Windowtrackworld - how to choose thermally efficient windows

You'll probably find triple glazed UPVC have the lowest thermal transmission. uw<1 (that's "mew" not "you" double you) is typical. The triple glazed windows will also have deeper frames which can mean less thermal bridging depending on how they are fitted into the wall.

But. You'll probably make more improvement by insulating the wall the windows a fitted in on the inside than by changing the windows. A typical UK cavity wall with an insulated cavity has a thermal resistance of R=1.5 ( R = 1/u ). Adding 100mm of insulation will give you a wall with at least R=4 depending on the insulation used. Dividing the losses through the wall by three will usually gain you more than reducing the heat loss through the windows by 30-50% with state of the art windows.

Good point - thanks.

I m planning new windows. 1 local co has just done a runner with clients money and COVID loans. Top tip, only use fensa reg company as deposits are protected, or put the deposit on credit card.

Said company has done it 3 times now.

Decent curtains ! Far cheaper.

As long as your current windows aren't wrecked, replacing them is extremely unlikely to ever pay off from an energy perspective. Even double Vs single glazing has an roi of something like 20 years.

Triple glazing is not always thermally better than double. Check out the gap between panes. Some triple glazing reduces the gap so that the overall frame depth remains the same, the net result is a triple glazed window that is less thermally efficient than its equivalent double, but paradoxically more expensive.

As it happens, were were at a window company on Friday, picking what we want.
The only option I would go with is Thermalite glass. It is almost as good, insulation wise, to standard triple glazing, but half the price. Carlisle Windows have a demonstration unit in their showroom, one side has a standard K glass DG unit, the other has Thermalite DG. Both have a heat lamp underneath them. When turned on, the standard DG unit gets hot straight away, the thermalite stays cool. A total difference between the units, and that transfers to less heat loss in your house. Not many DG sellers do it, so you’ll need to shop around to see who can supply. Calisle Window systems do it if you are local.

Speak to a few, ask about the specification of the glass and ask which manufacturer they use for the frames. Id use a local company personally but one I'm happy with. The national companies can be as variable as small ones.

If they aren't prepared to talk about the specs and only focus on price walk away.

The only option I would go with is Thermalite glass. It is almost as good, insulation wise, to standard triple glazing, but half the price. Carlisle Windows have a demonstration unit in their showroom, one side has a standard K glass DG unit, the other has Thermalite DG. Both have a heat lamp underneath them. When turned on, the standard DG unit gets hot straight away, the thermalite stays cool. A total difference between the units, and that transfers to less heat loss in your house

How does that affect solar heat gain ? Can see it being brilliant in hot country's where your avoiding solar gain but YMMV dependant on where you live

How does that affect solar heat gain ? Can see it being brilliant in hot country’s where your avoiding solar gain but YMMV dependant on where you live

I’d say it would be really good. We’ve been in conversation with them for a number of months now, and finally got to their showroom on Friday. I was sceptical about their claims of super insulating glass, but was very surprised at how good it is. The test pieces, the normal K glass was too hot to touch after 2 minutes, the Thermalite (could be spelt differently, I’ve since googled it, and there is little information out there about it.) was still cool after the same time. It is amazing to feel the difference.
The Owner of the Company also had doubts about its efficiency when it was trying to be sold to him, he had his house fitted with it to try it out, and he said he could feel the difference immediately. He recommends it to every client now.

I meant does it allow solar gain

I meant does it allow solar gain

That's a good question.... my guess is 'not as much' but I'd like to know. I love a bit of solar gain!

Edit:
From this it looks like it does reduce solar gain. So maybe better not to use it on SW/S facing windows, but do use it on other windows to reduce heat loss??

Aim for a decent frame too. I had aluminium windows from a company that didn't install them very well plus they had awful condensation on the bottom of the frame and glazing. Now replaced most with triple glazed (44mm) aluminium frames that are actually a larger plastic inner profile rather than the usual slim polyamide thermal break. I now have no condensation in my ensuite which was really bad before even with whole house heat recovery extraction.

Forgot to say, Old glazing units were planitherm too. Just shows how important the frames are too and make sure they are fitted well. Remove any draughts in the reveals so that they don't cool the inner frame.

The more glass and treatment you add the lower the coefficient of solar transmission "sw" will be. It's sw = .37 for the triple glazed treated units I've used for E and W facing windows but sw = .51 for the standard double-glazed window used in a S facing room. The uw are .11 triple (Edit: the manufacturer now claims .12) and .14 double (it's got uw = .14 on the sticker on the window but it's now uw .17 on the manufaturer's website)

The triple glazed windows definitely make the room noticeably darker when I shut them.

Having checked some numbers for this post I've realised that manufactuers have lowered the performance claims for their windows since they've adopted the latest EU normes.

My window supplier told me it's the air gap that does the insulating, if the frame allows, uprate the glass air gap.

Standard 28mm total, 40 odd fitted so I bought those. Not much more in price and tbh I can't tell difference but I bore all visitors showing them.

I posted on a thread recently as I have a similar problem to wooksterbo. I wouldn’t recommend aluminium “aluk” windows on that basis.

Part of the problem is I think there is nothing but air in the bit of the frame at the bottom to allow for drainage. There is then a small gap between the sill and frame on the outside which allows water out but also presumably cold air back in. Which the contacts the inner aluminium layer which accordingly conducts the cold straight through.