Airtightness

[MDC]

I'm discussing my potential new build with a timber frame company.

 

I want an air tight house. I've looked up 24 EPC's of houses they're associated with, and have discovered their average air tightness as tested on the EPC's is 3.2. The best result was 2.

 

They seem to think this is good. I previously lived in a house with a recorded air tightness of 0.8, so would like to get nearer that if I can. 

 

The figures for the roof, walls and floor generally are 0.14, 0.16, 0.14.

 

Without getting into the politics of EPC's [as I think they're the only publicly available measure of such things], how might it be possible to improve the air tightness for my build? 

 

Is it just about the discipline of the contractors or is there more to it?

[Thorfun]

without wanting to sound rude but i think the answer is to find another TF company that WILL get you to 0.8ACH if that's the level you want to get to. otherwise there are things you can do but it'll cost you time and money on top of what the current TF company will charge.

 

or maybe negotiate with them to get below 1ACH as a target. then they have the knowledge and kudos of achieving that and can then market it as a reason to choose them

 

[PeterW]

13 minutes ago, MDC said:

The figures for the roof, walls and floor generally are 0.14, 0.16, 0.14.

Those are uValues not airtightness values 

 

14 minutes ago, MDC said:

have discovered their average air tightness as tested on the EPC's is 3.2. The best result was 2.

OK but air tightness isn’t about the frame - it’s about everything including windows/doors/ventilation etc. You need to look at like for like.

[MDC]

46 minutes ago, Thorfun said:

without wanting to sound rude but i think the answer is to find another TF company that WILL get you to 0.8ACH if that's the level you want to get to. otherwise there are things you can do but it'll cost you time and money on top of what the current TF company will charge.

 

or maybe negotiate with them to get below 1ACH as a target. then they have the knowledge and kudos of achieving that and can then market it as a reason to choose them

 

Second paragraph is the one. I have a couple of alternative timber frame supplies in line. I like the people I'm dealing with at the moment, so would enjoy setting them the challenge.

[MDC]

41 minutes ago, PeterW said:

Those are uValues not airtightness values 

 

OK but air tightness isn’t about the frame - it’s about everything including windows/doors/ventilation etc. You need to look at like for like.

I put the uValues in, to show what they do generally achieve. We'll be having triple glazing, insulated doors, MVHR etc. Assuming the uValues are reasonable, the equipment and fittings are efficient, what is air tightness about?

[SteamyTea]

1 hour ago, MDC said:

what is air tightness about?

Assuming this is a serious question, it is about not replace heated air in the house with cold external air.

Basically controlled leakage, not uncontrolled.

 

Trouble with buildings is that the cold external air can also bypass the insulation. This is why it is all about design and detail.

[MDC]

22 minutes ago, SteamyTea said:

Assuming this is a serious question, it is about not replace heated air in the house with cold external air.

Basically controlled leakage, not uncontrolled.

 

Trouble with buildings is that the cold external air can also bypass the insulation. This is why it is all about design and detail.

Thank you. It is a serious question. 

 

We have designed a rectangular two storey timber framed building. I'm aware of the importance of form, so have no dormers or other bits that create corners and awkward shapes.I'm also aware of thermal bridging.  I think the uValues quoted are acceptable, so as you say,  it's all about the design and the detail. Is there a method for keeping the cold external air from entering the building? 

[PeterW]

Floor needs to be 0.10 to be reasonable. ACH of 2 is fine but what’s your end goal..? PassivHouse..? Or just something with low bills ..??

[SteamyTea]

16 minutes ago, MDC said:

Is there a method for keeping the cold external air from entering the building

Well not one single method.

The main thing is to make sure there is not an airpath that can bypass the insulation.

 

It would be easy to make an airtight house by just covering all internal areas with polythene, then tape all the joints carefully. Probably get an air test done at below 0.2 ACH.

But if the external air can bypass the insulation, and get onto the outer face if the inner wall, then the temperature difference will be high, and heat loss is proportional to temperature difference.

[Bramco]

1 hour ago, MDC said:

Is there a method for keeping the cold external air from entering the building? 

Facetious answer is buy a plot in a warm country...

 

But basically, it's no use having an airtight barrier on the inside if as others have noted, you don't have an insulating envelope around the barrier. So SIPS, timber frame with insulation, there's many ways to make sure that the cold stays on the outer surface of the building. So check the U values of the walls, floors and roofs.

 

Another point about airtightness is to make sure that the windows you specify don't have trickle vents.

 

They can be opening, you might want that in the summer but they can't have trickle vents.   

 

We actually made a mistake in our design. Bathrooms upstairs have rooflights that we can open. The downstairs en-suite windows don't open. Wish we'd made one of them open, so that when showering the window could be opened a crack.    MVHR should do this but if you think about it, the MVHR is aggregating the extract from all over the house, so that one bathroom aggregated with the rest never gets above the humidity protection threshold, in our case 75%.  We'll end up fitting a wireless humidity control in the en-suite to force a boost based on that room.

 

Simon

[MDC]

1 hour ago, PeterW said:

Floor needs to be 0.10 to be reasonable. ACH of 2 is fine but what’s your end goal..? PassivHouse..? Or just something with low bills ..??

Just something comfortable with low bills, please!