Condensation on roof

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Blog entry by ToddE posted 01-06-2009 08:27 PM 2392 reads 0 times favorited 9 comments Add to Favorites Watch

Hey, I just wanted to put this out to you guys and see if you have any suggestions. I have never run across this situation, as everything I have ever built was either totally replaced with new stuff, or built new. So, here’s my problem. I had to marriage an addition onto a double wide. In order for the roofs to line up, I had to use 2×4’s for the rafters because the ceilings were to line up as well. Anyway, my basic problem is, when I built the roof, I was concerned about moisture building up on the underside of the roof sheeting. So I put rafter mates in between the rafters and it left me very little room for insulation. The ceiling was to be tongue and grove pine, so I used that aluminum foil (bubble wrap) stuff that was supposed to give an equal insulation value as R-19. So, I have the shingles, the felt paper, the 3/4 sheeting, rafter mates, insulation and tongue and grove pine.
I think I am getting moisture from inside, getting sucked into the rafter mates and causing condensation. I believe the culprit is mostly because there hasn’t been any finish trim installed yet. So pretty much all the way around the ceiling there is a 1/2” to 3/4” gap. So I think warm air is getting sucked up into the sheeting and condensing and running down a rafter, because the water is seeping out of only one spot an one rafter. It is right over where the door that leads into the double wide from the addition is. So this leads me to believe that as the door from the house is opened, warmer air is exiting the door, rising and condensing in the ceiling and running down the rafter to the wall.
This was a concern of mine from the get go, so when I installed the “bubble wrap”, which I had never used before, I taped it onto all the walls with aluminum tape. Here’s my thinking, before installing the 2” wide trim, I wanted to seal this gap between the walls and the pine on the ceiling with something, then cover it with trim. I was initially thinking tape, but i wanted to see if you guys had any other ideas….Any thing would be appreciated. Thanks in advance, Todd

-- Allegheny Woodshop

9 comments so far

View TraumaJacques's profile


433 posts in 2316 days

#1 posted 01-06-2009 09:10 PM

You can try expanding foam insulation but be careful that stuff is strong and will lift wood off it’s mooring.

-- All bleeding will eventually stop.

View Todd A. Clippinger's profile

Todd A. Clippinger

8799 posts in 2915 days

#2 posted 01-06-2009 10:26 PM

My background is that of a remodeling contractor.

As I understand you applied the foil bubble wrap against the ceiling joists under the T&G ceiling. Right?

Your issue really has nothing to do with the missing trim in my estimation and I see two potential problems with your situation. An explanation of roof and ceiling basics should explain both.

A good example of this situation is a vaulted ceiling or shed roof addition.

If you have a roof cavity that cannot be vented (free air flow top to bottom) then you have to use a spray-in expanding foam insulation. The entire cavity is filled with the foam so that there is no air space. In my area of the country (Billings, MT) this is code. This was also the case when I was working in Ohio. This is a situation where you call in a professional insulation company or you will go through a pallet full of the small spray cans.

If you have insulation with a small dead space that does not ventilate properly, moisture will build up and deteriorate the structure. Mold spots may show up on the ceiling and I have seen ceiling sheetrock fall into the room below after building up enough moisture. This moisture is trapped in the cavity.

Another problem is this, the foil is a moisture barrier. A basic principle is that you never place plastic or solid moisture barrier like foil faced insulation on the ceiling. You should in fact, not even use kraft faced paper even though I see people do this all the time.

Moisture vapor rises and hits the plastic barrier, then condensation forms between the ceiling material and plastic. This moisture is trapped between the finished ceiling and solid barrier. The ceiling material in this case is the T&G instead of sheetrock but it does not matter.

The foam insulation does not act exactly as the plastic. The condensation occurs when warm, moist air hits a cooler surface. The foam insulation creates enough barrier that prevents this warm to immediate cold surface contact. Other than that I can offer no detailed explanation why it is different. Someone has it figured out because of the construction codes.

This is all said assuming that the roof is not leaking. When a roof addition is tied to a house it is not uncommon for someone to have caused a leak situation. If there is a leak on the roof it could be draining into the room down lower as it travels the rafter mates.

I am not looking at it, but from what you have described to us, you have put your ceiling together improperly. In Montana we run a very low humidity level but I am familiar with the humidity levels in Ohio. I am assuming that PA is the same so moisture issues manifest quickly.

-- Todd A. Clippinger, Montana,

View Todd A. Clippinger's profile

Todd A. Clippinger

8799 posts in 2915 days

#3 posted 01-06-2009 10:39 PM

If you need to call me feel free.

-- Todd A. Clippinger, Montana,

View Bob #2's profile

Bob #2

3808 posts in 2837 days

#4 posted 01-06-2009 10:48 PM

I agree with Todd.
You will have to pull the inside ceilng down and foam the rafter spaces in to prevent moisture and mold.

Ideally the insulation should have rested about an inch to two away from the roof side with stryofoam vents laid gutter to gable to allow an air flow. The roof peak should have been finished with a vented roof cap and the ceiling side sheeted with 10 mil poly and sealing tape.
Those are the only two methods that can work in extreme cold climates.
Sorry but…


-- A mind, like a home, is furnished by its owner

View Bob42's profile


456 posts in 2605 days

#5 posted 01-06-2009 11:22 PM

I agree also with Todd and Bob. The foil is not to be used in that area, you would need an air space. Foam is the way to go. Some supply houses sell the spray foam in larger containers with a sprayer, all you need is a compressor, although I haven’t seen them lately, that might save money by doing it yourself.

-- Bob K. East Northport, NY

View Todd A. Clippinger's profile

Todd A. Clippinger

8799 posts in 2915 days

#6 posted 01-06-2009 11:45 PM

Bob 42 is right, you can purchase small “pro” kits of the expanding insulation. You will just have to check on pricing in your area.

As a professional contractor, I have found it cheaper to sub it out. When I add the material plus my labor it is always more cost effective to hire it out to an insulation company. They have the buying power and they deal with the mess.

If you are doing it yourself, the dollars may not figure the same.

-- Todd A. Clippinger, Montana,

View lew's profile


10250 posts in 2570 days

#7 posted 01-07-2009 12:12 AM

There is a guy now here in Chambersburg, PA that is spraying a “green” foam. I understand it is made from soy beans.

-- Lew- Time traveler. Purveyor of the Universe's finest custom rolling pins.

View Bob #2's profile

Bob #2

3808 posts in 2837 days

#8 posted 01-07-2009 12:21 AM

Hi Lew:
That’s a new source for me. I wasn’t aware of the plant source for poly urethanes.
With that and the pursuit of ethanol based feul additives your morning donut could end up being $5.00! :-)


-- A mind, like a home, is furnished by its owner

View ToddE's profile


143 posts in 2750 days

#9 posted 01-09-2009 05:58 PM

Hey guys, thanks for the input. I don’t know if I clarified this, but the roof is a vented roof. The material I used is a code material for minimal spacing. I agree with the foam spray and everything, but I found the possible culprit. When the addition was put on, they drywall in the adjoining room was never put up on the top part, above the entry door. The heat from the house was going into the air space of the voids above the door, traveling up into the union between the addition roof and the old roof and causing the warm air to settle in a location where there was no insulation. Apparently before the addition roof was put on, the old roof’s hangover was never insulated. Oh well, I put some drywall up real quick and the leak ceased immediately. Thanks for your input guys. Todd

-- Allegheny Woodshop

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