Ma is still considering building a new house. She wants a super-efficient house and has been considering using SIPs. I’ve never built a new house (just tear up old ones), so I didn’t really have answers for her SIP questions. But I figure folks here will have the answers.
Two questions:
1. If she wants an R-60ish roof, does this mean she should opt for truss (not a SIP roof)? I think the thicker SIP’s only get up around R40ish
2. She is concerned about the OSB skin of the SIPs. She is wondering if once the SIPs are up, if she could spray the exterior with paint/sealer/some-such in order to improve its moisture resistance…. prior to having the siding put up.
I can see where’s she is heading on #2. She has seen enough old houses to know that EVERYTHING gets wet eventually! So as a precaution, coat the OSB in sealer/paint so that if it happens to get damp, it won’t have a structural failure/deformation. But I think my gut reaction would be to get the siding on as soon as possible if I want to keep the OSB from getting wet. Seems like adding a painting/sealing step would add several days to the closure process.
But as I said, I’ve never used SIP’s, so don’t know squat.
Replies
1. SIP roof is very tight, so the R needed is not as much as the typical leaky roof design that most construction uses.
Airflow of warm air to cold is a much larger source of heat loss than R value. Owens Corning spends a fortune promoting R value because their fiberglass products have no resistance to air flow. SIP construction, foam insulations and cellulose all resist air flow.
2. SIPs are made of OSB. They can stand exposure to the weather for some time. Painting the outside with a sealer of some kind shouldn't hurt anything. Might be belt & suspenders but if she wants it why not?
my two cents,
Norm
1. SIP roof is very tight, so the R needed is not as much as the typical leaky roof design that most construction uses.
So I assume you prefer the SIP roof over a truss roof? In the case of a SIP roof, do you go for the hot roof or a cold, vented one?
I saw in TOH where Silva was putting furring strips on the SIP roof panels and then adding a layer of OSB to create a vented SIP roof. I liked the idea of a vented, cold roof, but it seemed like an expensive way to create a vented roof, not to mention the added weight.
jt8
R-60 is way more insulation than even a well insulated house needs unless ma is in the arctic or something. I think the money spent going above R-40 or so would bring a better payback by making sure doors and windows were top quality and the appliances were efficient. The basement should be insulated well, too. Products like ICF's are a good value with regard to energy savings. Premium waterproofing and drainage for the basement are worth every penny.
She is considering an ICF basement and Loewen windows, but hasn't decided on doors yet. She will probably design the entries with "air locks".
My personal preference would be to skip the ICF basement. Use regular forms, and then come back and put 4" or so of extruded poly on the exterior. That puts the 8-12" of concrete on the interior side of the equation rather than being somewhere in the middle.
I know you reach a point of diminishing returns on insulation, however, depending on what kind you're using, insulation is comparatively cheap.
And I agree with you on the basement waterproofing. Not cheap, but worth the $$.
jt8