Is there no record of existing Phs to see how well they have maintained air-tightness of the long term, with details of their construction?

I think Tom has hit the nail on the head. How can we make OSB airtight but maintain its vapour permeability?. What coating would do this?

what about painting with acrylic gloss paint to make OSB air tight but vapour permiable?

http://www.paintquality.com.au/press/newspaper/boapt01.html

Posted By: joe90what about painting with acrylic gloss paint to make OSB air tight but vapour permiable?

i really like this idea, seems to be based on sound scientific testing. well done Joe, that's the 3rd time you've help me on this job, i owe you a drink. anybody else like or dislike this he says holding his breath

undecided at the moment, but probably going for inside side of structural timber work with 25mm counterbatten service void and PB and skim

What are you trying to achieve? You don't save any energy in a building by reducing the airtightness from 1.5ACH to 0.5ACH, you just get a sticker and a pat on the back for your effort, with the fellow who sold you all the tape telling you you're great! The point I'm making is that OSB is fine, leave it alone! You're just playing into the arms of the guys selling the more airtight board, they started all this propaganda!

Down from 10 or 3 ac/hr - that's about heat.
On down from 1.5 - 80% (for MHRV) of a fraction (for real-life pressures, mainly stack effect) of 1.5ac/hr doesn't carry a lot of heat. It's more to do with:
1) according to PH, uncomfortable draught sensations, leading to people turning the heat up; and
2) according to Fraunhofer/WUFI, transport of water vapout to condensation zones by bulk air movement.

I got these results running PHPP on my house design.

Pressurisation Test Result Specific Space Heating Demand
0.2 12.1
0.4 12.3
0.6 12.5
0.8 12.7
1.0 12.9
2.0 14.0

I started a thread on the AECB forum a couple of years ago about this 'problem'.

http://www.aecb.net/forum/index.php/topic,3928.0.html

Edit: sorry about the formatting of the table.

Posted By: Mike GeorgeI find the AECB site too agressive where questions/discussions go against their single mindedness re PH

Tended to be, yes.

Posted By: Mike GeorgeSo a law of diminishing returns then? Very much like u-values?

Exactly, as far as heat loss (but as I say there are 2 other gd reasons for lowest poss leakage). However, there is a difference, when pushing into that 'diminishing returns' territory:
It's easy to do with insulation, just costly, to add more thickness.
It's very difficult to do with airtightness - lots of hard detail work.

Also, need to be clear about the 2 different types of air leakage:
a) concentrated leaks, cracks missed e.g. around windows, tapes parting etc, and
b) dispersed, slow all-over leakage - could be via a network of taped joints begining to fail, or via the permeabilioty of the sheet/board materials themselves.

Of those 2 reasons for airtightness:
1) according to PH, uncomfortable draught sensations, leading to people turning the heat up; and
2) according to Fraunhofer/WUFI, transport of water vapout to condensation zones by bulk air movement,

for 1), b) doesn't matter - only the larger more concentrated leaks a) will be felt as draughts
for 2), a) is not dangerous, as there's enough oomph in the airstream to warm up the walls of the crack so it doesn't condense, whereas b) is the case that causes interstitial condensation.

So the need for high airtightness still stands, and both a) and b) must be good.

Posted By: Viking HouseWhat are you trying to achieve

For me its not just about saving energy, but just as much about hitting a target. albeit self imposed aim high and get as close as you can. excepting 1.5 , 2 or what ever, is the thin end of wedge.

That`s why its called the Green Building Forum not the General Building Forum