Wrapping the wrap

26 06 2010

The foam insulation (all those sheets you saw in an earlier post) is finally being put to use. Now that housewrap is on, the walls and the roof will be covered in a double layer of 2″ polyisocyanurate, foil-faced foam. The foil reflects heat, and the thickness of the foam keeps the dew point toward the outside of this “cool” wall system for retrofits–addressing condensation concerns. The roof/wall geometry will get resolved and the whole outside of the house will uniformly gain a few inches. This is one instance where getting fatter is better!

Beginning to wrap the house in a 4" layer of foam insulation

They work so fast!

A fastener - this is what keeps the foam attached to the wall

Hanging around - it was hot outside!

The extra exterior wall width plus the need to address the thermal bridging potential of the porch joists extending out, required that the porch be moved further away from the wall.  The guys just cut off the porch joists, continued installing their polyiso foam up the wall, installed a ledger over the poly, and then re-attached the porch to the ledger. They seemed to be enjoying the work since the porch offered shade, even as it was detached from the building!

Hard at work

Posing for a picture (with the older, leaky doors in the background)

Expanded door frames to accommodate insulation

Ready to be insulated (note new, more energy-efficient exterior doors)

Slats of wood (1×4’s) are fastened to the front of the foam to create a 3/4″ gap, allowing for drainage. This strapping is also what the future cladding will be attached to. The 4″ of polyiso foam (two 2″ layers) is attached over the house wrap. The house wrap provides something of a “back-up” drainage plane–should moisture manage to get past the poly. There are probably some tiny “gaps” between the housewrap and poly because of the unevenness of different surfaces–even when tightly pressed against each other. Having a hierarchy of drainage planes is Building Science and Synergy’s “belt and suspenders” approach. Gaps other than the one at the strapping, should be small and discontinuous enough to not permit convective air-loops which would defeat the insulation. Any water, however, will find its way down or simply just not touch the sheathing. The windows, as we will discuss later, are “outies”–air sealed and flashed to the front face of the polyiso. Any venting penetrations are sealed and flashed at both the house wrap and at the front face of the poly.

Wood slats



One response

27 06 2010

First, thanks for putting this up for us all to follow, great work!
Curious as to why there is a gap between the wall sheathing and foam. You say for drainage, but isn’t the drainage plane between cladding and foam? What is the rationale for 2 drain planes? How is the gap created and how deep is it? Is there any concern about bypassing the insulation with this gap?
Perhaps an illustration or 2 would help.

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