Replies

1. GBA Editor
   Martin Holladay | Jul 16, 2016 04:54am | #1

   Peter,
   All construction costs are local. The only way to answer your question is to call up a few contractors to get bids. The work will be far more expensive in San Francisco than in rural Maine.

   On a green building web site like GBA, we're going to try to talk you out of installing R-40 of spray foam, since spray foam is very expensive. Moreover, many types of spray foam use blowing agents which are environmentally unfriendly.

   Depending on where you live, we are probably going to try to talk you into installing R-60 cellulose instead of R-40 spray foam.

2. Expert Member
   MALCOLM TAYLOR | Jul 16, 2016 11:30am | #2

   I'm not sure how you make this an apples to apples comparison. A SIPS roof still needs a framed structure to support it so that would have to be deducted from the cost of the trusses. The soffits costs are a wash. Both need fascias and finished materials on underside.
   For a simple gabled house that size the trusses would be around 7Gs here. You are looking at about $800 for the OSB and another $250 for bracing and the blocking for the soffits.
   Whether the trussed roof mimics the SIPS and uses scissor trusses , or has an unconditioned attic, it would probably use blown cellulose insulation, which is a pretty low cost alternative to foam.
   As to the labour costs. If a four man crew can't stand and sheath a simple gabled truss roof in two days they should be shown the road.

3. 
   Dana1 | Jul 16, 2016 01:32pm | #3

   R40 of half pound density open cell foam runs about USD$4-4.50 per square foot (installed), and is about as green as it gets in the virgin-stock foam biz due to the low impact H2O blowing agent and mimimal polymer per R. (But it's nowhere near as green as using reclaimed rigid foam board.)

   R40 in 2lb density closed cell foam runs about $6.50-7.50 per square foot and is the opposite of green at ~2x the polymer per R and the use of HFC blowing agents (with only a very few vendors using water or H2O blowing agents.)

   R40 in open blown density (attic floor) cellulose runs about $1.20-$2 per square foot. Damp sprayed on a roof deck at mid density is about twice to 3x that much depending on the particulars. Dense packed R40 runs $3.50+ per square foot, and is often more expensive than open cell polyurethane.

   Clearly the better value is in open blown cellulose on the attic floor, since it's dramatically lower cost/R-foot and there are fewer square feet to the attic floor than the roof deck.

4. 
   jackofalltrades777 | Jul 16, 2016 04:01pm | #4

   The reason I referenced spray foam for the stick frame roof is that the SIPs are using foam. No way can one use blown cellulose in this application since it is a shed roof design and the thickness of the roof is about 6" top to bottom. No way to use cellulose in such a roof design.

   The SIP utilizes closed cell spray foam (yes I know about the blowing agents) but it gives an R-40 value in a thin roof profile. It also provides the strength and fire protection for the roof.

5. Expert Member
   MALCOLM TAYLOR | Jul 16, 2016 07:11pm | #5

   The comparison becomes harder if the goalposts keep getting moved. Now the trusses have become a 6" deep stick-framed roof. I don't think any meaningful comparison can be made without seeing the drawings.

6. GBA Editor
   Martin Holladay | Jul 16, 2016 07:25pm | #6

   Peter,
   It's hard or impossible to get R-40 of foam in a roof framed with 2x6s.

   If you frame it with rafters that measure a full 6 inches -- and remember, it's hard to buy those these days -- you can only get R-40 if you assume that closed-cell spray foam has an R-value of R-6.7 per inch. That's a little higher than a realistic estimate for spray foam.