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howard.wheaton 11-21-2010 07:44 AM

No vapor barrier
I was in my 2 attics to figure out how to install more cellulose insulation and noticed that there is no vapor barrier over the floor of the attic. i live in Chicago and is one required?

ccarlisle 11-21-2010 09:48 AM

Well, in Chicago, sure. It should be on the warm side of the insulation in the attic so right against the ceiling gyproc and underneath the joists. You should be able to see it if you pull away the insulation on the floor up there.

I mean it depends on when your house was built, they knew nothing about vb's in the 1800's but they do now. In your climate, like in ours, 6mil plastic is the norm.

howard.wheaton 11-21-2010 09:50 PM

So I need to remove all my existing insulation and put the 6mil down?

ccarlisle 11-22-2010 06:14 AM

Well, technically, yes. Practically, not neccessarily so as it might be easier to repace the ceiling below...

Ideally, if you were standing in the room below and looking straight up with X-Ray vision, you would see: the ceiling drywall or plaster, then a vapour barrier (typically 6 mil plastic stapled to the joists), acoustical sealant here and there then insulation between the joists, then the attic space. What you are trying to prevent is warm moist air escaping from the room into the attic, where it will condense on the cold surfaces up there and promote mould.

Now this isn't the only way - and each house is I'm giving you a typical set-up; not necessarily the best setup nor a modern one - but a typical one in homes in cold zones. Your attic should be as cold up there as the outside and ventilation to keep that cold air moving, a priority. There should be lots of insulation to keep the transfer of heat down as much as possible and air leaks around pipes and fixtures, sealed.

That's the bares bones of it. Now in your case, it is a pain removing the insulation and putting plastic vb in between each joist and sealing it. It would be faster replacing the ceiling - but nowadays, guys replace ceilings in an hour.

But I am a bit surprised your house doesn't have a vb....when was it built? and are syou sure there isn't one? of some kind?

howard.wheaton 11-22-2010 12:19 PM

House was built in 1979 and under the insulation is the drywall from the ceiling below. I was reading that they make a primer called "Vapor Barrier" that your apply from down below. From what I can see, that will meet code?

Gary in WA 11-22-2010 01:27 PM

Check with your local Building Department, according to Build America one isn't required, map on page 26: 5. Zone 5 requires a Class III (or lower) vapor retarder on the interior surface of
insulation in insulated roof or attic assemblies.

What This Means From A Practical Perspective
Polyethylene is a Class I vapor retarder. A kraft faced fiberglass batt is a Class II vapor retarder. Latex painted gypsum board (one coat of latex paint) is a Class III vapor retarder.

You are fine without doing anything if the ceiling is already painted once. Imperative to air seal, as Cc said:


Michael Thomas 11-22-2010 02:07 PM

As others have said, in an older house you are likely introducing as much or more moist air into the attic via poor air sealing as via a missing vapor retarder, here's one example, the discoloration at the roof sheathing is from interior air rising from a "typical" attic access in our Chicago climate:

you can also see condensed water vapor (frost) lower on the roof near the eaves.

You can use techniques such as infrared imaging to locate such leaks, but in many cases you can find many of them in winter by holding a hand over suspect openings to feel for movement of warm air and at any time of year by turning on lights on the floor below and then observing the attic floor for light leaks.

Every significant leak you plug will increase energy efficiency and reduce the entry of moisture into the attic, thus reducing condensation.

Air infiltration into attics due to poor air sealing can cause condensation, sheathing damage and mold growth– Paragon Home Inspections Chicago/Northbrook/Buffalo Grove

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