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Rick, General Contractor
Category: Home Improvement
Satisfied Customers: 21770
Experience:  Licensed construction supervisor with 35+ yrs. experience.
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Are you familiar with insulation and moisture issues?

Customer Question

Are you familiar with insulation and moisture issues?
JA: Thanks. Can you give me any more details about your issue?
Customer: Sure. My house originally had a flat roof. A previous owner put a gabled roof on top of the original flat roof using rafters to create an attic space. There was no roof insulation. I recently installed Roxul R-30 insulation in the rafter bays for insulation. There are no ceiling joist bays above the ceiling in the house because it was originally a flat roof, so there was no way to insulate the ceiling - I had to insulate the bottom of the roof instead. Anyhow, I've been checking in between my new Roxul insulation and the roof decking and I've noticed that I've got some mold starting near the ridge. $#(*$%(#%$(.
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Submitted: 1 year ago.
Category: Home Improvement
Expert:  Rick replied 1 year ago.

Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.

You're seeing mold because you have condensation occurring. This is due to an inadequate vapor barrier between the living space and the new attic. Also where your insulation is installed is contributing to this problem by creating a warmer space in the attic. Insulation is never installed in the rafter space in an unheated attic.

Customer: replied 1 year ago.
Hi Rick,Thanks for getting back to me. A couple more details:
1) My forced air heating system is installed in the attic
2) My goal of insulating the rafters was to move the building envelope to the roof line, rather than having it stop at the ceiling (because there is no way to insulate the ceiling anyhow).
3) The Roxul insulation is unfaced (they only make unfaced insulation).Would it solve my issue if I were to install a plastic vapor barrier on the heated side of the insulation in the attic (to prevent the moisture from going through the new insulation and condensing on the bottom of the roof decking, as it is now). I could staple some heavy duty, clear plastic sheeting to the bottom of the rafters, right underneath where the insulation is.Thanks!
Expert:  Rick replied 1 year ago.

Yes, a vapor barrier would help. And a poly barrier is best. But the rafter space insulation must be packed tight if that space is not vented. You don't want to trap moisture in the rafter space behind the vapor barrier

Customer: replied 1 year ago.
Am I correct in assuming that the mold is being caused by vapor coming from the now-heated attic and carrying moisture through the insulation where it is condensing and causing mold? I've been actively checking for this since I installed the insulation so the mold has just started in the last couple of days in a couple of places (very little so far).
Expert:  Rick replied 1 year ago.

Yes, you've created a warmer space in the attic. The air there, since it's warmer, can hold more moisture which is condensing on the colder surface of the underside of the roof sheathing.

Expert:  Rick replied 1 year ago.

I wouldn't be too concerned about the mold at this point

Customer: replied 1 year ago.
Ok, so I pulled all of the insulation down in the effected area, brushed off the mold (which was easy to do because it just started growing), replaced the insulation and put some 6 mil poly vapor barrier tight to the joists under the insulation. I'll put the rest of the poly vapor barrier up in the non-effected areas in the next day or so.
Now that the moisture can't travel to the underside of the roof decking, what happens to it? I know that spray foam installers spray closed cell foam directly on roof decking and gable ends to seal and insulate the attic and they don't have moisture issues - but I have no idea how that works exactly.
Expert:  Rick replied 1 year ago.

If the moisture doesn't have a colder surface to condense on it just stays in the air. Sprayed foam insulation is best for un-vented rafter spaces because it is its own vapor barrier and the entire space is covered with insulation so no cold surfaces. So the same principles apply with what you're using.