A question regarding U-values

Mike-Graham

Hi,

We are going to be building a new extension next year, and are going down the route of eco home, with the aim of getting as near to passivhaus as possible.

Obviously triple glazed seems to be the way to go, but getting such low passivhaus u-values seems near on impossible in the UK (Passivhaus requires 0.8U, best I've managed is 1.0U).

My question was related to a BiFold door we've been quote for, an Aluminium one with triple glazing, £2800 with a u value of 1.5, but we can get a wood one with double glazing for around £1700 also with a u-value of 1.5 for the whole product. I'm guessing between these 2 there will be no difference right?

I've just sent off for a quote for a Lacuna bifold which claims to reach 0.7U and Class 4 airtightness, but im guess the ££ will be astronomical.

In terms of the difference between 0.7U and 1.5U, on a 2m by 2.1m bi-fold, how much would it actually save me in energy?

Regards
Mike

HappyMJ

The difference between 0.7 and 1.5 is 0.8 multiplied by 2m by 2.1m is 3.36 then you multiply by the temperature difference between outside and inside let's say 21 inside and 4 outside which is 17 and comes to 57. Divide by 1000 and you have extra kWh required per hour. Multiply that by the cost of your heating (taking efficiency into account) and that's how much more per hour it will cost or how much it will save...depending on how you look at it. It's not an exact figure though. It's just an estimate. My guess about £5 per year.

Mike-Graham

Wow, thanks for the explanation, very helpful! :-)

So basically, its going to take me a hell of a time to recoup the additional costs of triple glazing, unless this new quote comes back at less than I thought, i'll keep my fingers crossed!

With regards to double glazing, its quite well known that its leaky i.e. the argon leaks out of the unit over time, I assume this also affects triple glazing, but is it affected as much, or more?

Smiley_Dan

Bi-fold doors are generally pretty difficult to find that are half decent. Getting them in a Ph is a challenge.

U for windows here: http://www.greenbuildingstore.co.uk/page--energy-saving-windows-doors.html goes down as low as 0.68. Plenty of other suppliers.

You'd be best off on a specialist forum btw.

Mike-Graham

Yeah I got a quote from GBS for a window, again for the same extension, and it was going to be around £3000 I think, way too much money in my opinion for the added benefits.

I've tried a few different forums and none really seem that good, some give odd replies, and on others no replies at all... :-(

Do you know of any good ones?

Smiley_Dan

Good windows are expensive. Ph is expensive.

Green Building Forum.

Mike-Graham

Indeed, but if as HappyMJ says, I only stand to save £5 a year on going for something much lower in U value, there doesn't really seem to be that much point. I guess this kind of stuff is going to take off in a big way relatively soon, especially with CfSH 6, and prices will tumble, and better products will come to market.

I'll give Green Building Forum a try, thanks

Smiley_Dan

I make it £12 a year for my values for degree days

Mike-Graham

Yeah will still take a lot to make back the difference i'd say.

I know one of the other points of the PH stuff is longevity, some units lasting 50years, which got me thinking, why would anyone want to be stuck with something for 50 years, even if it is good, 10 years time there'll be something better out. So I think although i'll go triple glazing where I can, im not going to go too far.

Robwiz

I think there's an incompatibility between a bifold door and Passivhaus because a true PH design has to limit the window area due to reduce both winter heat losses and summer solar gain.

I'd be more concerned about air tightness on a bifold door – with good insulation to floor, walls and roof, the fabric losses come right down but unless you specify PH windows/doors, your ventilation losses will be high.

Smiley_Dan

It's diminishing returns though. U values don't go negative so how much better will it get? I suppose they could improve in different ways; integrating solar PV etc.

A lot of people put 3G on the north side and 2G on the south side. That's possibly misguided, but just putting that out there.

There's a comfort factor, I'm told. I have 2G and I can still feel the convection currents next to the window, and feel cold with my back to it (heat radiating from my body to outside). This simply doesn't happen with 3G, so I'm told. I'm talking high quality 3G as linked above, not mid grade uPVC stuff.

I have inspected the eco contract GBS windows. They are very high standard to my untrained eye. Indeed, they almost don't look like wood they appear so "polished" with a factory applied paint (10 year guarantee IIRC). One thing is re-painting them would be a pain because they are so heavy. You'll need about three rugby players to take them off their frames.

All Ph units should essentially be immortal... or did you mean time to repaint? Alu-clad claim much longer times between maintenance but you need to be careful about the profiles, allowing air movement etc.