Having read the forum for a while I get the definite impression that air tightness is important. However, I have to admit at being a little bit confused and it might be a case that a little bit of knowledge is dangerous...

I introduced my project on the forum last week. It is going to be a traditional construction with a beam and block floor, brick and block walls and a standard cold roof:
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13175&page=1#Item_8

I'm happy with my architect and builder, but neither of them have built a new build before and (I don't think) are as concerned about air tightness as I am, although they know it is important to me and don't dismiss it. We have an air tightness target in the contract of 4m3 per h/m2

I'm hoping the forum can help me with the below:
1. What practical difference will it make if we won't hit the target of 4m3 per h/m2 and ended up at, say 6 or 7m3 per h/m2? Will it make much difference to my energy bills? We're installing underfloor heating running from a gas boiler?

2. Any advice on the best way to help my builder achieve good air tightness? I had thought about getting an intermediate air test, but speaking to someone who does air testing a lot of the tightness is achieved at the second fix and there isn't much point testing too early. Would it be worth getting a 'consultant' in to speak with my builder? And if so how do I find one?

Thanks,
Richard

Posted By: tony...don't seal skirtings to floors...

Woah, woah.... I thought this was considered a good thing to do? Or only in retrofit to avoid air leakage from wall/floor junction?

Ok I agree with that. Fix the problem at source. I suppose that's why it needs sorting before second fix! Blocking internal holes may cause other problems.

It's also quite difficult to seal skirting to floor, particularly if the floor moves. Can use a highly compressible foam strip but you also need enough of a gap to fit that in.

4 does seem on the unambitious side. What are the different certifications' recommendations (considering they are sometimes expressed in different units)...

Passivhaus: 0.6 air changes / hour [by volume]
EnerPHIT: 1 air change / hour [by volume]
AECB silver/gold???
CFSH levels???
Building regs: (10 m3 / hour) / m2

No recessed downlighters, unless you have an airtight service void!

No hole in the wall ventilation!

Hi Richard,

Ventilation losses can be in excess of 40% of the overall heat loss in well insulated buildings - though a percentage is a little simplistic as doesn't put an actual number on it.

The attached runs through a calculation from first principles..

too big file, pdf it?

Dunno, but have strong impression that forum attachments are borked at the moment.

Slight confirmation: just tried to upload a .pdf file which I know ought to be OK as I downloaded from here and got:

Some problems were encountered
You are not allowed to upload (Rust_Bleeding_Welded_Parts_Galvanized_Steel.pdf) the requested file type: application/unknown

Now same thing as a .pdf

Okay, Experts replied above

The non expert answer is:
I live in house with no drafts.
It stays pretty Warm
A year ago this time, upstairs, if i picked the right point i could see outside, Pretty rubbish.
We still have no true heating, only electric fan. This evening, no fan on, outside around 12/13 inside 16. Warm enough. I imagine, like this morning, that will sink to around 14/15. Normally is about that, when i get going at 5am. I am unsure doors are in fact closed/not too bothered, dear wife has habits of leaving inside doors to garage open.
Thats what sealing a place is like, but I think the wife is amazing not too complain too much
when doors are closed this place hits air passive house levels...
Any way, i shall now turn the heating on. too cold 16...

File size was too large even for pdf. Part 2

OK, here's what I have gleaned from this wonderful forum and experiences with my new build.

Posted By: richardelliotI'm happy with my architect and builder, but neither of them have built a new build before and (I don't think) are as concerned about air tightness as I am, although they know it is important to me and don't dismiss it. We have an air tightness target in the contract of 4m3 per h/m2

That is a rather unambitious target, do hope we can persuade you to aim for better.

Generally architects and builders are disinterested/uninformed regarding airtightness, seeing it as an unwanted hurdle and something delivered retrospctively by lots of "goop" should the test fail. Even my air tester was disinterested in attaining good air tightness as long as the house passed reg/SAP requirements (which it did by a mile even with a poorly performed test). There is a long way to go before good practice becomes mainstream.

You are going to have to lead this, specify how it is done and ensure it is delivered. A good builder can deliver what is desired, mine did, but they probably won't know how or why. You will have to teach them.

1. What practical difference will it make if we won't hit the target of 4m3 per h/m2 and ended up at, say 6 or 7m3 per h/m2? Will it make much difference to my energy bills

Energy saving can be quantified to an extent, as others above have indicated, from the dimensions etc. Of course the actual effects will be wind dependant too, and you can get into what can seem like deminishing returns. But keeping warm air inside the house is only one way that air tightness contributes.

Another harder to quantify factor is comfort. Much like 3G, removing cold drafts (even tiny ones in flow terms) results in greater percieved comfort at lower room temps. New air tight house is rediculously comfy at 18C even in a gale, while old somewhat leaky house needed 21C and above never feeling warm on a stromy day. Guess that someone could quanitfy the savings of running 3 deg cooler all winter, can't be bad can it.

And last, but not least, there is avoiding thermal bypass - the "plasterboard tent" effect mentioned by Tony. Let's be clear you can pass the air test and still loose out badly to thermal bypass, but well designed air tightness measures also avoid thermal bypass where air movement around/through the insulation or between air tightness envelope and insulation significantly reduces its effectiveness.

There were moments, especially as a lone voice on site, when I wondered if all the fuss was worth it. I can say now that it was. You only get one chance to build this in, it is foolish not to take it. As encouragement my masonry cavity build achieved better than 1m3/hm2 @50Pa without noticeable extra costs, just attention to design details and careful implementation by the builder.

2. Any advice on the best way to help my builder achieve good air tightness? I had thought about getting an intermediate air test, but speaking to someone who does air testing a lot of the tightness is achieved at the second fix and there isn't much point testing too early. Would it be worth getting a 'consultant' in to speak with my builder? And if so how do I find one?

Air tightness needs to be designed in. You need to provide your builder with details of how every aspect of the air tigtness envelope is achieved e.g. pipe penetrations, window & door installation, ceiling to wall joint, floor to wall joints, joist ends etc. It is along list and you need to specify all of it. It is not about running around before/during the test with a caulking gun. The requirements effect the plumber and electrician too. Good trades men will do what you want, but they will need to be told exactly what that is.

Early testing is of limited use in a masonry build. The 2nd fix does not deliver air tightness - it is not the socket fronts, skirting boards etc. that provide the seal (I hope). But you do need all the elements of the envelope in place e.g. plastering done, loft hatch in place etc.. I found that by time all the house was ready we were at the final test anyway.

No idea how to find a consultant to provide all this advice, do they exist? You need someone that has done your kind of build and achieved PH like standards. I became my own "expert", with lots of invaluable help from others on this forum. The information is all here if you do the research.

Good luck!!

The last paragraph should start "Finding resources....", but funding them is likely to be a challenge too!

I would suggest building to a target of about 1ac/h, and design in MVHR.

You say: ''He said he doesn't think the current speficiation will achieve what I want and had been looking at a product call Soundcoat Plus from British Gypsum to create and airtight skin inside the building before plasterboard is put on top.''

I think a lot of experience on this forum will tell you that you don't need fancy products, but careful application. One of my clients did a very 'tight' refurb of a 1960s cavity-walled house (already retro-filled cavity, and he added IWI) and got 0.75m3/m2/h. He did not use a fancy prodduct but, having stripped off the existing gypsum plaster, did a parge coat of lightweight lime plaster (baumit RK38 - dead light and easy to use - alternatives (incl sand and lime and weak sand/cement) exist). With new-build I'd like to see you aiming at least for that. Excuse incompatible units, but there is a discussion on here somewhere which looks at ac/h vs m3/m2/h. (Anyone remember where it is?).

The m³/m²/hr @ 50 Pa for air permeability is expressed in cubic metres per hour per square metre of envelope area.