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Insulation & vapor barrier questions....... whats right/wrong?

Posted by jpgenius (My Page) on
Sun, Mar 23, 08 at 11:18

My wife and I own a c. 1910 millworker home in Pittsburgh. We are starting to remodel the attic half-story to turn it into living space (an office and movie-room, specifically). Since the plaster was water-damaged near one of the dormers and we are tackling heating bills, we've decided to remove the lathe and plaster, remove the tiny closet, put some insulation in, and drywall. I LOVE reading posts here, and many of them have come in handy, but there are quite a few differing opinions regarding vapor barriers and blown-in insulation.'s my sitch: the attic is a half-story, has three foot knee walls and one dormer (its on the side of the house where the stairwell to the attic is) I have easy access to the wall spaces for blown-in insulation and I think cellulose is going to be my insulation du jour. My only concern is making sure its not going to cause any sweating. Anyone done this in a cold climate? The attic rafters are going to be fiberglass batting and rigid foam board (i have 5.5" rafters, so board and batt will get me close to what i want, at a non-second-mortgage cost). Should I vapor barrier, or use vapor retarder? Also, I assume I have to install venting between the peak of my room and whats behind my knee walls to keep my roof from should i? There's a TON of opinion out there, and lots of it deals with attic floors being insulated, which isn't my case. Any advice/links/opinions will help a little more before I start. Happy Easter and many thanks!

Follow-Up Postings:

RE: Insulation & vapor barrier questions....... whats right/wrong


Use channeled foam blue board that is made just for your application. It will let the attic area breath and prevent rot, mold or mildew. If your 2 x 6 rafters are 16" O.C. then the panels will fit right in between them before you install the fiberglass insulation R 19 over them.
Follow mfrgrs' installtion reccomendations when installing the foam board.
I would make ceiling runs from the peak to the soffit below, leaving the knee walls inside the chamber of the room without any insulation.
Use of Ridge Vent is definetely recomended.
Insulate the gable end walls and dormers with regular wall insulation.
These are my recomendations.

All the Best,

The PorchGuy

RE: Insulation & vapor barrier questions....... whats right/wrong

Unfortunately my rafters are spaced at 26" intervals, but its easy enough to cut foam board I suppose. Channels facing the roof I assume? And should I install soffit vents as well? Or will just the gable vent suffice? Thanks again!

RE: Insulation & vapor barrier questions....... whats right/wrong

Correct, it is easy enough to cut the foam boards to fit. Remember the type of foam boards you need to use are not flat, they have channels to allow air flow - very important.
Yes you should install soffit vents as well.
All this combined will give you a long lasting renovation that will function properly.

Good Luck!

All the best,

The PorchGuy

Here is a link that might be useful: My Album

RE: Insulation & vapor barrier questions....... whats right/wrong

Here is my response to a similar question on one of the other boards. I reasearched this a lot, and though I'm not in a cold climate, most of the recommendations would hold for your situation:

I had a similar situation. Some genius had put 3 inches of blown fiberglass in the attic (over the knob and tube wiring!) and that's it. The room was hot in the summer and cold in the winter. Here is what I did:
- r13 on the floor between the soffits and the back of the kneewall - this was over conditioned space down below (I didn't cover it, but I could have if needed for storage)- rafter vents at the soffit-end of the insulation keep the soffits open
- r13 kraft faced insulation on the back of the kneewalls (2x4 wall construction)
- 2" isocyanate foam insulation (r14)with reflective surface on the angled portion of the ceiling, leaving 1.5" air space up to the unfinished part of the attic. My rafters were true 2"x6", so I didn't need the fancy channeled insulation. This insulation is R7 per inch, so I got R14 on the slanted portion.
- R38 faced insulation in ceiling of the finished space, with eyebrow roof vents to keep air moving through there.
- seal up all penetrations (electrical boxes, pipes, etc) with caulk or spray foam)

The principles you need to follow are that you should insulate any of the unconditioned space where air from the outside is flowing. Any vapor barrier (e.g. the kraft facing)should face the INTERIOR - don't put insulation then vapor barrier, because then any moisture from the interior will reach the cooler air of the unconditioned space and condense, leading to mold. If you use the foam insulation, you have to cover it with a fire rated material (e.g. sheetrock). You could also consider using the new reflective sheets on the bottom of the roof sheathing - I didn't do this, but apparently it has a huge impact on heat gain.

The impact - the upstairs is comfortable all year. It has no dedicated heat or AC, but never gets below 65 during the winter or above 72-73 in the summer. If it gets stuffy on the hottest days, I open a couple of windows to get a cross breeze.

Hope that helps.

RE: Insulation & vapor barrier questions....... whats right/wrong

The ventilation space at the underside of the roof sheathing should be at least 1" to be effective and that is also what is required by most building codes. I don't believe that can be achieved with grooved insulation board.

Consider not venting and using sprayed foam between the rafters.

RE: Insulation & vapor barrier questions....... whats right/wrong

Spray foam is a promising, though I recommend doing lots of research. There's plenty of information and misinformation on this stuff.

Check out

There are others, too. Also, speak with some spray foam contractors and get their opinions. This is my plan and I'll be updating here.


Here is a link that might be useful: Our Blog: Rehab Or Die

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