Some are advocating building joists into "boots" in walls of new homes. They say that this is to reduce air leakage. It only does half the job initially and slightly more as the joists shrink. However as a majority of new homes are 'blobbed and dobbed' still without the recommended parging coat and the questionable continuous ribbons of adhesive, air leakage is still very high. Sometimes it seems good as they say to caulk the skirtings to the floor! What is the point of this in first floor bedrooms!!! So a new home can literally have holes from under the first floor to outside and "pass" air leakage testing!

Even without holes to the outside the full combined air leakages of the walls are directed into the floor void.

Boots only stop air infiltration when perfectly fitted and not crashed off by plasterers or carpenters but do not stop air leaking through the block work itself.

Hi Bowman.
Good point. Aspen in the US have just released a thin strip solution based on Aerogel Space Loft for steel and timber frame buildings that helps to minimise even eradicate thermal bridging:
http://www.aerogel.com/Aspen_Aerogels_Insulcap.pdf
The first manifestation of this type of product in the UK is being used in a property in Bovey Tracey in the Bovey Climate Action initiative http://www.boveyclimateaction.org.uk/

That's right, does nothing for airtightness.

Aerogel has approx 3x the k-value of expanded polystyrene. So this 10mm thick strip does the same job as 30EPS. So why not use plasterboard laminated to 30 EPS or 25 XPS or 20 PUR - vastly cheaper, probably, and aids general insulation as well as making the same somewhat puny contribution to reducing cold bridging. Better still. do it with multifoil instead!

Posted By: Chris WardleCaulk around the end of the joists where they meet the plaster for good measure

What would be that caulking spec/description? To caulk the joists' underside, it'd have to be done before ceiling plasterboarding. And, where end joist runs parallel to external wall, what about plastering the wall face that it's close to? Even if there's ways to avoid such holey construction in newbuild, it's a very real problem in uprating existing buildings. Any solutions?

Perhaps that end joist parallel to the external wall needs leaving out until the wall has been plastered? Could all be a lot better without cavity walls I guess. If a single skin of blocks was just externally insulated in an air-tight manner then you've solved the problem with new build.

For existing, perhaps we need to be lifting floorboards (or dropping ceilings) and spraying the wall between the joists with expanding foam. The gap between the end joist parallel to an external wall could also be filled with expanding foam.

Very timely posts (for me)

I am just in the process of fitting kingspan (gave up on the intention to use thermafleece due to cost unfortunately) between the joists on the downstairs floor and (having ripped the upstairs floors up around the edges) was about to try to address airtightness upstairs as well. Downstairs I have an issue with the joists being just above the DPC so sealing between the joist that runs along an outside wall (nice 20 to 70mm wavy gap) and around the ends of the joists which sit into another outside wall gives me real concern with regard to damp so I am curently trying to decide how to finish the job off. The ends of the joists are allready suffering a bit due to 'serial driveway laying syndrome' which I will arrest shortly (dig the dam thing up!) but I am concerned at providing damp bridges. Also spray foam sounds good until you are flat on your back in a 21/2 foot void truning to spray up with a can you are supposed to hold upside down & the stuff just falls out anyway with 1970's DR WHO monster type consequences (same problem with my roof insulation on the end rafters in a dormer bungalo) - count to ten Simon my partner says through the floor at me floopy mumbo flogg phnumf I say back.

Seems to me I'm left with building a wall under the joists and cutting off the ends filling the holes etc (there goes another floor) but no real solution for the sidewall joists. Upstairs I am similarly worried about sealing around the ends of the joists and possibly creating a rot problem where there currently isn't one.

As for dealing with cold bridging from under the house - I don't know where to start with regard to the DPC

Help! I'm considering not completing the insulation/draughtproofing exercise fully as the least worst option.

The considerable upside is the draftproofing effect of the kingspan in the floors where it is nearly complete is astonishing - thanks Tony this would never have been so far up my list without your one man campaign!

S.

Sky, I would do the best job you can with the underfloor insulation then lay a membrane over the top and relay the floor ( presumably you took the g/f up?) then trap this air tightness membrane behind the skirtings. If you are working from underneath then may be foam the floor to the skirting afterwards? So long as the insulation is only touching wall above dpc then it is OK for it to make contact.

Upstairs with the walls exposed under the floor you will be able to see how to do a fantastically good job now. Continue whatever wall insulation you are adding down under the floor. (well done you are now my hero and if you need a hand I'm available to help)

Do you have cavity walls? Lime mortar? re-pointed with cement?