What is the consensus on this?  I'm thinking about spraying in 1-2" of foam then either blowing in insulation or perhaps using Fiberglas bats.  The foam seems to be the way to go to maintain air tightness.  Any thoughts?

lkg907,

I am currently researching the same question. I have heard yeas and nays on the topic of SIPs/insulated truss roof, but anyone who believes that a correctly applied 8" spray-polyurethane insulated truss system can have any reasonably less (or more) heat retention value than a correctly installed 8" (i.d.) polyurethane panel roof is ... well, that's some goooood stuff they got.

However, this is an apple to apple comparison, and life doesn't work that way. So since my current dilemma is the same as yours, maybe it will stimulate some educated opinions that will help us both out...

I am building 6"SIP (polyurethane) walls with semi-cathedral energy heel truss roof - about 6/12 over 3/12. I am using a truss roof design instead of panels because of the following reasons:

1)  SIP roof panels need structural support members to cover a 30x45 floor plan. Interior walls and   gluelams can do this, but using interior walls to support a roof means I have got to design my interior floor plan to hold up the roof (yea right) or use horrifically expensive gluelams (anybody wanna donate?). Trusses offer structural simplicity at minimum cost, while still providing semi-cathedral ceilings.

2)  A complete SIP roof (in my case) is at least 40% percent more expensive than an equally insulated truss roof, when comparing materials and labor as per the apple to apple comparison above. This cost increase can be linked to several factors, including:
        a) I am shipping my wall panels out of Portland, 1 40' flat at app. $5500.00 shipping to my North
            Pole, AK address. Roof panels would double the shipping. I can buy all trusses w/sheathing for
            $5500.00!!.
        b) SIP roof installations require a boom truck or crane, in addition to the standard forklift. Boom
             trucks run $500.00 per day, and Linkbelts run about $2200.00 per day w/operator. I can install
             all wall panels and trusses for this two-story house with a jobsite forklift w/jib. Boom trucks
             require equipment-specific insurance binders and certified operators up here - what's in your 
             wallet?

Now for the equal heat-retention bit. I can spray the underside of my roof sheathing with 8 inches of spray foam, all the way down to the top plate, for about $15000.00. This creates a hot roof design similar to a SIP installation, for an all inclusive roof cost of about $25000.00 installed, as compared to about $40,000.00 with SIPs.

This is cheaper, but not good enough, for the following reasons:
1) This is still 10% of the cost of the semi-custom, highly energy efficient 3300 sq. ft house. Not good.

2)  In addition, Fairbanks/North Pole has a +/- 150 degree F temperature swing, from -55F winter to +95F summer. Hot roofs need darn near perfect installations to hold up to these conditions.

3)  Code requires spray foam to be covered with a sprayed fireblock membrane in this case, because there is no directly applied drywall, as in a SIP application. This increases cost.

In order to avoid both the cost of spray foam, whether in SIPs or directly applied, and to keep a more conventional cold roof, I can hang second floor ceiling VB and sheetrock and spray foam the rock from above, before sheathing the roof. By using 2" of spray foam I create the air seal, and I could add R-50 of blown-in fiberglass for pennies on the dollar, as compared to SIPs or full spray foam. By using a full 8" of foam at the energy heel area I can ensure adequate isulation and ventilation at the soffit, and Insulfoam IV doesn't shift-in-place like the old blow-in stuff, so I know that R-value insulation levels will remain reasonably high.

If my premises are correct, we can both build equivalently insulated truss roof assemblies for our SIP wall houses, while reducing costs by up to 40-60%. What's not to like about this scenario?

Alaska,

Your points are well understood. And while it should be inexpensive to transport SIPs over water, I guess they will stick it to you until you build a whole subdivision. I might add that trusses offer an opportunity to use more efficient wiring paradigms and romex is a hot a commodity as OSB was a few years ago.

I have made some posts of the same subject and tend to get beat up a lot over it. I am in central TX and my issue is that the most I can gain in efficiency is about $25/month over 12 months because my worst bill for AC compared to no HVAC months is $30. But the AC will have to turn on. Thus the reality is that even if I could increase my efficiency another 50%, I only have $150 to gain in a year which is only 3k over a 30 year note. So it is likely that I would not ever see a reurn on my investment provided my loan rate and energy reamain close. And while this is not guaranteed, I still have the option of blowing in more down the road.

When there is no doubt of savings, use the SIPs. But when you can save 40-60% as you have found, you have to start looking at the ROI and your resale opportunities.

There are suppliers in Ontario we have used. They might ship for less??

I'm designing a small home with an attic room loft. The 1st level would easily adapt to 6" SIP walls but I'm not sure how to approach the truss design for the roof. The S. side LR will have a cathedral ceiling. If I have to invest in a truss system, which opens up over the Living area, then I may want to abandon the SIP's at the roof and just insulate the attic rooms and trusses at the peake. I'll be heating with a masonry stove and have a HP/air circulator for back-up and humidity control. It will also cool in the summer months. I'm only at 40 degrees latitude in WV.