hybrid roof assembly and conditioning requirements
In Eastern MA. Insulator is proposing a hybrid roof strategy with a layer of closed cell foam against the roof deck followed by a layer of open cell to the interior of that.
I am aiming for 50% of the aged R value to be closed cell, so I think that should avoid condensation issues on the roof deck from what I’ve read.
Is there any protection for the rafters? I never see rafter condensation discussed, only roof deck.
Do we need to condition the attic in the same way as if we’d had all open-cell insulation under the roof deck? Meaning, does attic need it’s own hvac supply and return? We could do this in the main attic, but not other attics on the house.
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brianjf,
As long as your closed cell foam is thick enough you will be fine. Moisture problems occur on the first surface warm interior air meets that is cold enough for it to condense. That is typically the sheathing. The rafters are fine, as are the studs in a wall, both of which have ample drying capacity.
Every part of the house inside the insulated area has to be conditioned, and reasonable levels of humidity maintained, but you are right, it isn't half as important with closed cell foam as with open.
Thanks, Malcolm!
We have a 2-story house. We are adding a 1-story bump-out addition with a shed roof that meets the house partway up the 2nd story. There doesn't seem to be a perfect solution here.
A vented roof can't really work because there's no ridge vent.
An unvented roof needs a conditioned attic which is not going to be accessible with hvac ducting.
Is that right? Is there an approach I'm missing for this?
brianjf,
You can vent attics that butt into walls a few ways. Gable Vents at each end, Box (or Turtle) Vents located near the peak, or Ridge Vents sometimes called Roof to Wall Vents made for exactly your situation.
A vented, unconditioned attic is the least expensive, and simplest solution.