How thick an SIP should I use?
I’m designing a house to be built in VT just north of Stowe. We had started out considering log homes but after investigating more now think Timber Frame is a better way to go because of insulation considerations. We’re looking into using SIPs (EPS, polyurethane or Neopor) outside of the frame but haven’t ruled out a spray on approach using low density polyurethane or cellulose.
The house will be in an open field and we plan to site it to maximize passive solar absorption by facing the high ceiling great room which will have a 24ft wide full gable window wall facing south. The house is basically a dormered cape with a bump out on the south side for the great room which makes it have a center part that’s approx 24×32 with wings on either side that are approx 15×24. So the footprint is about 1500sf plus or minus. The wings will also have windows on the south as will the upstairs dormers.
We are looking into the best way to heat the place above and beyond the passive solar. We considered electric radiant subfloor and solar PV but are leaning against this because solar PV is just too expensive still. So we’re thinking of doing a hydronic radiant subfloor system with solar thermal. Even though we think we’d need at least 300sf of solar thermal and probably about 1000gal of water for the thermal mass heat sink it’s still cheaper than solar PV. Not sure about whether we’d use propane or some type of electric coil heater for the backup but we will have a wood stove in the great room and are considering using a type that can heat air in a manner that it can be blown through ducts to the upstairs rooms which will have walls bordering on the great room.
We’d have a separate solar thermal domestic HW system of around 30sf for a 50 gal system probably using a Superstor tank with electric backup.
I’ve read a couple of articles about passive solar designing and they all seem to call for R values of R-45 or so for the walls and over R-50 for the roof. In order to do that with EPS SIPs (which is what almost every timber frame place in VT I’ve talked with uses) we’d be looking at 12 inch thick walls.
The catch with using 12in walls is that unless I go with a slab foundation I’d have to do something like use pilasters on the foundation walls in order to accommodate the extra thickness of the SIP and still make sure that the posts of the frame sit over the top of something that can support them. I’ve discussed this with the excavator I’m thinking of using and he didn’t see it as much of a problem.
However when I’ve discussed this with the timber frame folks they all seem to think that this is over kill and have been trying to steer me away from this. They all use 6in SIP standard which gives R values in the mid 20s. I My impression is that they’re just resistant because it’s not the way they usually do things and they’re not really familiar with designing a house that can minimize or eliminate fossil fuels.
So my question is does it really make sense to go with the extra thickness for the SIPs (or other insulation)?
We’re willing to spend a little more $ to try to really reduce the need for fossil fuel heating so in that regard the extra insulation doesn’t need to reach payback, but we don’t want to spend what could be a lot of extra $ to go with super high insulation values if it really won’t provide much return at all.
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