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Community and Q&A

Housewraps, cedar siding and moisture on interior of exterior wall

tbonus | Posted in General Questions on

We built a Timberframe home in 2002. This past winter we noticed that we had moisture on the inside of our exterior sheeting. I have read many articles and have received a lot of opinions. However, depending on who I talk with, the answers are different. 1) We put cedar siding over Dow weathermate housewrap without a rainscreen. This seems to be the majority of the problem. 2) one solution was to remove the siding and put 1 1/2 rigid foam insulation, a rainscreen and then the siding. The idea here is to move the duepoint away from the walls. The problem I have with this idea is that our local code required a vapor barrier on the backside of our interior wall. And, it seems that without beeing able to completely remove this, it could cause problems. The second idea is to put tarpaper over the plywood and create a rainscreen for the air to dry out the siding. We live in climate zone 5 (just barely). We have cold winters but not as cold as others that are also listed in zone 5. My question is….since it seems that the worst moisture problems are on walls close to the kitchen, bathroom and laundry room…would this solution allow the interior moisture to escape and not build up? And, if we chose the foam board insulation solution, wouldn’t there be potential for moisture build up behind the insulation (since we are adding another barrier? We have install bathroom and laundry room fans with a greater cfm just to make sure that they are removing as much moisture as possible…..

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  1. GBA Editor
    Martin Holladay | | #1

    We need more information.

    1. When you tell us that you have a timber-frame home, I assume that you mean that you have a post & beam home. What type of wall assembly do you have? It sounds like you don't have SIPs, so I'm assuming that you have stud walls. Are the stud walls installed between the timber-frame posts or entirely on the exterior of the timber-frame posts?

    2. How do you know that you have moisture on the interior side of your wall sheathing? Have you opened up your walls? Do you have rot, mold, or other problems?

  2. tbonus | | #2

    Martin, thank you for your quick response. I have been reading your other posts in regards to this issue and have been doing most of my processing through those posts. We built 2x6 stud walls between the posts (yes, it is a post and beam home). Yes, we did open up a couple of the walls, after suspecting the problem in one wall. We do not have rot....a little mold in one of the worst walls but the others are okay. We did remove the siding on three of the walls. Some of the sheathing indicates moisture but not all.

  3. GBA Editor
    Martin Holladay | | #3

    If your posts are visible on the interior, this is almost certainly an air leakage problem. Post & beam homes with infill walls between the posts are notoriously leaky. As the posts shrink (and, in some cases, swell again seasonally), the seam between the posts and the adjacent wall assemblies opens up, allowing air leakage into the wall cavity.

    The best solution is to install rigid foam on the exterior of the wall sheathing, because these types of air-leakage cracks are extremely hard to seal.

    It's a fundamental design problem, and no one should be building post & beam homes this way, in my opinion.

  4. tbonus | | #4

    I miswrote my last response. Our 2x6 walls are on the outside of the beams...they do not run between them..We have four post that run down the middle of the house. One post on each of north and south walls and two more in the middle of the house. Both east and west walls do not have posts and are 2x6 walls that carry the roofing beams.. We have SIPs over the ceiling (roof) beams.

  5. GBA Editor
    Martin Holladay | | #5

    Does the drywall run continuously past the exterior of the posts, or does the drywall stop at each post?

  6. tbonus | | #6

    It runs continuously past the posts. When they built the walls, I remember them holding the wall out so they could put the drywall between the wall and the posts.

  7. GBA Editor
    Martin Holladay | | #7

    OK, now I understand how your wall was built.

    The most likely sources of the moisture are rain penetration or air leakage from the interior. If the source of the moisture is rain, you'll usually see damage under the bottom corners of the windows.

    If the source of the moisture is interior air leaking through cracks, you'll usually see damage near the seams of your OSB or plywood.

    So, where are you seeing the damage or mold?

  8. tbonus | | #8

    It is not limited to seams or cracks. This is why I was leaning toward the theory presented on this site or another. The fact that we did not put an air gap between the siding and building wrap..leads me to think that during the summer as the sun hits the house, it turns the moisture to a vapor and the vapor goes behind the wrap...once it cools down and returns to its moisture state, it is behind the housewrap and keeps the sheathing wet. Then, if this isn't the problem, and it is air leaking through the cracks from the interior, wouldn't adding 1 1/2 of exterior foam just add to the problem because the moisture wouldn't have anywhere to go...? Or would it not be an issue because the dew point never reaches the plywood? I appreciate your time...I have been trying to wrap my brain around this for quite some time.

  9. GBA Editor
    Martin Holladay | | #9

    Exterior rigid foam will greatly lessen the risk of wall moisture problems, even if your wall has interior polyethylene, as long as it is thick enough and you do a good job flashing your windows, doors, and penetrations.

    A ventilated rainscreen will also greatly lessen the risks; the same caveats apply. (That is, as long as you do a good job with your flashing).

    I think that either approach will work to solve the moisture problem. The advantage of installing a layer of rigid foam is that you greatly improve the thermal performance of the wall.

  10. tbonus | | #10

    Okay, when I was writing earlier I was not at my house. Now I am home and have had a chance to inspect your concerns. The drywall does go behind the beams but it is not taped and does not fully connect. I cannot, however, see if the polyethylene makes a full connection. I am assuming that it does. If this is the problem at the north and south ends it doesn't seem to explain the other spots on the east and west. Another thing to add to the mix. We added a porch (not enclosed), after the house was built. The contractor had to remove some of the siding. In the areas that he removed and replaced the siding, he used Tyvek instead of the Dow Weathermate. There is a definite difference between the two areas. You can actually see a line as to where the Tyvek and Weathermate met, another reason I was thinking it could be related to putting siding directly over the Weathermate. Since the drywall has potential air leakage issues, are you still comfortable putting foam insulation on the exterior.

  11. GBA Editor
    Martin Holladay | | #11

    Q. "Since the drywall has potential air leakage issues, are you still comfortable putting foam insulation on the exterior?"

    A. Yes. Tens of thousands of Canadian houses have had such retrofit work performed, and even though they have interior polyethylene, the retrofit work does not appear to be leading to any problems.
    As you work on the exterior of your home, strive for airtight details, using high-quality tape for seams and, where necessary, canned spray foam.

  12. tbonus | | #12

    We have decided to go with the foam board, after changing our minds several times. We were able to get behind all posts and seel up all openings in the drywall. Do you recommend XPS or Polyiso...It seems that I remember you recommending Polyiso but XPS is more available in our area. We currently have 15 pnd this okay as an underlayment? Then, we also like the reverse board and batten concept. The only problem with this idea is that it would put a lot of penetrations in the foam board...thoughts? Again, I appreciate your thoughts. Not too many contractors in our area are using foam board yet and their knowledge is limited. Tom

  13. GBA Editor
    Martin Holladay | | #13

    Polyiso is more environmentally friendly than XPS (because polyiso doesn't have the bad blowing agents with a high global warming potential that are used to make XPS). Polyiso also has a higher R-value per inch.

    However, either type of foam will work, as long as the foam is thick enough to keep your wall sheathing above the dew point during the winter.

    #15 asphalt felt is an acceptable water-resistive barrier, as long as it is properly integrated with all of your window and door flashing.

    I wouldn't worry about nails or screws that penetrate the foam. They won't reduce the R-value or airtightness of the assembly by enough to cause worries.

    If you haven't read it yet, check out this article: How to Install Rigid Foam Sheathing.

  14. tbonus | | #14

    We used 1 1/2 of xps foam. Are there any spray can foams or caulks that are not compatible with the foam.....for the seams....and around the windows. Is there a particular tape you recommend? Thanks, Tom

  15. GBA Editor
    Martin Holladay | | #15

    As far as I know, any type of canned spray foam will work. Here is more information on tapes: Air-Sealing Tapes and Gaskets.

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