It is looking like a house, but I am dragging butt and reconsidering some of my belt-and-suspenders ideas. I planned on using duPont's Attic Wrap to seal the attics in my house. I am also stuffing 4 inches of XPS between the ceiling joists because I have it. It occurs to me that it would be relatively simple to caulk and foam the XPS (trusses on nailers on concrete walls.)  The foam would also make it relatively simple to seal penetrations (five HRV vents and one 3' run of fire sprinkler pipe.)  At that point venting the attic would make no difference and the attic wrap would be superfluous, it seems to me.  I am blowing cellulose over the foam and the HRV runs, so assuring airflow under a low roof could be tricky.

In the minus column, the XPS would be vapor impermeable at 4 inches. So central pa. does not lack for humidity. That said, I am anticipating that humidity issues are a given in a super tight house and I am prepared for them.

Posted By toddm on 14 Oct 2010 09:06 AM
It is looking like a house, but I am dragging butt and reconsidering some of my belt-and-suspenders ideas. I planned on using duPont's Attic Wrap to seal the attics in my house. I am also stuffing 4 inches of XPS between the ceiling joists because I have it. It occurs to me that it would be relatively simple to caulk and foam the XPS (trusses on nailers on concrete walls.)  The foam would also make it relatively simple to seal penetrations (five HRV vents and one 3' run of fire sprinkler pipe.)  At that point venting the attic would make no difference and the attic wrap would be superfluous, it seems to me.  I am blowing cellulose over the foam and the HRV runs, so assuring airflow under a low roof could be tricky.

In the minus column, the XPS would be vapor impermeable at 4 inches. So central pa. does not lack for humidity. That said, I am anticipating that humidity issues are a given in a super tight house and I am prepared for them.

With a ventilated attic, with the cellulose on the cold-in-winter side and the XPS on the warm side of the stackup the low-perm aspects of XPS would be on the PLUS column, not the minus.  In winter the moisture is coming from the interior, not the exterior.

But 4" of XPS would be an expensive way to add R to the attic if there's room for ~7" more cellulose.  An inch of XPS is as-good an air barrier as 4", cut'n'cobbled & SPF-sealed at the edges.  But if you have it, sure, it works there. If it's multiple layers, seal each layer as you go, and lap the seams between layers.  It's a lot of cut'n'cobble though.

No air-barrier is perfect (or will STAY perfect after decades of use), and the additnal air-barrier of a high-perm attic wrap would keep the attic nice and dry while reducing/eliminating infiltration.  The radiant-barrier aspect of that product will make a modest but measurable reduction in cooling load too- even more so since your HRV ducts run in the (now cooler in summer) attic.

Humidity issues are NOT a given in a super-tight house that has an HRV.  You do have to manage the duty cycle with the indoor & outdoor humidity though.  In winter putting it on dehumidistat control set to 30% keeps it from over-drying the house. In summer you have to watch the outdoor dew points and  cut back the ventilation rates whenever that humidity would be over the healthy relative humidity level when reduced to  the indoor temp.  Tight house or not, you'll need active dehumidification to keep it below 60% RH indoors all summer in so. cent. PA. Don't count on the air conditioning to always keep it dry enough, especially if yours is a high-R low gain house.  If you plan on keeping it below 50% RH in summer, go with an ERV instead of an HRV.

Thx, Dana. Like I say, the XPS is sitting there. Attic Wrap also involves lots of fussy work, and it is a particular PITA on hip roofs.
As John Maynard Keynes once quoth famously, in the long run we are all dead. I am trying belatedly to make my demise later rather than sooner.

Posted By toddm on 14 Oct 2010 04:01 PM
Thx, Dana. Like I say, the XPS is sitting there. Attic Wrap also involves lots of fussy work, and it is a particular PITA on hip roofs.
As John Maynard Keynes once quoth famously, in the long run we are all dead. I am trying belatedly to make my demise later rather than sooner.

Good luck with that!

Do a good job foam-sealing the XPS, blower-door test & spot-remediate before laying down the cellulose, and it'll be 10x tighter than typical US construction.   If you end up at R55-60 in the attic the benefit of a radiant barrier will have long-since disappeared into the noise. (4" of XPS + 10 or 12" of cellulose and you're there.)