We still cant answer this simple question so how will we ever be able to build tight?

Recently saw a new home pass the air leakage test ( just ) yet within a couple of weeks of occupation there were patches of condensation on the ground floor ceilings! Though the internal air in the rooms was sealed there was access from outside to under the first floor void. All the insulation was outside the air barrier! was this right??

Many of us seem to think that the decorations or the plaster is the air barrier but I am coming to the conclusion that we need it to be outside the insulation but how this can be achieved is difficult to see. In many parts of Europe they get hugely better air tightness than we do and are often using poured concrete walls may be we need to radically change our solid construction methods?

Any takers for suggesting where the air barrier should be? Any ideas better than caulking skirtings to floors would be most welcome.

My understanding is that it doesnt matter if the air barrier is on the outside of the insulation or on the inside so long as it is continuous.

The difficulty with the internal method is the junction of the first floor and the external wall. This needs careful detailing whether its in timber frame construction or in masonry.

The difficulty with external is the junction of the roof and the external wall - the same issue arises.

Passing the air test is not a demanding level of air tightness any way so I suspect the qulaity of workmanship was probably an issue as well.

Mike, from bottom to top, plasterboard/wood joists built into plastic pockets in block work walls/flooring sheets/carpet/skirtings caulked to the floor. (there seems not to be any sound deadening quilt present).

That's it -- holes or gaps into the floor/ceiling void allowing cold in in a big way yet in a relatively so called air tight house.

Nice idea. So you are advocating a barrier in the cavity but only at joist level?

I did propose that joists should be laid on a strip of breather membrane which was then built back in above the joists and plastered in at the ceiling and behind the skirting. This tray would also need to run parallel to the joists. It would link air tightly from one floor level to the next. This was in a letter to BFF a good while ago.

I am still seeking an answer to where the air leakage barrier should go in masonry construction? Any other ideas?

I am kind of thinking that the air barrier would be a lot better on the outside of the insulation. We have already talked about quilted loft insulation under performing when it is windy. So a windproof layer on top of may improve the insulating properties and if we could link this to a similar one in the walls and seal it to all openings etc we would be there. Easy to say not so easy to do.

My nervousness about solid with external insulation is that it would be difficult to stop wind getting in amongst the insulation layer. The cladding could even be brick couldn't it? Then build the inner walls, insulate, air barrier, then bricks could be a nice way to go so long as the air barrier was inspected.