Mineral wool in an attic
GeorgeH3
| Posted in Energy Efficiency and Durability on
Hello,
This is a complete newbie question.
I read the article about Mineral Wool in Fine Homebuilding 254 (pp. 48-53 September 16, 2015) and it says that if you use it for insulating the floor of an unconditioned attic, you need to use a vapor barrier underneath it.
Would the painted drywall ceiling in the living space below the attic perform that function or do I need to install something in each rafter bay before putting in the mineral wool? Would that thin bubble-wrap-like insulation sheeting work?
Thanks,
George
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Trying to detail a vapour barrier into an existing attic isn't going to be effective. Most of the moisture transport will be through air leaks. So, if you have a completely uninsulated attic, go to town with caulking and foam to seal wire penetrations and around the plumbing stack. Use sheet metal and high temperature caulking to seal around chimneys. Over light fixtures, use 6 mil polyethylene, acoustical caulking and staples to make and airtight seal. Caulk or foam the cracks between the ceiling and partition wall top plates. A blower door test by a qualified energy auditor could be helpful here.
The paint on the ceiling below might be a vapour barrier depending on how old the house and how many layers of paint there are. If the house is not so old and you want to be sure, you can get apply a vapour barrier primer to the ceiling and repaint.
Thanks David Baerg...it's a 1954 house with almost no insulation in the attic. i was planning on air-sealing the attic floor/wiring holes, chimney chase, etc. as you describe. Once I do that, I can just put the mineral wool in?
George
George,
There are no requirements in the building code for vapor barriers on the interior side of a ceiling. In colder regions of the U.S., some building codes require a vapor retarder -- a less stringent layer than a vapor barrier -- on the interior side of ceiling insulation. This requirement can easily be met with vapor-retarder paint.
In any case, regardless of what the building code says (or what the Fine Homebuilding article says), moisture problems in attic insulation or other attic components are not caused by vapor diffusion. They are caused by air leaks.
For more information on these issues, see:
Do I Need a Vapor Retarder?
Forget Vapor Diffusion — Stop the Air Leaks!
Air Sealing an Attic
Thanks Martin...great info! Now I guess I have no more excuses to put off doing this any longer...