- The supply chain for windows
- Building Regulations for windows
- Basic considerations
- Fitting sealed units
- Passivhaus window standard
- Guarantees on windows
- Upgrading sash windows to take double glazing
- Secondary double glazing
- Repairs of fogged up sealed units
- Window liners / cavity closers
- Toughened glass
- Roof Lights
The supply chain for windows
If the UK is to meet the 2021 EPBDEnergy Performance of Buildings Directive. This is the EU directive that all new buildings shall be "Nearly Zero-Energy Buildings’ from 2021. standards then the window manufacturing industry will need to go through a minor revolution. Most companies are working to antiquated designs. A major exception is the Green Building Store who are both importing and manufacturing doors and windows to PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. standard. This tends to involve a very much higher specification than current Building RegulationsThese are the mass of regulations that cover safety, health, welfare, convenience, energy efficiency etc. in the way buildings are constructed. Not to be confused with Planning consent (which is more to do with whether you can put up the building in the first place). See more on the regulations cover especially as regards frames and air tightnessA measure of how leaky a building is to air. In other words, how draughty it might be. There are now standard fan pressure tests to check how air tight a house is and the Building Regulations have minimum standards for all new houses (L1A – Conservation of fuel and power in new dwellings (England)). A much higher degree of air tightness is covered by the Passivhaus standard. Most of the high quality doors and windows are now coming in from the continent, especially Scandinavia, Germany and the Baltic countries. They tend to be triple glazed and of very high quality timber with double strips of draught seals. PVC frames to a high standard are available but see the drawbacks of PVC. A particular problem concerns repairs; the almost honeycomb nature of modern pvc extrusions means that the fixing of hinges and other ironmongery is dependent on special types of anchor points and these can be difficult to repair compared with timber (although there are some specialist repair contractors).
The Wood Window Alliance do a good guide on the advantages of timber windows and how to specify and use them.
Building Regulations for windows
The Building Regulations cover the necessary insulation standards for windows under Fabric standards
The technical details about the energy rating of windows are supposedly at the BFRC British Fenestration Ratings Council web site but in fact the details are all wrongly marked…… see more
Most of the explanations on the right do not correlate with the example label on the left. For instance an energy index of -14 is confused with -3. (except that they have mixed up the term ‘energy rating’ with ‘energy index’). The effective air leakage of 0.01 W/(m2.K) is confused with 0.10 W/(m2.K). It’s easy to make mistakes on a web site and no doubt this will soon get updated but the point is that this is symptomatic of where the industry is at. Their web page has been like this since March 2009 and they are supposed to be the people in charge of making this information clear! That’s over four years that homeowners have been getting confusing information presumably without any of the BFRC’s trade members getting in touch! What is one to make of this?
Update 17/02/14. Yes the example has been updated, lost its TV personality and become slightly more confusing with minus values now being plus values.
- Is this seen just as pseudo science which acts as window dressing along with a TV personality?
- Does no one really care?
- OR is it kind of in the interest of a rather unprogressive window industry to have confusing information floating about?
The Superhomes"Opening doors to low energy refurbishment" The homes, which achieve at least 60% less CO2Carbon dioxide is a gas which is given off when carbon based materials such as fossil fuels (coal, oil, and natural gas) are burned. It is called a greenhouse gas because it works like the glazing of a greenhouse and causes global warming emissions, have open days. see the Supehomes website web site corrects most of the mistakes of the BFRC except they confuse the Energy Index of -14 with that of -3. These mistakes and confusions grow legs!
There are a few basic things to bear in mind whether you are doing the odd replacement or involved with new build:
Whole window standard
Whereas the insulating properties of windows used to be based on the glazing unit, they now include the frame to give a whole window figure. (With Passivhaus design there is also a factor included for how easily heat can escape round the edge of the window due to the way it is installed. See details)
As a minimum try to use sealed units with a spacing of 16 or 18mm. rather than 6mm. This is the optimum gap for heat insulation. However the frames need to have sufficient rebate for this. See window design. Use a filler of argon gas rather than air. It gives better insulation (krypton and xenon gasses give even better insulation but are rather expensive). Specify ‘Low Erefers to glazing with low emissivity. A special coating on the glass prevents radiant heat escaping by reflecting it back into the room.’ (low emissivity) glass on the outer pane. This helps to reflect heat back into the house.
It is important that sealed units are properly glazed into their frames. Glazing tape and setting blocks must be used to support the units and to keep them free draining. Glazing beads should be used as putty or most mastics will have a harmful effect on the spacers which separate and seal the panes of glass.
Competent persons (for window replacements)
See the FENSA competent persons web page. A competent personUnder the Building Regulations, a Competent Persons Scheme allows individuals and enterprises to self-certify that their work complies with the Building Regulations as an alternative to submitting a building notice or using an approved inspector. See more at the government Planning Portal can self certify so it does not need to go through Building Regulations approval.
Passivhaus window standard
If you are aiming for Passivhaus standard then you will probably be using argon filled triple glazing with ‘Low E’ and insulating spacers
This standard has two related parts to it
- high level of comfort
- avoiding cold radiation see moreWhen you are close to a cold surface your skin radiates heat towards that surface and you sense this even if the air around you is warm. The Passivhaus standard aims to keep the inner surface of all windows no lower than 17°C when the outdoor temperature is minus 20°C so that there is virtually no discomfort. There is more detail about this here
- avoiding the flow of cold air which falls down a cold surface see moreWhen the warm air in a room meets the cooler surface of glass in a window it starts to sink and flow down the window surface. This can be quite uncomfortable if you are sitting close to the window. The Passivhaus standard ensures that the speed at which the cooler air flows downwards is less than 0.1 m/s. which is hardly noticeable to most people. There is more detail about this here
- avoiding cold radiation see more
- the overall whole window value (Uw) has to be less than 0.8 W/m2°K. see moreWindow manufacturers achieve this by working with a combination of four factors concerning insulation which make up the overall value –
- the glazing U-value Ug and the surface area of the glazing Ag
- the U-value of the frame Uf and the surface area of the frame Af
- the thermal bridge coefficient at the edge of glass Ψg (warm edge spacers) and the glass edge length lg,
- a forth factor is the thermal bridge due to the installation of the window in the exterior wall ΨInst and the length lInst where the window meets the wall
The Passivhaus Planning Package ensures that individual windows are up to the correct standard and that the house as a whole achieves sufficient insulation.
As window design becomes more sophisticated (e.g. Passivhaus windows tend to be triple glazed) the catches, handles and ironmongery generally become heavier and this, along with multiple layers of draft excluder, can make the opening and closing of windows more difficult. It is important that handles are firm enough and have enough leverage that they work easily.
Guarantees on windows
It is worth buying sealed units with the best possible guarantee and ensuring that they are fitted as prescribed so as not to invalidate the guarantee. It is particularly important to sit the units on glazing blocks and maintain the draining area at the bottom of the unit so that it remains dry.
Most sealed units now come with a guarantee of at least ten years. Initially this sounds like a good deal but when you consider the cost of changing units every ten years it not only makes a mockery of cost saving but also the energy used to make the units and the energy to replace them etc. would probably start to approach the energy saved over their short life time. It is not uncommon for poorly made units to fail within a few years.
Failure first shows up when units mist up in cold weather. If this happens within the guarantee period then all the glazing should be checked by an expert and complaint made to the supplier. A gadget exists which can check whether argon has leaked out (assuming argon was used in the first place) and this can be a good indicator of the condition of the unit.
Following a complaint of failure the first thing the supplier will want to do is check that the units were properly installed – particularly that the lower edge is free draining and that the correct edge sealants were used. (See next section)
Upgrading sash windows to take double glazing
All too often good windows are ripped out and replaced when it is possible to upgrade them. If timber windows are in good condition there are usually ways of at least double (and occasionally triple) glazing them. see more
Next is the decision about what sort of sealed unit to use. There is seldom enough depth of rebate to go for a 4/16/4 mm. unit so a compromise can be reached. One way is to use a ‘stepped’ unit which gives a good insulation gap but leaves a visible edge around the inside of the window and this may need hiding with a timber bead. This is often used on larger sash windows (rather than Georgian ones). It may also be possible to route out a deeper rebate. A joiner can advise on this.
On small sash windows such as Georgian ones it may be possible to use a thinner glass with a narrower gap, say 3/6/3 mm. providing the rebate is deep enough. Otherwise you will probably need to resort to new sashes or, in the case of a listed building, secondary glazing.
Of course, with vertical sliding sash windows the total weight is increased by the extra glazing and this affects the balance with the sash weights. One method is to use spring loaded balances to support the sashes. Alternately it may be possible to replace the existing sash weights with heavier ones. This is not an easy business. One way is to cast heavier weights by melting lead and pouring it into short lengths of steel tube (with sash chord attachment wire eyes cast in). This produces a heavier counterweight to match the increased sash window weight. It all needs calculating to work properly. There are now companies making heavier weights to order. A worse but cheaper way is to hang extra weights onto the existing cast iron weights. This can be done by slitting short lengths of lead pipe and sliding them onto the top of the iron weight. The problem with this is that it restricts the travel of the sash because the sash cord is effectively shortened.
Secondary double glazing
There are many cases where sealed unit double glazing cannot be used.
- It is often not allowed on listed buildings.
- If existing windows are being retained and upgraded the glazing bars may not be sufficiently deep for sealed units
- It may be impossible to use sealed units with vertical sliding sash windows.
- Sound insulation may necessitate a larger gap between panes, or thicker glass.
The answer then might be to use secondary glazing, usually with the new glazing fixed to the inside of the window frame itself.
Frames for secondary glazing are nearly always aluminium or PVC with timber seldom if ever used. This is because of the general need for a thin profile to the glazing section to allow it to sit within the existing frame. Aluminium is usually preferable to PVC as it lasts longer and can be recycled. It is common for the secondary glazing to slide horizontally to allow for cleaning between the panes and for opening the windows.
The U valuemeasurement of how much heat escapes (or gets in). The units are W/sq.m./°c. see Insulation properties of secondary glazing is about 3W/m2/deg.C: not as good as sealed units which are about 2W/m2/deg.C or high spec triple glazing which might be down around 0.8W/m2/deg.C
Repairs of fogged up sealed units
Seal double glazed units may fog up with moisture after a number of years due to the seal on the spacer becoming partly detached or simply due to moisture slowly permeating the seal. The normal solution is to have the pane replaced but it may be possible to have it repaired in situ. see more
possible repair solutions
This is something that has always been done in colder countries such as in the far north of Europe and Canada but may or may not be available as a service in the UK. We are only aware of one UK company doing this at present (see the Crystal Clear Window Works who repair fogged up windows). There may be others around (if you hear of any we would appreciate an email )
Repair will not be as good a solution as a new unit for three reasons
- if it was an argon filled unit it is not possible to replace the argon – only air.
- it will probably not be possible to get the inside of the glass totally clean (depending on whether the seal has broken or just leaked)
- you will be left with a small plastic valve in one corner of the window
However there are also a couple of factors which may be in favour of such a repair
- the embodied energythe total amount of energy it takes to make a material (or a building). See more on embodied energy involved in repairing a window is much less than for replacement – glass is high in embodied energy
- if the window in question is very difficult or expensive to replace then repair may be a better option. For instance a very large window in a very inaccessible position may be exorbitantly expensive to replace.
- If future repair work is planned for the house, this may provide a short term remedy.
There is an interesting discussion on a Canadian forum about the pros and cons of such work
Window liners / cavity closers
see also Window design
With the emphasis on thicker wall insulation and less thermal bridgingthis is a pathway where heat can easily escape (or get in) through some part of the structure. It is usually caused by some element of structure such as a steel lintel or wooden studwork. Also known as a cold bridge. see more on thermal bridging the subject of window and door liners takes on a new look. At the lower end of insulation values such as traditional masonry walls with wide cavities there are proprietary closer products which will allow for a cavity up to 150 mm. They tend to be plastic extrusions with an insulated core and built in wall ties.
With timber frame construction, particularly post and beamSubstantial, usually horizontal structural member. there may be the option to simply span the outer and inner layers with a ply closer, say 15mm ply. This is a simple method which makes a strong joint with very low thermal bridging. The plan view below shows a fully filled 200mm wall with ply closer.
With Passivhaus design this potential route for heat loss is taken into consideration along with the window frame itself, the edge effect of the spacer on the glazing and the centre pane value of the glazed unit.
The building regulations require that toughened glass is used in certain places where people are liable to injure themselves on impact with the glazing
The darker areas show where toughened glass is required. The image is taken from a page of the building regulations which explains several other legal requirements of glazing.
Roof lights are not always used as creatively as they might be. It is worth looking at the opportunities and drawbacks with them:
Roof windows may be useful for several reasons:
- to let light into a room see more
Roof lights tend to be very efficient in terms of allowing light into a room compared with the same size of window in a wall. They not only have an unimpeded view of the sky but they are more centrally placed over the area they are lighting.
Using natural light whenever possible saves a considerable amount of energy. The amount of light entering a room through a window is partly to do with the window size but more importantly is to do with how much sky is visible from the window and the size and shape of the room. The light entering directly from the sky may be hundreds of times that which gets reflected from the ground or trees (although sometimes water can be used to reflect light up into windows). The maximum amount of light is provided by roof lights positioned over the middle of a room. One of the ways of measuring the natural daylight falling on a particular area in a room is by calculating the daylight factor (DF). This is the ratio, as a percent, of the daylight that reaches a point inside a room compared with what light would be there if there was no roof or building, in other words just an open sky. (It is measured under an overcast sky and is measured at table top height, 850mm, and is averaged over the room).
The measurement takes into account how much direct sky is visible from within the room, how bright the external surfaces are outside the room and how reflective they are within the room. The diagram shows how the light coming in through a window tends to be much more restricted by surrounding shading than is the case with a roof window.
It works out that a DF of lower than 2% looks gloomy and artificial lighting is required. Between 2% and 5% is usually suitable without artificial lighting provided it is not for a working area (such as reading or in a kitchen) and anything above 5% looks quite brightly lit. There are various formulas for calculating this but it usually comes down to the experience of the architect.
If you are very concerned about a particular lighting situation you can ask your architect for a calculation to be done or you can use the BRSBuilding Research Station Daylight protractor and the Simplified Daylight Tables described here. The brightness of the sky is assumed to be 5000 lux (lumens/sq.m.) and so a DF of 6% requires a minimum level of lighting of about 300 lux for a horizontal working area. Offices are often lit at 500 lux whereas domestic situations may go down to around 50 lux where only a little background lighting is needed.
- to provide a view see moreThis of course depends on how low down in the roof the roof window is situated. Quite often with older houses a low roof light will interfere with an existing timber purlin. Cutting a purlin is usually a serious structural matter although a structural engineer may be able to work out a way round this.
- for ventilation see moreMost proprietary roof windows such as Velux have built in ventilators.
- as an escape route if there is a fire see moreThe Building Regulations define define various means of escape[for the purposes of part B of the Approved Documents] - Structural means whereby [in the event of fire] a safe route or routes is or are provided for persons to travel from any point in a building to a place of safety.
- for heating see morefor passive solar heating of a house (especially when MVHRMechanical Ventilation with Heat Recovery. This is usually a double fan arrangement which extracts stale air from the house and sucks in fresh air at the same time. As the warm stale air is blown out, heat is extracted from it and passed over to the cool incoming air by means of a heat exchanger. With the latest technology, over 90% of the heat can be recovered. (see Passivhaus standard) is used).
- gaining room height over landings in attic conversions see more
Occasionally, when new stairs are provided to serve an attic conversion there will be limited headroom if the landing is near the eaves. A carefully situated roof window may give slightly more headroom along with a brighter space. The Building Regulations cover the headroom required over stairs in K1 paragraph 1.10 It can be reduced slightly for loft conversions.
- to provide a roof window balcony see moreE.g. the Velux Cabrio or the Fakro equivalent
There are several possible drawbacks with roof windows
- heat loss see more
- possible overheating see more
- security risk see morefrom a person entering through a window which has been left open
- the need to clean them see moreopening roof lights can usually be turned right round to enable cleaning. Fixed ones can be glazed with self cleaning glass. Dome roof lights are usually constructed of formed polycarbonate or acrylic, in two or three skins. Dirt does tend to adhere to plastic surfaces, especially as they start to age and roughen slightly. Usually they cannot be cleaned from the inside. As they are normally employed on flat or nearly flat roofs it may be possible to easily access them from the outside for cleaning. They often work well with flat(ish) living roofsA roof with a covering of soil or growing medium and plants. They tend to be divided into turf roofs with a 150mm layer of soil and sedum roofs with a thinner layer (about 40mm). see Living Roofs which tend to deposit bits of plant material on them when birds pick around for food. Of course you don’t get a clear view through a plastic roof light but that might not matter on a flat roof.
- problems with planning permissionthe legal basis for being allowed to do some form of development such as building a house. (not to be confused with Building Regulations which are all about whether the building is properly constructed). see more on Planning
the legal basis for being allowed to do some form of development such as building a house. (not to be confused with Building Regulations which is all about whether the building is properly constructed). see more on Planning see moreIn certain sensitive situations such as conservation areas planners don’t like the look of large areas of glazing on roofs. It may be acceptable to use the smaller ‘conservation style’ roof windows which have a vertical dividing bar to visually split up the glazing in the manner of the old cast iron roof lights. It may also be necessary to let the roof light into the roof slightly so that its surface is level with or very slightly above that of the surrounding roof finish.
- cost including structural see moreEspecially with loft conversions, it may be necessary to cut into a purlin to be able to insert a roof light at a level low enough to provide a decent view. As the purlin is one of the main structural members holding up the roof joists it will be necessary to insert some other means of support. This can become quite expensive.
- shading see morenothing worse than trying to sleep off a hangover with the sun belting in through a roof light without a blind. Providing the top of the window is reachable then a normal blind is fine. If it is too high up in the roof then motorised blinds are available.
- rain running in see moreIt’s surprising how annoying it can be to leave a tilting roof light (just above your bed) slightly too far open during a thunder storm, so that all the rain runs back and into the bed. And you didn’t realise and you come back home and climb into bed……..urgh! A restricting chain can be fitted to limit the tilt of the window.