Use poly with Roxul (on ceiling)?
Hello,
I am zone 4A, about 40 miles south of St Louis, Missouri. I am gutting my 1937 bungalow. It has two attic bedrooms (so it is a 1.5 story home), and it has an unvented attic. I want to leave it unvented, because of the multiple dormers. Eventually, the roof will be insulated with closed cell foam, but that will be a long ways off. I am not gutting the upstairs for quite a while. I have gutted the first floor and insulated the walls with Roxul R15 rock wool. I just ordered Roxul R30 to put in the first floor ceilings.
My main question is: will it likely be ok to put up some 4mil plastic before I drywall the ceilings? Half the total 1st floor ceiling is below floor above, and about half is below attic. I am primarily wanting to put the plastic up as an added insurance in case a dormer flashing fails and leaks into the attic.
My secondary question: would there be a benefit to also using 4mil on the walls before drywalling? The wall assembly is set up like this now, since I redid things (from the outside, in): Smartside lap siding with a satin exterior paint on it, then tyvek, then tongue & groove truly 1″ thick, random width planking, fir 2×4 wall with r15 rock wool insulation. My thoughts were, the 4mil would help stop air movement through the wall, since it has planking for sheathing and is far from air-tight.
The plastic on the walls isn’t a big to me either way, but I would really like to use it on the ceiling if there are no drastic issues with me doing so. Thanks for any insight into this
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Replies
Lonnie,
I have never heard of anyone installing polyethylene in a ceiling to handle future roof leaks. If anything, I've heard an opposing theory: namely, that you don't want polyethylene in your ceiling because it might disguise the existence of a roof leak (and create the undesired problem of storing large amounts of water where you don't want it).
My vote: you don't want the polyethylene there.
If you are worried about air leaks in your wall assembly, you should take steps to limit air leakage. The usual air barrier on the interior side of your wall assembly is the drywall layer. To ensure that your drywall is relatively airtight, you need to use caulk at the perimeter of the drywall, and to limit air leaks at electrical receptacles. For more information, see Airtight Drywall.
Interior polyethylene is not recommended in your climate zone because the polyethylene can become a condensing surface when the room is air conditioned. It is a wrong-side vapor barrier in the summer.
Martin, thank you for your reply. I was thinking that would be the case, but wanted to make sure before I buttoned up my walls on the inside. Much better to ask now, than worry later after the walls are finished lol. Caulking the drywall perimeter is a good idea, and I will definitely do that. Thanks for the link, I will read that on how to better seal up the receptacles too. I'm already addressing the windows and doors for air infiltration, so with your suggestions, I should get a fairly tight structure. I put new windows and doors on three sides of the house, and will be disassembling and redoing the original wood windows and door on the living room/dining room side of the home, then adding good storm windows over them.