Double or Triple glazing

[Lesgrandepotato]

We are in the process of a major renovation project of a 1980 split level bungalow. We are going to install Velfac windows we have some fairly large areas of glazing and a couple of sets of Bifolds.

The question remains whether we should go for the triple glazed units at a cost of about 15% extra or not. House is in the south lakes so we get a bit of weather. Concerns are the possiblity of the external pane getting condensation, reduced light, the upsides being better insulation u of 1.0 vs 1.4, but will it make a real difference?

[944fan]

Triple glazing is usually only applied where noise insulation is needed. Whilst triple glazed units have better u-values they have worse g-values. This means the house gets less free heat from the sun as they let less of it through. The heating system then has to work slightly harder to heat the house.

Often this worse g-value offsets the benefits of the better u-value.

[Lesgrandepotato]

So the fact we have a two storey glazed elevation facing the morning sun should suggest we go double glazed? Then maybe triple on the shady side of the house?

[944fan]

Tricky. A significantly large glazed area is going to lose a lot of heat so the better u value might be more desirable.

You might want to get someone to do a heat loss calculation for both using SAP. That way you can compare the different u and g values and see which gives you the best overall performance.

I used to write the software to do this but unfortunately I left that job a couple of years ago so no longer have access.

[mel]

I've just had Triple 37mm units fitted upstairs in the bedrooms and 25mm Doubles downstairs, from the outside everything matches and you wouldn't notice the difference. The reason being is I live on a main road and although I'm used to traffic noise and sleep well it was a concern to reduce noise as much as possible in the sleeping rooms. It's certainly done the trick and muted almost everything, the company I spoke to were open and honest in saying that the difference between 2 & 3 layers of glazing from an insulation point of view didn't justify the extra cost and they'd only advise it for noise benefits. This is in a pair of 1800's cottages that are converted into one house, so solid walls and no wonky window fixing bolts

[roofer]

We've just fitted some units in a School under Heathrows flight path. Outer pane is 10mm thick ! The effect on noise reduction is amazing.

[Mr GrimNasty]

You don't really get any worthwhile improvement with triple glazing units over double, what makes all the difference is having 1 pane in a much thicker heavier glass (doesn't have to be 'special' acoustic glass), then for the final kill secondary glazing 4-6" back from that.

It's the glass weight differential and the big gap (that you can't get in a triple glazed unit) that takes out the penetrative base frequencies.

[Blue62]

I'm thinking of triple glazing to keep the heat out, as we are glazing top to bottom and all the way across on a new build, south facing. I am worried about heat build up in the summer, but maybe there are better ways to keep the house cool.

[944fan]

Blue62 said:

I'm thinking of triple glazing to keep the heat out, as we are glazing top to bottom and all the way across on a new build, south facing. I am worried about heat build up in the summer, but maybe there are better ways to keep the house cool.

Whilst triple has a lower g value than double it is only slightly lower and still int eh region of 0.5. That means half of all solar energy hitting the glass will transmit through. Double will be more like 0.6/0.7.

I would be more worried about heat loss in winter with an entire glazed wall, although you should probably think about summer cooling too.

If its a new build you could spec an air source heat pump with underfloor heating that provides cooling also. However, reversible heat pumps are not eligible for the RHI so it may not be cost effective.