site wide search

related pages


Green Building Store ULTRA

Green Building Store ULTRA

The supply chain for windows

If the UK is to meet the 2021 EPBD 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 Passivhaus standard. This tends to involve a very much higher specification than current Building Regulations cover especially as regards frames and air tightness.

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 part L1A and part L1B of the Approved Documents.

The technical details about the energy rating of windows are supposedly at the BFRC web site but in fact the details are all wrongly marked……  see more

Here is a copy of their ‘Homeowner’ page as of 12/10/2013.

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 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!

Basic considerations

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.)

reasons to build ecologicallyMultiple glazing

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 E’ (low emissivity) glass on the outer pane. This helps to reflect heat back into the house.

Fitting sealed units

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 person 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 more
      When 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.

    • avoiding the flow of cold air which falls down a cold surface   see more
      When 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.

  •  the overall whole window value (Uw) has to be less than 0.8 W/m2°K.   see more
    Window 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 Ψ(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.

There is a good forum discussion on the more technical end of multiple glazing at the Green Building Forum, and a good explanation of how low energy windows work

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

Note that windows are a Controlled Fitting for building regulation purposes. See Section 7 of Approved Document F

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

Removing the old glazing can be done by taking out the sashes and stacking them on the floor on top of each other with a thick layer of water based paint stripper applied to the putty all round each pane. Then wrap the whole pile tightly in polythene or shrink wrap (so that the stripper does not evaporate) and leave to soak for 12 to 24 hours. After that the panes should come away easily and old putty be scraped out. This works well for a number of sashes in a stack.

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.

Fensa now do a useful Guide to Compliance with Building Regulations for replacement doors and windows in dwellings

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 value 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 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 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 beam 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.

plan view

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.

Toughened glass

The building regulations require that toughened glass is used in certain places where people are liable to injure themselves on impact with the glazing. see diag. 5.1 below.

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

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 BRS 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 more
    This 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 more
    Most proprietary roof windows such as Velux have built in ventilators.

  • as an escape route if there is a fire     see more
    The Building Regulations define define various means of escape

  • for heating     see more
    For passive solar heating of a house (especially when MVHR 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 part K, paragraph 1.10. It can be reduced slightly for loft conversions.

  • to provide a roof window balcony     see more
    E.g. the Velux Cabrio or the Fakro equivalent


There are several possible drawbacks with roof windows

  • heat loss     see more
    This is covered in the building regulations parts L1A and L1B along with their revisions. The Passive House criteria are considerably stricter

  • possible overheating     see more
    The Building Regulations take this into account, (L1A paragraph 46) for new buildings as does the Passive House standard

  • security risk     see more
    from a person entering through a window which has been left open

  • the need to clean them     see more
    Opening 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 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 permission     see more
    In 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 more
    Especially 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 more
    Nothing 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 more
    It’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.

3 comments to Windows

  • Neil Green

    Hi John, we can help with this for you. we are FENSA registered company, visit our website and build your own price for this in minutes

  • Jan Merquis

    I live in the middle of a block of 5 flat roof houses and have just had a new roof and two skylights. One skylight is over the bathroom and one over the stairs. My old skylights were made of glass with wire and impossible to see through. My new ones are clear plastic were anyone on the roofs, our local low flying police helicoptor can see through and watch anyone in the shower. Is this legal? I was not consulted or advised that clear plastic would be fitted and living on top of a hill my house is subject to the sun all year round and the bathroom is like a sauna after 12 when the sun moves over to the back of the property.

  • John

    Where can I source stepped double glazing units?

Leave a Reply

You can use these HTML tags

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>



What is 13 + 13 ?
Please leave these two fields as-is:
IMPORTANT! To be able to proceed, you need to solve the simple calculation above. (so we know that you are a human) :-)