function. Typical design considerations include:

 

1. Opening Style:

 

a. Outwards – this is the traditional kiwi window opening style, and has the advantage of not needing to worry about what is happening on the inside of the building envelope.

 

b. Inwards – this is the more traditional opening style in Europe, and has several advantages:

 

i. It makes window cleaning on multi-storey buildings easier.

 

ii. It offers a “tilt” function which provides good secure ventilation.

 

2. Locking points – the number of locking points in the frame will affect the air tightness, security and acoustics. A “tighter” frame will decrease air movement and sound through the frame and improve security.

 

3. Size – it is important to match the hardware specification to the sash weight. All good window manufacturers will have recommended specifications from their system provider.

 

4. Seals – window seals make a big difference to window performance, they affect water tightness, air permeability, noise insulation and thermal insulation. Most well-constructed windows include synthetic rubber (plastic) gaskets. Higher specification windows have multiple gaskets to further improve performance.

 

5. Colours – one of the advantages of buying a complete window system from a single supplier is the ability to colour match frame and hardware colours. This is easier with typical New Zealand aluminium systems.

 

6. Coastal Zones – it is important to consider upgrades to “marine grade” hardware for more corrosive coastal environments. This will increase the life of your window and ensure it keeps working properly.

 

GLASS

 

Introduction to architectural glass

 

Virtually all glass currently applied architecturally into/onto buildings is the “float glass” variety – optically the best quality glass from current technologies meeting quantity, thickness and colour considerations in modern construction. All float glass processed into downstream products and applications (windows, balustrades, safety glass, insulated glass etc.) within New Zealand is imported from abroad as there is no local production of float glass.

 

The float glass process is the most common method of flat glass production in the world. This process basically involves melting sand, limestone and soda ash in a furnace and floating it onto a large bed of molten tin, hence the name float glass. This mixture slowly solidifies over the molten tin as it enters the annealing oven where it travels along rollers under a controlled cooling process. From this point the glass emerges in one continuous ribbon and is then cut and further processed to customers’ needs.

 

Thermal Insulation and Glass Choices

 

For additional information the New Zealand Window Efficiency Rating System WERS can be found here

https://www.designnavigator.solutions/WERS2.html

 

Thermal Insulation – denoted by U-Value

 

Glass U-Value measures the thermal insulation performance of a glass type. It is alternatively denoted as Ucog to depict the U-Value at “centre-of-glass” i.e. independent of the effects of (a) the spacer used along the periphery of insulated glass and (b) the framing into which the insulated glass is installed.

U-Value Definition (from AS/NZS 4668).

 

“The U-Value is a measure of air to air heat transmission (loss or gain) due to the thermal conductance of the glazing and the difference between indoor and outdoor temperatures.

As the U-Value decreases so does the amount of heat that is transferred through the glazing material. The lower the U-Value the better the thermal insulation.”

 

Glass U-Value Units, Determination and Denomination

 

Metric U-Values are measured in Watts per square metre per degree Celsius (or Kelvin) i.e. W/m2C or W/m2K. The R-Value is thermal resistance and is the inverse of the U-Value i.e. U-Value = 1/R-Value and R-Value=1/U-Value), both U-Value and R-Value being centre-of-glass values.

Note: Tabulated values can vary slightly based on calculation tool used and across products from different sources. The values will also be different when comparing across standards eg. European standard (EN 673) Vs American standard (NFRC 2010).

 

As may be noted above, Glass U-Value reductions are much more sensitive to the inclusion of Low E Glass than to the type of cavity infill (air or inert gas).

 

Thermal Resistance

 

Thermal Resistance – denoted by R-Value

 

For a window application its R-Value, denoted as Rwindow, signifies the overall window thermal resistance inclusive of glass and framing. To obtain Rwindow, Uw (Uwindow) is first determined through calculations combining:

 

i. Ug (or Ucog i.e. Centre-of-Glass U-Value) and Ag (glass area coverage).

 

ii. “Ψg” (spacer linear thermal transmittance value in W/m.K - for insulated glass only) and lg (peripheral edge cover with spacer by length).

 

iii. Uf (Framing U-Value) and Af (frame area coverage).

 

Uw = (Ug x Ag + Uf x Af + Ψg x lg) / Aw

 

where Aw = Total Window Area = Ag + Af, others as above.

 

Uw (Uwindow) is then inversed to obtain Rw (Rwindow) i.e. Rw = 1/ Uw.

 

As such, Rw or Rwindow must be distinguished from Rcog (Rg or Glass R-Value), the latter being inverse of Glass U-Value (Ug or Ucog) and which excludes the thermal effects at the peripheral areas (from IGU spacer and Framing).

 

High Performance Glass

 

Low Emissivity (Low E) Glass

 

Low E is an abbreviation of “Low Emissivity”, which is the ability to radiate absorbed energy. Low E Glass has a unique Low Emissivity coating designed to effectively reflect a significant portion of long wave radiations incident on the glass from the inside or outside of the house or building. Based on their primary capability to reflect long-wave energy, these Low E coatings help “trap” a proportion of the internal heat energy inside the building. Hence, Low E Glass reduces heat losses through the glass to the outside and makes it easier to passively maintain internal temperatures and comfort levels.

 

Low E Glass is of two types – sputter coated (soft coat Low E, also known as high performance Low E) and pyrolytic coated (hard coat Low E, also known as medium performance Low E). The thermal insulation benefits – denoted by lower U-Value – are considerably better with the use of high performance (soft coat) Low E Glass types which comprise Low E coatings inclusive of one or more layers of pure metallic silver as the functional layers within highly sophisticated multi-layered coating stacks (silver has a low emissivity property).

 

The level of thermal insulation provided by Low E coatings varies depending on the Low E surface emissivity, and the Glass U-value can vary from around U2.0 down to U0.9 (in W/m2K) for Low E double glazing, subject to spacer width and cavity infill(s). The lower the emissivity of the coating the lower the Glass U-Value. Uncoated float glass surface emissivity is 0.84, coated surface emissivity in pyrolytic coatings is of the order of 0.15 to 0.3, and coated surface emissivity in sputtered Low E coatings commonly ranges from around 0.06 down to 0.01 depending on silver content in the Low E coating layer structures.

 

Overseas, high performance Low E insulated glass has been extensively applied in thermally-efficient glazing systems for over three decades. Low E glass use has been systemically required by regulatory codes. Green building initiatives due to the drive for lowering carbon footprints have also supported the selection of Low E Glass in new construction.

 

In New Zealand, over the past few years high performance insulated glazing has become increasingly prominent even though local codes do not require it.

 

Low Emissivity (Low E) Glass and Solar Control

 

Along with addressing thermal insulation, Low E coatings can be combined with tinted glass in insulated glazing to help with solar control. For double glazing, this normally means the tinted glass forming the outer pane and the Low E glass the inner pane, with the Low E coating positioned on surface #3. For triple glazing, the tinted glass would form the outer pane with the Low E glass forming either the middle pane and/or the inner-most pane. In such cases, the tinted glass outer pane absorbs (subsequently re-radiates a portion of it externally) and reflects the short wave radiations (solar heat energy) more than would clear glass and helps lowers solar heat gain. The insulated glass thermal efficiency (U-Value) remains unaltered.

 

In New Zealand, some great opportunities lie in addressing solar control performance in glass based on regional climates – with greater emphasis required on lower values for solar heat gain in regions marked by warmer climates. There are currently no regulatory requirements for solar control. This impacts comfort levels for building occupants in summer months and/or necessitates spending on mechanical cooling to prevent over-heating of interiors. Building solar control capabilities into glass, based on climatic requirements, will help lower the carbon footprint of construction particularly in regions more affected by solar energy ingress during the warmer months.

 

Other Performance Attributes of Glass

 

Other performance attributes of glass that can be custom built into energy efficient glass are:

 

Acoustic insulation:

 

● Thicker glass provides better sound reduction.

 

● Inclusion of one or more laminated glass pane(s).

 

● Wider spaces/gaps between two or more glass skins – in double glazing, triple glazing and secondary glazing.

 

● Asymmetry in glass thicknesses (different glass thicknesses correspond to different natural frequencies, hence attenuate sound frequencies differently).

 

Glare control:

 

● Enhanced by lowering visible light transmitted through the glass. (Tinted glasses effect).

 

Ultraviolet (UV) control:

 

● The use of laminated glass blocks reduce UV transmittance by 99% or more.

 

● Body-tinted (coloured) glass types and coated glass types lower UV transmission – the extent to UV filtering varies significantly depending on the glass type.

 

Privacy:

 

● Coated Coloured glass increases privacy during daytime when external light levels are generally much greater than internal lighting.

 

● Note: During night-time, when it is dark and internal lighting is used, the glass is see-through. The see-through effect is never zero unless the glass is opaque.

 

● Patterned glass / textured glass enable translucence (lack of transparency) without reduction in light transmission.

 

● Laminated glass with an opalescent interlayer helps achieve translucence.

 

● Glass types incorporating surface treatments such as acid-etched glass and sandblasted glass also block vision through the glass.

 

Structural Integrity:

 

● Designed to satisfy wind, snow and live loads etc.

 

● Allows for minimal glass thicknesses and glass sizes to meet regulatory requirements.

 

● Structural requirements over rule other performance aspects.

 

Security:

 

● Examples are intruder-resistant (anti-bandit) glass, cyclone-resistant glass, bullet-resistant glass (BRG), anti-ballistic glass etc.

 

● As for most laminated glass applications, laminated glass providing security is only as good as its frame and the protection offered depends as much upon the design, fixing and maintenance of the window, as it does upon the glass itself.

 

Spacer Technology

 

In an insulating glass make-up (eg. double glazing, triple glazing), the glass panes are set apart at the edge by means of a spacer profile and seal. This creates the cavity (or cavities) between the glass panes which may be filled with air, inert gas or a mix of air and inert gas to enhance the insulating properties of the glazing make-up.

 

Traditionally, hollow aluminium sections have been used as spacers. Due to the higher thermal conductance of aluminium, heat losses via conduction through the edges have been high. If the surface temperature of the room side glass edge area drops below the dew point temperature of the surrounding air, condensation can form on the glass surface at the edge area. Such internal condensation is often seen on very cold mornings, normally around the lower part of the units and may also be identified on other colder surfaces in the vicinity such as aluminium framing.

 

Warm-Edge Spacers

 

Warm-edge spacers are normally made of stainless steel, special engineering plastics, butyl, thermal plastics, silicone foam or combinations of these. These new warm-edge spacers contribute to increasing the energy efficiency of a building by reducing the total window U value (Uwindow) and thereby increasing the window R Value (Rwindow).  By maintaining higher inner glass surface temperatures, warm-edge spacers reduce edge area condensation especially when used along with thermal-break (or thermally improved) framing systems. In accordance with the EN ISO 10077 standard, a spacer is considered thermally improved or “warm-edge” if its thermal conductance is ≤ 0.007 W/K.

 

Warm edge spacers are required in all Healthy Home construction.

 

Design Detail Considerations

 

E2 – External moisture

 

From a window and door perspective this is the most important clause to consider. If watertightness is not done properly, almost any thermal performance increase will be negated due to water and mould build up within the wall framing.

 

Performance of individual units is covered by NZS4211 testing and occasionally NZS 4284. Overseas testing regimes vary greatly and are not covered by an Acceptable Solution or NZ standard.

 

The main consideration and complexity in watertight design for windows and doors is the junction between cladding, framing and window. E2/AS1 is the current acceptable solution covering both direct fix and cavity clad situations. There are of course many other ways to detail the plethora of different junctions that arise in the built environment. The key principles for water management are the 4 D’s in order of importance are:

 

1. Deflection – divert water away from the risk area.

 

2. Drainage – if water were to get past a line of deflection, allow it a path to escape.

 

3. Drying – water that does penetrate an enclosed location or where small amounts may be built up can dry out through evaporation.

 

4. Durability – if the above elements cannot completely contain the water, ensure exposed elements are sufficiently protected or inherently robust to withstand prolonged exposure to water.

 

To take the window example, the glass and frame are clearly deflecting water away from the building. The head flashing above deflects any potential water ingress from the cladding out before it gets to the opening. Drainage through the trim cavity and cladding cavity around a window allows water to drain out below e.g. through a flashing or the cavity closer below. A sill flashing allows water ingress behind the jamb and head to drain back out. An open cavity allows for drying of the framing if water is absorbed at any stage. Sill tapes and aluminium sill flashings are durable to withstand prolonged exposure given drainage can be limited on flat sill trimmers.

 

G4 – Ventilation

 

G4/AS1 gives minimum standards for building ventilation. Windows and doors should always be considered as a part of the whole building system. In some cases it may be prudent to limit openings.

 

H1 – Energy efficiency

 

H1/AS1 reference NZS 4218 – Thermal Insulation – Housing and Small Buildings, for minimum standards of thermal insulation. There is nothing in here that limits the specification of better performing windows and doors.

 

E2/AS1 – Typical window detail

 

The Acceptable Solution E2/AS1 includes a standard installation detail (see Figure 16 below). This detail was developed primarily to deal with the risk of windows not being watertight. Placing the window out over an exterior cavity allows any water ingress to drain way if required without entering the insulated part of the wall structure.

Typical NZ Building Code E2 detail.

 

The challenge with this detail is that it places the window frame and glass outside of the insulation line, which is the warmest part of the wall. In cooler climate zones this keeps the frame and glass colder, increasing heat loss and the chance of condensation on both the frame and the glass.

 

By recessing the window frame back in to the insulation line, the window frame and glass will be warmer, reducing heat loss and the chance of condensation. Thermal modelling of PVC frames for example, indicate an increase in window R-value of 15-18% depending how far the frame is recessed.

 

All Healthy Homes must have recessed windows.

 

Below are a couple of typical details: