View Full Version : Insulation: vapor barrier a problem?

06-04-2011, 01:22 PM
I own a cabin that I bought with the intention of retiring to in a few years. It had no insulation in the ceiling. I am no handyman by any means, but my brother has some skills, though he is not particularly familar with insulationing. Since he is going to live with me, he's providing the labor and I'm buying the materials. The plan was to put in 2 inch rigid foam board, then lay fiberglass batts on top of that to increase the R-value. The foam board has a foil vapor barrier on both sides. A good portion of this is already in place. When we bought the batts, we wanted plain fiberglass (no vapor barrier). However, we discovered that the batts we were sold have a vapor barrier on one side. Taking them back is not an option. My brother feels that if we lay the vapor barrier side down on top of the foam board (vapor barrier against vapor barrier) it won't be a problem. However, I'm concerned that air will still get between the layers and moisture will become trapped there, causing mold. And if that happens, we won't know about it till after everything is done and sealed in with drywall, and we'll have to rip everything apart again to fix it. I think the safest thing to do is just peel the paper off. We've been arguing about this (my brother is very stubborn!) So, what do you think? Am I worried over nothing? Or is it a real concern?

06-04-2011, 08:16 PM
Yes, remove the paper facing, do not install it against the rigid foam board. (not sure why your installing rigid foam first that seems a waste as fiberglass insulation has a far greater r value per inch than rigid foam board.).

06-04-2011, 10:46 PM
Thanks for the reply. The reason for the rigid foam is kind of a long story. The original ceiling was a drop ceiling, and everyone that looked at it said it wouldn't support the weight of insulation. So we thought to lay the rigid board across the beams and then put the fiberglass on top of that so we wouldn't have to put in a new ceiling. But after we got going we found that we weren't going to be able to re-install the drop ceiling anyway, so now the plan is to put drywall up across the bottom of the beams for a new ceiling. If we had known we'd have to do that, we probably would've just gone with blown-in insulation - it would've been a lot simpler. But you know what they say about hindsight. On the plus side, we should have a dead air space between the drywall on the bottom of the beams and the rigid board on top, which will (hopefully) increase the R-value at least a little.

06-05-2011, 07:36 AM
On the plus side, we should have a dead air space between the drywall on the bottom of the beams and the rigid board on top, which will (hopefully) increase the R-value at least a little.

No you wont have a "dead air space" between the joists, infact you will create a humidity nightmare. Unless you can seal all sides and ends of the foam board against the ceiling joists you will trap hot air in an area it cannot ventilate from fast enough, this will most certainly lead to moisture issues in the 8" ceiling joist cavity.
If you have the foam board and cannot or do not want to return it your best option is to cut it to fit neatly inside the joist cavity (14.5" if they are 16" on center) and lay it down between he joists directly on the drywall then install your batt or blown in insulation, batt insulation minus any face paper.

06-05-2011, 10:03 AM
What my brother has been doing is trimming the foam board to fit tightly around the rafters, etc,, including cutting the edges at an angle where they meet the roof, and then sealing everything (including the seams between boards) with foam sealant. This has been very time-consuming and a large part of this is already done. Do you think this will create a problem even with it sealed like this? Now you've got me worried! There's no way I'll convince my brother to rip that out after he's spent weeks on it. If you still think it will be a problem, can you suggest a way to mitigate it short of undoing all that work, maybe something other than drywall?

06-05-2011, 05:33 PM
Spray foam is not designed for high heat areas so this is one problem, it will become hard, crispy and shrink, the other problem is still going to be my original concern and that is going to be that fact that it is almost (if not completely) impossible to "seal" the rigid board to rafters, and joists alike if there is a small gap somewhere hot air (summer) will fill the joist cavity and will meet cold air from the house AC at worst this will condense and cause moisture issues at best will cause the drywall to sag as it absorbs the moisture, the sagging drywall is most often seen in drywalled garages that have no insulation.

When you say your brother cuts the ends of the foam board where it meets the roof I hope he is leaving at least 1 1/2" gap between the foam board and the underside of the roof deck it is IMPERATIVE that there be good airflow from the soffits over the insulation and up into the attic.

06-05-2011, 06:27 PM
Im grateful for your insights, I'm sure your advice is saving me from future problems. My brother has been at the cabin working on it this past week (about 200 miles from where I am) and I just spoke with him prior to your last post. He said if you didn't feel it was possible to seal the foam board well enough, how about putting vents in the drywall ceiling every so often to allow airflow? Also, ref.the soffits, according to him, there ARE none in the living room / kitchen area, but there are further back in the building along the sides of the bathrooms and bedrooms. (Evidently whoever built the place was not a pro.) He is providing for air flow in those areas. Also there are 3 gable vents. He feels that there will be enough airflow even though there are no soffits in the living room/kitchen. But I'm coming to realize he's operating more on "feelings" than knowledge. Do you think that will be suficient? Also, what are your thoughts on the idea of putting vents in the drywall ceiling? Again, I really appreciate you taking the time to educate me on this.

06-06-2011, 06:20 AM
Installing a couple of vents in the ceiling drywall in each joist run would help your moisture problems, however if there is leaking air from the "foamed in place foam board" small gaps/cracks etc... then you will be allowing hot air (or cold depending on the season) into the "conditioned" living area.
If it were me and your brother wont be happy, cut out the foamed in place foam board and recut it to fit in the joist runs then install your batt insulation, if he's not willing to do that then at the very least STOP doing it that way now and continue on the correct way.
Four simple rules for insulation in attics:
1. Right insulation
2. Right installation
3. Never allow insulation to touch the underside of the roof deck
4. Air flow...air flow....air flow

06-06-2011, 10:10 AM
The roof has to breathe to inhibit the formation of condensate on the opposite side of a roof. Firstly a ridge vent must be installed to get rid of the stagnant air at the peak or ridge of a roof. A means of air movement must be installed from the soffitt to the ridge vent. Styrofoam spacers are installed on the undersides of the roof sheathing to maintain this air space and promote air circulation. Regular insulation with a kraft face facing the inside of the roof is installed. If there is an air current then condensation shouldn't be formed. Pushkins - back me up on this.

06-06-2011, 07:10 PM
Thanks for sharing your knowledge. Looks like some decisions to be made, and no doubt some arguments with my brother. I'm glad I learned these things, no doubt you saved me from a lot of problems down the road.

06-06-2011, 09:19 PM
Yeppers I agree, although a ridge vent isn't critical, if there are other ways air can enter and exit a roof. Most important things I mentioned in my last post.