0

enter image description hereTuff Shed has 2x4 rafters on 24 center. I was going to spray foam 3.5 inches of closed cell foam for r19 value but then read it would defeat the silver lined radiant barrier and that heat would transfer to the foam.

So would it be better to leave a gap at the barrier and install a 1/2 inch rigid foam insulating board 1 inch away from the barrier between the rafters and then spray 2 inches of foam onto the foam board? Which would be more effective? First way would be much easier but is it wrong?

Also there are no vents at all in the shed and it was wrapped with some kind of barrier before the exterior was sheeted. Should I use faced insulation in the walls and the facing towards the interior?. It will be air conditioned in the hot Phoenix summer and slightly heated in the winter. I should add it will be sheet rocked.

Thanks for your response.

Improve this question
2
  • Is the radiant barrier something that came as part of the shed, or something you added?
    – ThreePhaseEel
    Aug 14, 2021 at 3:12
  • It came with the shed. Supposed to help keep it cool.
    – PHXScorpion
    Aug 14, 2021 at 4:02

2 Answers 2

Reset to default
1

Lots of views but unfortunately no answers.

This is what i think i am going to do. I touched the radiant barrier this afternoon and it was very hot so i do believe insulating right against it will transfer the heat to the insulation and be way less effective.

So I think I am going to install a ridge vent on the roof peak and then put soffit vents on the bottom of the eaves and then cut slots in each wall rafter space up above the sill plate. This way air will flow from the soffit into the wall above the sill and up the roof to the peak and exit via the ridge vent. Then I will install 1/2 inch rigid foam 1 inch from the roof. I will drill 1 1/2 screws into it so the one inch sticking out will keep it one inch from the roof. Then I am going to tack 2x2 lumber on each rafter interior face extending them so that I still have 3 1/2 inches to spray my closed cell foam for r19.

So the barrier will have a one inch channel of air space for the hot air to escape. That should keep the insulation cooler so overall I think it should be better. I read the air intake square inch needs to equal the ridge vent square inch and I need about 80 square inches on each.

Improve this answer
2
  • 2
    I like your proposed plan. Here’s some alternate ideas: 1) Rather than drill holes sideways through the roof joists to help transfer vented air, you could use continuous soffit vents and match it up to the ridge vent. (The stipulates “cross ventilation” so use soffit vents on both sides of building.) 2) Code says use 1” minimum vent space between roof deck and insulation, so you’re fine there. 3) Rather than install inside vapor barrier on walls, etc., Code puts vapor barrier on side that is heated most of the time. So, because of your location, no vapor barrier required inside.
    – Lee Sam
    Aug 14, 2021 at 4:20
  • Thanks. I'll see if I can buy ridge shingles from Tuff Shed so That I can install a ridge vent and shingle over it. Im guessing i would tear up the old ones getting them off.
    – PHXScorpion
    Aug 15, 2021 at 13:27
0

In case anyone is interested here is my progress. I have added 5 soffit vents on each side. I cut slits in each sill plate area to allow air to flow in. I cut some 1/2 foam board and used 1 1/2 screws for spacers with some small foam pieces in the center of the board to allow 1 inch space for airflow to the ridge. My ridge vent should arrive in a week or so and then I will cut a 15 foot slot for the hot air to escape. The radiant barrier of my foam boards will face the roof which will allow me to spray closed cell foam against the inside. I will face the rafters with 2x2 lumber so I can spray in about r19 of insulation. A lot of extra work, but hopefully worth it.enter image description hereenter image description hereenter image description hereenter image description here

Improve this answer
1
  • 2
    Interesting project and solutions. When you finish, you should make a short YouTube video of what you did.
    – Stay-at-home-dad
    Aug 23, 2021 at 20:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.