Adding an addition after AirKrete and polyiso – now what?
1960’s ranch near Rochester, NY
A couple of years ago, the home was resided with vinyl. I took this opportunity to have the home wrapped in Tyvek and covered in 1.5″ reclaimed polyiso with black kraft facing. The walls, which had minimal fiberglass insulation, were filled with AirKrete.
Overall this has performed well, the home is quiet and the walls are warm in winter and comfortable – despite problems with the local contractor.
Now I am looking to add an addition, 10×20. The addition will bump out one wall and require part of another to be modified.
Unforseen problem: the 1.5″ reclaimed polyiso is no longer available. So what to do with the modified wall, to keep the vinyl siding even with the existing exterior surface?
I see my choices as:
1. Redo the entire modified wall with some other thickness of reclaimed polyiso. 1.25″ foil faced is available at $13 per panel.
2. For only the modified part of the wall, use 1.5″ Foamular XPS at $24 per panel. This seems like the best alternative – materials are expensive but installation is easier, thus less expensive.
3. For only the modified part of the wall, use the 1.25″ foil faced reclaimed polyiso at $13 per panel plus .25″ fanfold Foamular at equivalent $8 per panel to get to 1.5″ thickness. I assume the Foamular would be installed next to the Tyvek wrap, and seams would be staggered with the polyiso. Very fussy installation, more opportunities to make mistakes, I can see local contractors balking at this one.
Will any of these alternatives (foil facing, XPS) cause moisture problems inside an AirKreted wall? If so, what should be done instead?
Other thoughts/ideas? TIA.
Replies
Rochester is zone 5A, and a minimum of R5 is good enough for dew point control at the sheathing for 2x4 framing with fiber insulaton but you'd need R7.5 minimum if the new framing is 2x6./R20. The R-value of the AirKrete as a bit squishy since it can't be tested in an ASTM C518 test plate without breaking, but guarded hot box tests give it about R6/inch, so at 3.5" so it could be as high as R21, which means even in a 2x4 framing you need at least R7.5. to keep the sheathing dry. Reclaimed polyiso at R5.5/inch would be ~R7 at 1.25" but R8+ @ 1.5". Stick with 1.5" polyiso.
You can also get there 1/4" fan-fold XPS siding underlayment in conjunction with the 1.25" polyiso to make the thickness match, and it will also add enough R to provide sufficient dew point control, no need for interior vapor retarders.
At 1.5" XPS would be labeled R7.5, but as it loses it's HFC blowing agents over a few decades it will drop to ~R6.3, which may be fine on the warm edge of climate zone 5, but not on the cold edge..
AirKrete will not air seal the cavity the way expanding foams will, so be sure to caulk the sheathing to the framing inside every stud bay, under the bottom plate, and between any doubled up framing such as top plates, jack studs etc with a decent quality polyurethane caulk.
Thanks, Dana. Glad I asked!
Do I understand correctly that you would place the fan-fold directly under the siding? So from the outside in, we have vinyl siding > fan-fold > foil faced 1.25" polyiso > Tyvek wrap > sheathing?
J,
One more point: It seems to me that your only challenge is to make sure that the vinyl siding of your addition is co-planar with the vinyl siding of your existing house.
But everything on your addition will be different from the details on your existing house. Your wall studs will probably be thicker. You don't even have a foundation yet.
On your addition, the studs don't even have to be co-planar with the foundation. Studs can overhang a foundation or be recessed if you want.
In short, build your wall in the right location (and locate your studs on the foundation) so that the vinyl siding is co-planar -- even if the rigid foam on your addition turns out to be thicker or thinner than the rigid foam on your existing house.
Martin writes: "Your wall studs will probably be thicker."
Deeper studs will require deeper foam. AirKrete's high R6/inch and high vapor permeance means that the IRC chapter 7 prescriptive R7.5 insulating sheathing for 2x6 framing doesn't apply.
AirKrete hits R21 with 2x4 framing, and the IRC prescrtiptive R5 isn't sufficient for dew point control in all climate zone 5a locations.
In 2x6 framing AirKrete runs R33, and would need ~R11.75 insulating shething to have adequate dew point control.
If using fan-fold XPS to the stackup to add another 1/4" or R1 to the exterior insulation it can be on either side of the polyiso layer, but it's probably easier to staple it to the structural sheathing, with the polyiso closer to the siding.
Thanks very much, Martin, for the idea, and Dana for the clarification.