Posted By: HairlocksI am building a timber frame house on a concrete base with block work up to the DPC.

What do you mean by concrete base? If you have concrete strip footings, dwarf walls with a concrete infill to be followed by insulation and UFH in screed, then he is right. It does partly depend on what sort of blockwork. The edge insulation, if just laid against the edge will provide an airpath assuming the screed is finished to the same level as the edge insulation. Paint would make little difference I suspect. A form of render would.

Although a different construction at foundation, I will glue and fix the sole plates as this is another area of potential leaks. If the sole plate has gone straight onto blockwork, with DPM in between, then that is a very leaky area (I suspect).

Air will certainly leak through exposed blockwork - found this when testing large buildings over the last 10 years. Impressed that your builder understood this - maybe knowledge is spreading out there.

I'm wondering where the DPM will be placed in your build. I would have expected that to be lapping up the inside of the blockwork, and over the top, with the DPC and then sole plate sandwiching it? So that should be already in place and doing its job, withouth the insulation/screed. Maybe his concern is around drain penetrations, but they need to be water tight, so that'll do for air tightness, considering potential water pressure differentials.

That would give you a theoretical air barrier starting at the sole plate, so blockwork is outside the air barrier.

It is a very difficult balance testing early enough to be able to see/repair any air leakage, yet not so early, that you have temporary patches everywhere for unfinished bits. That's the beauty of having the air barrier as far to the ouside of the shell as possible (as per Viking House and FosterTom) - no need for plasterboard etc to be fitted before testing.