Does polyethylene installed in the ceiling cause any problems?
user-914555
| Posted in Energy Efficiency and Durability on
I live in Minneapolis Minnesota. Is there any problem with installing a polyethylene barrier in the ceiling? The insulation installer wants to use it to hold up the blow-in cellulose before the drywall goes up. I was planning on just using a vapor retarding primer/paint. One concern I have is that If I ever do have a leak in the roof I won’t know about it because the poly sheeting will block the water from reaching the drywall.
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Replies
David,
In your climate, it is acceptable to install polyethylene. However, it isn't necessary.
You're right that polyethylene in a ceiling can delay the discovery of a roof leak. But roof leaks have a way of showing up eventually -- trust me.
It's essential that your ceiling have a tight air barrier. Pay particular attention to the gaps between partition top plates and drywall, gaps around electrical boxes, gaps at can lights, gaps at plumbing vent penetrations, and weatherstripping at attic access hatches.
If the main purpose of the poly is to hold up the cellulose, you can choose an air-permeable membrane like InsulWeb instead of polyethylene if you want.
If you don't install polyethylene, you'll still need a vapor retarder. Vapor-retarder paint is one option.
David,
InsulWeb is better than poly for installing cellulose, because it holds in the fiber while letting the blowing air escape. Here's a very informative article about cellulose installation: https://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-cellulose-insulation
David,
Are you insulating an attic or cathedral ceilings? If it is an attic, your contractor can blow the insulation after the drywall is in. If you are dealing with cathedral ceilings, in my opinion you must vent between the cellulose and the roof deck. Dense packing cellulose against roof sheathing is a dangerous practice unless you have sufficient foam board insulation on the outside of the sheathing.
Torsten,
We are building a second story addition with one main shed dormer and two gable dormers. We are using dormers to minimize the overall height of the house. In fact the dormers will really have no attic space except where the ceiling drops down in the bathrooms and closets. The roof is constructed with 16” trusses 24” OC with 23” wide vent baffles. We did not plan to “dense pack” the truss space but rather blow in with settled depth at about 14”. We are using spray foam for walls, rim joist and any other vertical spaces.
Hey. So can I resurrect this? I have enough polyethlene on site to put above my drywall on my ceiling (Half of which is 3/12 pitch scissor truss, half flat) doing R-32 batt and then heaping cellulose on top - have 8’ of vent to soffit- to get close to R 50 for entire ceiling. I’m doing intello (airtight/vapor permeable membrane) on interior walls. But don’t see why I can’t do poly on ceiling since I will have a well vented attic space.l- it should be able to dry>out. Framer says everyone in Iowa just completely stoped using plastic about 6-7 years ago- cause of mold. But I want extra chance for air tight living space. ( Will have HRV). So return poly and get mor intello if I can fit it in budget, or just use poly. Or skip interior membrane on ceiling and pray drywall crew is awesome?
spearrock,
A lot depends on your climate. Poly works fine in heating dominated regions, but if you are somewhere that uses summertime cooling for an extended period of time, poly can end up being a cold-side air/vapour barrier and have moisture condense on it.
I took a quick look at the dewpoint in Des Moines in the summer and the peak very rarely goes above 75F, the average is well bellow that.
For you to have condensation on the poly in the ceiling, the ceiling needs to be colder than the dewpoint. About the only time you would have serious condensation is if somebody ran the AC at 60F.
In heating climates usually drywall mold under ceiling poly happens from lack of proper insulation and very high indoor humidity in the winter time.
If you are looking for a piece of mind you can also go with a variable perm membrane for the ceiling. I find a membrane behind drywall is easier to detail properly as a proper air barrier plus you can visually see if it is continuous. Of course it doesn't hurt to also install the drywall in an airtight manner.
Also don't forget to seal any ceiling device boxes. You can get vapor barrier hats for these or use vapor tight device boxes (plastic ones with a gasketed flange).
Yeah. Lots of AC in July. So if someone turned off the ceiling fan this could be an issue.
I doesn't really have anything to do with the ceiling fan. Air can seep into insulation. When it's 90 degrees outside and high humidity, if some of that outdoor air seeps in and makes contact with your air-conditioned interior condensation will result. If the spot where that contact takes place is a sheet of impermeable plastic in the ceiling that condensation will get trapped and accumulate and over time cause problems.