Home>Discussions>INSULATION & HVAC>I screwed up roof insulation, help!
2 posts / 0 new
Last post
I screwed up roof insulation, help!

I finished two rooms in an unfinished attic, roof construction is 2x6's, roof has only two vents, length of the roof is about 35ft. long. I used R-13 to leave an air gap. I didn't know about the Styrofoam spacers, I insulated from the knee wall to not quite to the top of the roof about a 7ft. span. Since then I checked it through the crawl space and noticed that the insulation expanded and is touching the bottom of the roof sheathing. Hence, causing condensation. What can I do to correct this mistake. The roof is old so we will have to replace it in the next couple of years, in the meantime what can I do until then to avoid rot or mold?? any advice will be appreciated.

Re: I screwed up roof insulation, help!

This is a tricky situation ... here's some thoughts.

Likely the condensation is from warm moist air leaking from the heated living space into the colder space behind the ceiling and roof. This can be from the living space below leaking into the attic space behind the knee wall then this moisture is traveling up into the sloped section and being trapped and condensing. Another area could be from the heated area you have recently added is leaking the warm moist air into this space. Or a combination of both.

It's not clear what method of vapor barrier was employed in the new space when you were insulating before the walls went on. If you were simply using paper faced insulation this is an example as to how poor they are as a vapor barrier.

From the description you have an avenue for moving air to circulate within the sloped part of the ceiling which will lower the temperature significantly in this area. If this moving air can't remove the moisture fast enough the moisture will be trapped and condense in place.

Another factor is you have only R13 where you may have been better off using R24 bats designed for the 2x6 and filling the entire cavity. This would provide a better Resistance for a differential in temperature in the ceiling space which would help prevent the condensation to form. The R13 is not providing enough Resistance for the heat loss from the living space where the R24 would likely be enough to maintain enough Resistance for a drastic temperature differential.

You might try this to see if it helps as a temporary solution .... I know it sounds contrary to what is commonly thought for the need of ventilation.

Block and seal off the gaps you left at the tops of the knee walls as well the point of where the slope meets the flat part of the ceiling..... preventing any moving air from circulating up the sloped part of the ceiling ... as seen in this diagram.

This will create a static air space which will help since fiber glass bat insulation performs best in this enviorment. In a moving air enviorment fiberglass losses it's insulating performance .... which can reduce it's R value 20% to 40%.

As for a permanet fix short of tearing out the ceiling and walls you may have to correct things from the top of the roof. This one of those situations where closed cell spray foam lends itself perfectly.

Hope this helps and makes sense.:)

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.