Velux window - glazing options?

redser7

We're getting a small extension soon and need French doors and a Velux window in the vaulted roof section.
I was talking to a joinery about double or triple glazing options for the French doors. The salesman made the argument that we should just go for double as the heat will be just p*ssing out of the Velux window anyway and we wouldn't see the benefit of the triple glazing. He's not providing the velux btw.
Does he make a good argument here?
Also, it got me thinking about the velux window. I'm not sure what the builder plans to provide us with but are there glazing options for Velux windows and can someone recommend what we should go for so as not to have all our heat p*ssing out
The velux will be approx 1.2 m square.
We currently have 18 year old double glazing in the rest of the house. I guess we would like to upgrade that in a few years.
Cheers

redser7

Also, the triple is only 150 extra so it's not too much of a hardship to pay for it.

hfallada

You can see the u values for velux on their website. There isn't a whole load of a difference between the u values for triple and double glaze considering the large difference in price. Some on this has already done the sums and apparently it's cheaper in the long run to get double glaze considering the heat loss.

A lot of Americans don't use velux style windows due to their poor energy efficiency

redser7

Thanks for that. It is a 1.8m set of doors with one sidelight of 0.425m. The triple costs 150 extra.
The U values given for the full units are 1.3 for double and 0.85 for the triple. Is that a significant difference would you know?
Cheers

Clseeper

Redser7,

I did some really quick calculation using PHPP Dublin climate data. If I just assume a door 2.25x2.00m with the two different U values you quoted, this is what I get;

Heat loss (1.3U) = 364 kWh/yr
Heat loss (0.85U) = 240 kWh/yr
So difference of 124 kWh/a

So depending what you use to heat your house with you would save roughly €9.30/yr if oil versus €25/yr with Electricity by going with the triple glazing. Prices from SEAI domestic fuel Jan'15.

This doesn't include heat gains from solar, thermal bridging losses on install. Plus my guess is those U values are for the glass alone and not the whole unit including frames.

So there are the rough figures. My take is speed the money on things you can't change at a later date (Roof, walls, windows, insulation etc). You can change a sofa or kitchen far easier.

redser7

Thanks a lot for that detail! It gives me real world perspective on the differences. btw those are the real full unit values (frame and glass combined). In each case the glass was lower.
The other thing I'm thinking is that the triple has 5 hinges and the double has 3. Maybe the triple is a sturdier door with the extra hinges.

Clseeper

The triple glazed units will be a good bit heavier so that probably accounts for the extra hinges. I have some large sashes with 48mm triple glazing and there was no way I could lift it on my own. Works fine on the hinges though.

So obviously my bias is towards the triple but the other thing I can think off it the comfort factor. The higher heat loss could mean a larger temperature difference near the window. Not something I could comment on without looking at the thermal data and possibly not something you may notice much in real life.

Good Luck.

redser7

Thanks again!

Calahonda52

U might also get a better roof U value if u drop the Vaulted roof and just box out for the velux

redser7

Thanks for the suggestion but we're building onto an existing sloped party wall. Don't want to get into squaring that off.

Southern Comfort

Hi Redser. I presume you're thinking of the 114 mm X 118 mm Velux window.

I think you should go for a triple glazed French door and a quadruple glazed 114 X 118 Fakro roof window. I think you'll find this interesting.........

Let's assume you go for a white pvc roof window. The best triple glazed Velux (GGU 0062) has an overall (glazing and frame) u value of 0.81. It costs €1002. The flashing is €81 and an insulation collar is €144. That's €1227 plus VAT.

The Fakro FTT U8 Thermo 114 X 118 is a quadruple glazed window with krypton gas, which is superior to argon. (The Velux GGU 0062 triple glazed also contains krypton). The glass unit in the Fakro is 4/12/4/12/4/12/4 (52 mm) and the glazing u value is 0.3. (The Velux glazing u value is 0.5).The overall (glazing and frame) u value of the Fakro window is 0.58. It comes with a special thermal, highly insulated thermo flashing with a total price (window and flashing) of €1200 plus Vat. And it's certified as passive.

Heat rises so your roof will need a very good u value. The Fakro window will give you a superior u value to the best Velux window and for less money.

I have no connection to either Velux or Fakro. I have both Velux and Fakro double glazed windows in my house and both are excellent products.

To add a bit more balance, Velux do make a passive window also. You won't find it in their brochure but it is available as a special order. It has a triple glazed outer section and a double glazed inner section, separated by a wider air gap. It's the GGU SK06 008230. It has an overall u value of 0.51 but, including the flashing, the 114 X 118 costs €1710 plus VAT.

And, with regard to the triple glazed French door, I'd go for something like Vista Therm Elite SE 44 mm glazed units. They will give you a glazing u value of 0.6.

Hope all this helps to contribute to a cosier house!

redser7

It does indeed. Thanks so much for all that effort and advise!!

Southern Comfort

You're very welcome. 👍

Southern Comfort

Another option, Redser, would be to go for the Fakro FTT U6 window. It's the triple glazed model and, including thermo flashing, the 114 X 118 cm window in white is €875 plus VAT. It has the same u value as the best Velux triple glazed one (0.8) but is €352 cheaper before VAT.

I popped into their showroom in Naas a few days ago. The entire Fakro range is on display there. The FTT U8 is passive but the U6 is a serious product also.

redser7

That's great! Thanks again for the advice.