multiple glazing

Types of sealed unit

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 too 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.

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 the uncomfortable cold radiation feeling you get next to a cold surface more +/-»
• avoiding the flow of cold air which falls down a cold surface  more +/-»
• high level of insulation to save energy more +/-»

The Passivhaus Planning Package ensures that individual windows are up to the correct standard and that the house as a whole achieves sufficient insulation.

There is a good forum discussion on the more technical end of multiple glazing at the Green Building Forum

Guarantees

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)

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. A useful guide can be found on page 18 of Howarth Timber‘s web site.

Upgrading sash windows to take double glazing

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).

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. To rectify this it is usually necessary 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.

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 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 1W/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. 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 not aware of any companies doing this at present but they may be 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 on the eighth floor of a block of flats may be exorbitantly expensive to replace.

There is an interesting discussion on a Canadian forum about the pros and cons of such work.

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.

cavity closers / liners

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.

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

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.