MATERIALS FOR WINDOW FRAMES

Wooden frames

Redwood pine is the usual material. The slow growing pines from the cold climates of Scandinavia and Russia are preferred.

Solid timber can be a little unpredictable in use, so wood frames may be made of 'engineered wood' for more stability:

• Knots weaken timber and are prone to weep resin. Remedy:Knots can be cut out, and the resulting short lengths of timber can be 'finger jointed' together with adhesive to give long lengths.

• The moisture content of wood varies with the season, causing it to expand as it gains moisture and contract as it dries. The extent of warping depends on the orientation of the grain. Remedy:Laminated wood has sections of wood stuck together with opposing grains – it remains more stable during the variation of its moisture content with the seasons. (It can also result in more economical use of wood when compared to the machining of a complex profile out of a thick piece of solid wood.)

The preservative treatment and coatings applied in a factory are likely to be better than anything that can be done on site, and some companies are claiming an eight or even ten year period before their windows need to be painted. They should then be repainted every five years, or so. (Stained wood needs to be re-coated more frequently.)

The Wood Window Alliance are claiming that British window manufacturers apply better preservative treatments than many continental manufacturers, as windows in the UK have to cope with more challenging weather.

Triple glazing is thick and requires a deep glazing platform if there is to be space fora glazing bead. Some manufacturers, especially those in Scandinavia, incorporate aluminium glazing beads as these can be thinner than wooden ones – so reducing the depth of the glazing platform and hence the frame.

Oak is a hardwood alternative to redwood. It is costly – Jeld-Wen's oak windows cost about 50% more than their equivalent windows made with engineered softwood. But oak is very robust, and it can remain in good condition even if neglected for years. (I've known oak windows which were neglected for decades and they had become stripped of much of their paintwork – but the frames remained substantially sound.) Indeed, oak windows may deliberately be left completely uncoated, so that they weather to a grey appearance.

'Modified timber' is an intermediate choice between softwood and hardwood – intermediate in cost and in longevity. Modified timber is softwood (or possibly hardwood) that has been treated to make it more rot-proof and more dimensionally stable. A bonus is that paint coatings last longer.

There are a variety of processes for producing modified timber, with varied brand names:

• Chemical modification – Accoya (a sort of 'pickled pine') and Kebony. • Thermal modification – Thermowood and Platowood.• Wood cell impregnation – Keywood and Lonza.

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The most widely available of these is Accoya. This is derived from Radiata Pine, grown in New Zealand and then shipped to Holland for the acetylation process. Currently, windows incorporating Accoya, Kebony and Thermowood are available from particular manufacturers.

Composite frames

Most composite frames consist of wood in conjunction with some other material(s). The intent is to improve:

• WeatheringThe outside of the frame is clad with sections of powder-coated aluminium or fibreglass. The window no longer needs to be periodically painted.

And/or:

• Thermal insulationWood is only a moderately good thermal insulator, and the frames of wood windows (accounting for about 30% of the window area) are, by what might be called Passive House standards, thermally inadequate. So some window manufacturers have responded by producing wooden frames that incorporate insulation material such as polyurethane foam or cork.

The 'Progression' window imported by Green Building Store.(Tilt & turn. Uwindow = 0.68 W / m2

°C.)

The illustration above shows a composite window that incorporates a fibreglass section for improved weathering, and some cork for improved thermal resistance. (Cork's thermal conductivy, λ, is 0.06 W/m°C, cf, 0.26 for redwood pine). The window is certified by the Passive House Institute.

Note that the diagram shows the use of external wall insulation – a favoured method for building Passive Houses. The insulation covers the jambs of the window (and the head). Even with cork incorporated into the frame, the less frame exposed on the outside, the better.

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The Vale PHI window

(Made in Wales by Thomas Joinery: Inward opening – Tilt and turn)

The above illustration shows a cross-section for a British-made composite window certified by the Passive House Institute.

uPVC

PVC stands for PolyVinyl Chloride, a material which is sometimes referred to simply as 'vinyl' (eg, when used for gramophone records and floor coverings). PVC is a thermoplastic, and that means that it can be re-melted and potentially recycled. (The European PVC industry has set up the VinylPlus programme to increase the recycling of PVC – scrapped window frames form a substantial proportion of PVC waste.)

uPVC ( or 'PVCu', or 'PVC-U') is an 'unplasticised' version of PVC. Without plasticisers, the material is relatively hard and rigid, and uPVC products are widely used by builders – for guttering, drains, and waste pipe, as well as for windows.

Cheap windows for a new build can be made by a local window manufacturer using sections of pre-formed uPVC profiles. (The uPVC profiles may well come from Germany – eg, REHAU, Veka – though some profiles are produced in the UK.)

Typical uPVC window profiles (frame and sash)

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The interior of a profile is divided into multiple chambers to give both strength and thermal resistance. (Small chambers reduce convection currents in the air inside.) A longish section needs to be reinforced inside by a length of steel or aluminium, and this obviously reduces the thermal resistance of the profile.

Example PVC window profile (Veka Softline 70mm)Note:Steel reinforcement in frame and sash (coloured red).Five chambers (numbered). More chambers result in greater width.Width of both the frame and sash profiles (sometimes called depth) is 70mm.

There is a European standard applying to window profiles (BS EN 12608:2003). Of particular interest is the thickness of the exterior walls of the profile:

Class Wall thickness (mm)

A More than 2.8 mm

B 2.5 to 2.8

C Less than 2.5

Thickness classes for walls of uPVC profiles

Most profiles are in Class A or B. Class A profiles generally perform better – and cost more. (The Softline 70mm profile above belongs to Class B.)

Corners of frames are invariably mitred and welded together. Joints with a mullion or transom may be welded or screwed together. 'Fully welded' is preferable.

uPVC sections tend to be bulkier than their wooden counterparts. There tends to be more frame and less glass, and this has an adverse effect on the thermal characteristics of the window.

Nowadays, glazing beads are usually on the inside, so that the Insulating Glass Unitscan be fitted from the inside – which is easier than from the outside and which gives more security (arguably) against intruders.

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The Passiv uPVC Tilt & Turn window, by Munster Joinery

Some uPVC windows have Passive House certification – with PU foam insulation inside. (See the diagram, above.) The foam not only insulates, it also gives some rigidity, allowing the metal reinforcement to be dispensed with.

Note that uPVC windows do not have to be white. Some profiles are available with laminate foils that give a woodgrain effect, imitating oak or mahogany, for example. A variety of plain colours may be available, too.

Aluminium

The aluminium window industry is rather similar to that for PVC windows. There are a few big manufacturers that extrude the profiles and many more local assemblers of the windows.

Aluminium has a high thermal conductivity, and it may seem surprising that an aluminium window frame can have a thermal performance that meets modern day requirements. In fact, many aluminium windows are listed on the BFRC website as having an 'A' Window Energy Rating. How do they do it? –

Such a window profile is composed of two slim aluminium profiles, one for the outside and one for the inside. The two are joined by strips of polyamide plastic, giving a thermal break between inside and outside.

Aluminium is stronger than PVC, allowing windows to have slim frames. Corners may be crimped together or welded – the latter being stronger and more costly. The frames not only look elegant, they also allow in more solar gain. They can be powder coated in a wide variety of colours – it may be possible to have different colours inside and out.

A different treatment is to anodise the frame. An electric current is passed through the frame while it is suspended in an acid. This causes a film of aluminium oxide to be formed on the surface which is very inert and hard. This metallic finish may be coloured, too.

(If you are building by the seaside, be aware that while sea air may be good for your lungs, it is not good for aluminium, which becomes pitted in a salty atmosphere.)

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Fibreglass

Another name for fibreglass is Glass Reinforced Plastic (GRP). Profiles in this material are somewhat akin to their PVC counterparts, but in their

manufacture they are pultruded (pulled) rather than extruded (pushed). Fibreglass is stronger than PVC, so the fibreglass profiles have a smaller cross section. Fibreglass windows have elegant thin frames that are rot resistant. If a frame is damaged, it can be repaired – damage to PVC frames is difficult to repair.

Fibreglass expands and contracts at a similar rate as glass, reducing the stress between glass and frame in extreme temperatures. (PVC expands at seven times the rate of fibreglass.) And as with PVC, frames can be insulated with PU foam. The corners of fibreglass frames and sashes are held together with moulded plastic blocks.

The use of fibreglass for windows has been pioneered in Canada, but they are now becoming available in the UK.

Next month: Pre-election special.July issue: More about windows.

FURTHER INFO:

Wood Windows – Designing for high performancePatrick Hislop Third edition published by TRADA, 2009. 86 pages, £42.

Green Building StoreEcoContract Ultra PHI certified windows, made in Lithuania.They also supply the Progression range of PHI certified windows, with Thermowood and cork. Made by Slavona in the Czech Republic.www.greenbuildingstore.co.uk.

Thomas JoineryMake the 'Vale' PHI certified window in Pembrokeshire, Wales.Frame: larch with PU foam.www.thomasjoinery.co.uk.

The Window ManHe sells uPVC windows – and his idiosyncratic website tells you what to look out for ifyou are buying uPVC windows. The website indicates that he has links to a network of local suppliers and installers (for wood and aluminium, as well as uPVC).www.thewindowman.co.uk.

Senior Architectural SystemsProduce aluminium, composite, and fibreglass windows.www.seniorarchitectural.co.uk.

Munster JoineryThis Irish company produces windows and doors in the UK. Its window ranges include varied PHI certified windows (tilt and turn or outward opening casement) – in AluClad pine, hardwood, uPVC, and aluminium. Double, triple, or even quadruple glazed.www.munsterjoinery.co.uk

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VinylPlusVoluntary programme by the European PVC industry to improve the sustainability of PVC. www.vinylplus.eu.

Words: 1845.

© Copyright article by Robert Matthews in SelfBuild & Design magazine, May, 2015.

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