Is insulating going to cause rot?
I put this post in the energy, heating and insulation section because it deals mostly with insulation, but it could also go under general construction techniques too. Last week we pulled the vertical shiplap siding (NEVER a good idea-unless its a barn) off a 23 year old house. Owner wants to re-side with red cedar claps. In the course of pulling off the siding we found rot, and lots of it. The HO suspected it might be there, she has seen fine sawdust on the floor underneath some windows and found it between the kitchen window sill and her countertop. Our first thought was carp. ants and sure enough they had had quite the time in there, for many years. This house was built as a timber frame and then the builder basically framed in each bay between posts, beams and braces to give him somewhere to attach the windows, doors and as a surface to attach the sheetrock. Then he insulated with fiberglass in each of those bays and then wrapped the whole place in 1″ pink board (which the ants loved too). Keep in mind this was before the days of whole house ventilation. Essentially the interior exposed beams and posts acted as huge thermal breaks thru which the interior moist air has escaped, condensed on the back side where its cold (only an R-5 there) and then, trapped between the wood and pink board, proceeded to rot the wood. So now we are in the pickle of trying to figure out how to re-insulate this beast without causing the same problem down the road. SIP’s are out of the question, we are pretty much limited to re-insulating the existing bays and re-doing the 1″ of foam. All the doors and windows are furred out that inch to accommodate the pink board. We have stressed the importance of mechanical ventilation for the house, and she’s willing to have one put one in. But is this going to be enough? One thought we had was to pull all the insulation out and rewrap with the pink board, except where it passes over a post or beam. These areas would be strapped out to create an air space so that any exiting moisture has a chance to evaporate and not cause rot. We were going to create a rain screen wall for the new siding anyway. So thats the story, have at it!
Edited 5/26/2008 7:20 am ET by scramble
Replies
Your climate is an important part of this answer. I assume you are in a northern heating climate for this answer.
Insulation does not cause rot, but the overall placement in the design can lead to trapped moisture.
With interior partially exposed timbers, you have an uphill battle because the point of entry for moisture to pass into th wall system sounds like it is primarily at that union between beam and the interior wall surface. You will be ne3eding a flexible caulk well adhered there because the timbers will move, relative to the wall surface.
Rather than eliminate the foam panel over the timbers, I would be more prone to doubling it to eliminate the dew point at which the moisture condenses.
In short, the impossible mission, should you decide to accept it, is to do as much as possible to keep moisture from entering the wall assembly, and then to insulate well enough to prevent it from condensing. Try not to trap it in the assembly, but let it out again seasonally.
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Yeah- we are in Vermont, definitely a heating climate. I guess the real question is if adding sufficient interior ventilation to remove excess moisture (keep in mind, the original builder didn't even install a bathroom fan) is going to keep moisture from migrating thru the beams. My guess is no-there will always be moisture (vapor) in the inside of the house, esp. in the winter, and we know R-5 is not enough to raise the dew point-but we're stuck with this inch of play. So, maybe sprayed on iso. closed cell foam over the beams? Better R value and well air sealed?
iso is not closed cel foam. It is open celled and it will hold moisture like a sponge.Polyurethene foams are closed cell
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Like PFN suggested, the 1" of foam was not sufficient to keep the dew point out of the wall assembly. If the exterior foam is sufficiently thick, excess interior moisture is less of an issue. Not sure what the optimal foam thickness is for your case but it's obviously more than 1"
What about sprayfoam?
I have no idea if this is possible or not. Need to talk to some suppliers to see if the can mask off the posts and and keep it clean, but get a good seal behind the exposed part.
Another option would be to not use the foam, but fur out that inch.
Then use a monolithic spray insualtion such as BIBS or damp/dense pack cels.
And if you go back with foam you can go with ESP that you can get treated with borax so that the ants can't live in it. It is R4 rather than R5, but it is also a little more permable.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
The return key every now and then makes it easier to read ;-)
I put this post in the energy, heating and insulation section because it deals mostly with insulation, but it could also go under general construction techniques too.
Last week we pulled the vertical shiplap siding (NEVER a good idea-unless its a barn) off a 23 year old house. Owner wants to re-side with red cedar claps. In the course of pulling off the siding we found rot, and lots of it. The HO suspected it might be there, she has seen fine sawdust on the floor underneath some windows and found it between the kitchen window sill and her countertop. Our first thought was carp. ants and sure enough they had had quite the time in there, for many years.
This house was built as a timber frame and then the builder basically framed in each bay between posts, beams and braces to give him somewhere to attach the windows, doors and as a surface to attach the sheetrock. Then he insulated with fiberglass in each of those bays and then wrapped the whole place in 1" pink board (which the ants loved too). Keep in mind this was before the days of whole house ventilation.
Essentially the interior exposed beams and posts acted as huge thermal breaks thru which the interior moist air has escaped, condensed on the back side where its cold (only an R-5 there) and then, trapped between the wood and pink board, proceeded to rot the wood.
So now we are in the pickle of trying to figure out how to re-insulate this beast without causing the same problem down the road. SIP's are out of the question, we are pretty much limited to re-insulating the existing bays and re-doing the 1" of foam. All the doors and windows are furred out that inch to accommodate the pink board.
We have stressed the importance of mechanical ventilation for the house, and she's willing to have one put one in. But is this going to be enough? One thought we had was to pull all the insulation out and rewrap with the pink board, except where it passes over a post or beam. These areas would be strapped out to create an air space so that any exiting moisture has a chance to evaporate and not cause rot. We were going to create a rain screen wall for the new siding anyway.
So thats the story, have at it!
Thanks-
I tend to type the same way my mind runs, like one big streaming torrent. I forget it makes it hard to read for others.
"the same way my mind runs, like one big streaming torrent"Very poetic description of diarrhea there, LOL
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sometimes I think that's all I have in my head anyway-
Spray foam insulations should be good options. BIB or wet applied cellulose possibly, too.
Also think about vapor barrier rated paint ... but you still have cracks to deal with (e.g. wall/beam intersection).
Mechanical ventilation is/can be critical! You can't have well insulated w/out continuous vapor barrier and not expect to control interior generated moisture. That was the BIG lesson learned in the early days of higher insulation levels and tightly sealed houses.
Look at lifestyles related to moisture generation, too. Lots of aquariums? Plants? Lots of showers, bathing, cooking? Moisture in a basement?
If you find yourself needing lots of ventilation volume for long periods each day ... use a exhaust air heat exchanger to recover some of the energy.
Make sure ice dams don't create leaks at walls ... Murphy could be lurking out there.