Posted By: lineweightHowever, that only really works for newbuild. In most extensions or refurbs the newly constructed parts of the envelope generally enclose spaces that can't easily be sealed off from the rest of the (presumably more leaky) building.

So a blower test can't really be done.

There's no point in having part of a house airtight and the rest of it not! The whole point is exactly to achieve a good blower test result and thereafter minimal infiltration.

So if airtightness is a goal of a project, it has to be a goal for the whole building.

Which suggests to me that the best order of implementation is probably to do the airtightness work on the existing building first, and then set about building any new parts. So as far as the new parts are concerned the testing is straightforward.

Posted By: djh
There's no point in having part of a house airtight and the rest of it not!

Not sure I agree with this.

If work is only being done to part of a building it's usually still worth doing that to the best standard reasonably achievable.

It is quite possible that the remaining parts of a building will be upgraded and improved in the future in which scenario the benefits of good airtightness of the previously completed parts of the construction can be fully reaped.

When people make decisions about altering their houses, energy efficiency isn't the only or principle motivating factor. If, say, they need extra space, then it doesn't make sense to tell them that they should upgrade the energy efficiency of their existing building before adding what they need - more space. It makes more sense to give them what they need and at the same time, build in good energy efficiency because it's much easier to do so at the time of construction than retrospectively.

If the primary aim of a project was "improve airtightness" then yes I agree, you would start by improving the existing fabric. But in most cases if people want an extension, that's not their primary aim.

Also - I don't feel that the benefits of airtight construction only apply if applied to the whole building. Yes they will be compromised to some degree by connections to the non-airtight areas but I don't think it makes it pointless.

One scenario might be a loft extension where the new constructions only need affect one side of a pitced roof. In this instance, if only those elements were made airtight but the roof slope on the opposite side was left leaky then I can see much of the benefit would be lost. In which case, it is probably worth doing some work to that opposite face to ensure that within the loft storey, infiltration is minimised especially when a cross wind is blowing. But doesn't it still make sense to do that even if you don't make major improvements to the fabric of the building at lower levels?

I don't think it's as simple as all-or-nothing. Doesn't each scenario merit its own judgement?

Posted By: fostertomA lot of it can be really foolproof, if designed to not rely on tapes and membranes.

I too am a fan of not relying on tapes and membranes .... nonetheless there is still plenty of scope for sloppy practice.

For example the simple matter of filling gaps between abutting PIR boards. How many building sites do you see where this has been done conscientiously?

My experience is also that with or without a membrane based approach, when working with existing buildings there are always going to be tricky connections and details, and it's not possible to anticipate and cover them all in drawings, meaning that exactly how they are put together has to be decided on site.

Neither is it always possible to design those tricky bits out when you're working with existing buildings.

Posted By: ringiHow do you even get trade people to look at the drawings and do what they say, rather then do what they have always done?

I have only managed that by working 'with' them usually I say: "I want to learn how to do it myself.". Of course my time is my own.

Worth it even on part of the house, + 1

Shouldn't it be ideally outside the insulation layer; what about bypass?

Ignore me.

Personally I think testing should be regular. Like a car having an MOT.

Certainly it'd be good to know for sure how different air-tightness strategies work in the long run, and how the ones that don't actually fail.

I agree with most of the points being made.

But getting back to the original question - which is about dealing with these issues where you *can't* really test the airtightness - at least not with a blower test...

It seems to me the strategy has to be to use a method of construction which, based on testing in test-able conditions, is known to work if carried out properly.

And it has to be possible to know that it is being carried out properly by performing visual checks, during construction and on completion.

The ideal is for someone who has an interest in it performing well, to be closely monitoring it during the entire process, but while that might be possible for self-builders where the building owner/occupier can be on site fairly much constantly, it doesn't really work in other situations.

So really there needs to be a method of construction with one or more agreed points in the process where a visual inspection is made before work gets covered up.

And those stages - and what's agreed to be an acceptable standard of work - have to be specified in advance and written into the building contract if at all possible.

I'm wondering if anyone's had any success in developing such a strategy - that works in the conventional client-builder-designer contractual setup. And crucially that doesn't scare builders away completely (some level of compromise probably has to be accepted along the way).

Yes, it's all a bit despairing.

As far as the basic shell is concerned, I put my faith in OSB glued and screwed with polyurethane gapfilling glue, as a foolproof airtight barrier. Foolproof as in it's really hard to get it wrong. But not impossible - was it Mark Twain who said 'the trouble with fools is they're so darn ingenious!'

The joints (the usual fail point) are really taken care of, but there's some debate as to whether the body/face of OSB is really airtight - it varies from batch to batch of same manufacture - but Peter Warm, one of the 2 UK PH Assessors - says he's never had an airtightness problem with OSB. Anyway, I then back the OSB up with blown-in Warmcel, which has good (but not sufficient alone) airtightness as long as it stays in place - no slump etc (which is another topic).

Taken together, you have airtightness in-depth made of robust 'traditional' materials, not relying on a flimsy membrane, unknown-durability tapes and 100% perfect application which can go wrong with the slightest inattention.

In-depth means it's not a total disaster if someone pokes a hole, either during the build or during the next 100yrs or whatever.

A visual inspection helps but it can't tell you whether something is airtight. You need an airtightness champion on site all the time checking and with the authority to order rework and a workforce who have bought in to the idea at least for this job. And even then the only way to know it is airtight is to test it. Even building regs insist on tests!

So if you're only building part of a building to be airtight then at minimum you need to seal it off and test it, IMHO. Otherwise it's all just good intentions 'passive house principles', 'zero-carbon', 'green'. dynamic whatever, active bull****.

Yeah that's an equiv spec - nailing can be gd as long as guaranteed to pull down tight - and Icynene has strangely similar properties to blown-in Warmcel, the only thing missing being Warmcel's wonderful hygroscopicity.

Phil,

I second that re Pro Clima!

Pricey, though! I hope you have been buying from mainland Europe. I use www.baunativ.de. (No connection etc. etc.)

I have lots of pieces of tape stuck to my working jeans!

Yes, but the exchange rate is a bit hairy at the moment! Still a hell of a lot cheaper, though.

Tom, what is the gap-filling Pu glue you use? Is this a liquid-ish glue rather than a low-expansion squirty-foam?

I try not to rely on tapes alone, preferring tapes and clamping when I can get it. I take your point, though, very much. In the end, anybody's air- and vapour-tightness is only as good as the holes which subsequent trades put, or do not put, in the a/t or v/t layer.