Batts or Foam

[retiredmarine]

Wondering what everyone's thoughts are on foam or batt insulation.  I'm building a little over 800 sqft single level with a crawl space (24"), the roof will be trusses, and I'm building in Southern Missouri.  Looking for comments/suggestions for the ceiling and walls.  The foam is a bit over 2X as expensive, requires a tech, and an appointment but it is done for me. If I spray I can close up the roof all the way to the top and in the eves. The batts are less expensive, but there is my time. Conventional insulation does require venting at the top and in the eves.  Thoughts?

[Don_P]

I'm assuming between bearings at joints 11 and 13 in inside and the level bottom chords are over porch. You do need to frame/block between trusses over the walls to the underside of the roof there anyway. I prefer foam whenever the budget allows, it is the difference between slowing and filtering the air through fiberglass and actually stopping it with foam. its a pay me now or pay me later question in my mind.

[retiredmarine]

You are correct Don 11 and 13 are the outside walls the remaining is the porch. not having toe worry about adequate ventilation and being able to close up the peak was another advantage to foam.  As I finalize the pans I'm revisiting a few areas to see if I can save a few bucks here and there.  "pay me no or pay me later" resonates.

[NathanS]

Spray foam insulation is oversold in my opinion.

If you are only insulating the cavities, the foam will be undermined by thermal bridging of the wood framing. In other words, there is virtually no difference in total wall R-value of spray foam vs fiberglass.

If you are concerned about energy efficiency you should put rigid foam on the exterior of the walls and spend time air sealing either drywall or the sheathing. Fiberglass batt + 1" rigid foam will significantly outperform the spray foam.

[retiredmarine]

Well now we have a debate...  so in the ceiling your saying no difference, hmmmm.  And on the walls - air sealing by mil plastic on the inside before drywall?

[NathanS]

The most common 'air barrier' would be the exterior sheathing. After that, it would be drywall.

In Missouri I would think you'd want to avoid a poly vapor barrier if possible because you're air conditioning most of the time.

In the trusses there is not as much of a framing penalty to the r-value. It would depend on how much space there is for insulation. If you have 12"+, put in air channels and do a good job air sealing the the ceiling I really don't think you're going to see much of a performance difference. Much cheaper, extra labor for you would be blowing in cellulose, caulking the drywall to the top plate, caulking all the ceiling electrical boxes so they don't let any air through.

[retiredmarine]

The plan is OSB sheathing with Hardie lap siding. In side a band of poly bead sheathing along the bottom (4') and dry wall to the ceiling. The trusses are 24" OC.