Hello:

I am working on this addition to my home, shown in the picture. I was wondering if anyone has some advice/insights into insulating the new rooms (it’s ~ 11′ x 40′).

My plan was to used closed cell insulation in the 2×10 rafter bays, from my understanding I can do that and don’t need to leave an airgap and use a vapor barrier (the closed cell will keep moist air from going up to the roof and condensing). I was going to use regular fiberglass batts on the floor and in the walls.

If I do that

1) what should I do about the closed-off soffit area? (area B on the pic) It is separated from the inside by blocking on all rafters. Can I just fill that up with foam too and then close off the bottom? Do I need to vent it somehow? I was going to use T&G pine boards as my soffit, and wrap the housewrap up under them before putting them on.

 

2) what should I do about the cantilevered area under the wall? (are A on the pic) It is also closed off from the inside because of blocking. I was going to cover the bottom of that up the same as with the soffit, painted T&G pine and run the housewrap under there too. Can I fill that up with either foam or fiberglass batts, and if so do I need to vent it?

 

3) Finally, the other option would be the more traditional way of using fiberglass batts on the ceiling too. Then I could use soffit vents and put an air gap above the insulation, and it would have the added benefits of ventilating the house roof (since I can’t put soffit vents on the house roof now that the addition is there, the air gap could extend all the way up to the attic). 

I appreciate any advice.

Thanks! This forum has helped me build this thing so far from the ground up.

As requested, here is climage info:

• climate zone 5 according to IECC (Northern Colorado, Weld County).
• Minimum R38 on the roof, according our building department.
  

• Weld County, Colorado gets 14 inches of rain, on average, per year

• Weld County averages 34 inches of snow per year.
• Summer High: the July high is around 90 degrees
  
• Winter Low: the January low is 15
• elevation ~ 5000 ft
• roof is modified bit roll roofing, older house roof is asphalt shingles.
• There are gable vents on each side of the existing roof.

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