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Community and Q&A

Insulating a guest room in Portland, Oregon

WShawn | Posted in Energy Efficiency and Durability on

We’re in the process of remodeling a 140 sq. ft. guest room in the southwest corner on the ground floor of our 1928 bungalow. I’ve torn down and disposed of all of the lathe and plaster, revealing totally empty 2×4 stud bays covered with 9″ shiplap planks, tar paper of some sort, and sided with 8″ beveled cedar plank siding.

I figure most anything I add to seal and insulate the stud bays will be a big improvement. I won’t attempt to add any additional housewrap. I was thinking about going along the cracks between the shiplap planks with that Great Stuff Gaps and Cracks foam sealant. Probably not the ideal product, but I don’t think a more complex system like EcoSeal or whatnot is warranted in such a small space.

Would caulk be better in this case? If so, what kind?

I plan to install R-15 Roxul mineral wool insulation, which was used in our kitchen remodel a few years ago.

I’m up in the air as to whether I should install a 6 mil poly vapor retarder over the insulation. I’ve read conflicting opinions on that here and elsewhere. We do have central air conditioning. I think a smart vapor retarder would be overkill here, and kind of pricey.

I’d finish with 1/2″ drywall.

Any thoughts or suggestions?


Shawn Marshall
Portland, Oregon

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  1. GBA Editor
    Martin Holladay | | #1

    Trying to seal all of the cracks between your sheathing boards with caulk or canned spray foam would be time-consuming and frankly rather ineffective.

    You might consider purchasing a two-component spray foam kit for a flash-and-batt job.

    If that seems too expensive to you, another possibility is a cut-and-cobble job, using 1/2-inch or 1-inch EPS. ("Cut-and-cobble" refers to cutting strips of rigid foam and inserting it between studs.) Push the EPS up against the sheathing boards, and seal the perimeter of each piece of foam with caulk or canned spray foam.

    A third possibility is to insert pieces of housewrap in each stud bay up against the sheathing boards. If you go this route, you will need to seal the perimeter of each piece of housewrap with caulk or a high-quality tape.

    You don't need interior polyethylene. What you need to do is to pay attention to airtightness when you install your drywall. For more information on vapor barriers and vapor retarders, see:

    Do I Need a Vapor Retarder?

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

  2. WShawn | | #2

    Hi Martin:

    Thanks for the reply; I appreciate it.

    So if I cut 1/2" or 1" strips of EPS (extruded polystyrene?) and put those into the stud bays and seal around the edges what would happen to the Roxul insulation I plan to install? I'd be losing 1" or 1/2" of depth (depending on which thickness of EPS I use) and then have to compress the mineral wool into that now 3" depth (if I use the 1/2" EPS).

    Is this the type of EPS you're talking about:

    I'm thinking the housewrap approach might work best with the Roxul to prevent air movement (though I'm fuzzy on how it can allow moisture to get out while preventing air from getting through-sounds like magic to me). I wonder if taping the edges to the studs and plates would be sufficient, combined with the friction of the mineral wool pushed into the stud bays. Would that be Tyvek tape or something that would stick better to the wood? The dusty, dusty wood.

    I've been reading the pros and cons of poly barriers. I'm leaning against a poly barrier, making the drywall as air tight as I reasonably can, like caulingk the perimeters and using gaskets on the outlets. I don't think it's worth doing everything you list in your Fine Homebuilding article.

    What caulk would you use in this application?



  3. GBA Editor
    Martin Holladay | | #3

    EPS is expanded polystyrene -- the white stuff. It's a little more vapor-permeable, but just as airtight, as XPS (extruded polystyrene).

    The Foamular you linked to is XPS.

    Housewrap is vapor-permeable while still being airtight. It allows water vapor to pass through it. For more information on the difference between air barriers and vapor barriers, see Questions and Answers About Air Barriers.

    Housewrap tape should work if your studs are fairly clean and smooth. If they are rough, you might want to use caulk and staples to seal your housewrap to the framing.

    Either a polyurethane caulk or a silicone caulk should work fine; either is better than cheap latex painter's caulk.

  4. WShawn | | #4

    Great, thanks for the clarifications.

    Take care.


  5. mackstann | | #5

    If you use a half inch of foam, you could use 3" Roxul Safe 'n Sound to fill the remaining space without any squeezing required. It doesn't have a rated R-value but I think it's safe to assume it's similar to other Roxul products -- around R-4.2 per inch (which is also about what EPS is).

  6. WShawn | | #6

    So is that the only difference between the Roxul thermal batts and the Safe and Sound Batts: the thickness?

    Is this the kind of EPS foam we're talking about?



  7. mackstann | | #7

    I haven't dug into the specs on Safe 'n Sound vs. ComfortBatt. I would do that before substituting. As long as its density is close to or higher than CB, then I would think that a similar R-value would follow. But this is just my hunch as a moderately informed layman homeowner.

    That link is indeed EPS, but it has a poly facer, so keep in mind that it will act as a vapor barrier (shouldn't be a problem here, I don't think). Another good place to look is Knez. They have a few Portland metro locations and have a good selection of insulation that the big box stores don't often have. Wood Feathers is another one.

  8. WShawn | | #8

    I was wondering about the vapor barrier aspect of what Home Depot has. Per what Martin suggested I wasn't going to go with a vapor barrier here.

    I think since I have some Tyvek left over from another job, and I know the 3 1/2 batts or Roxul will work, I think I'll go with that solution. It would probably be easier to install the foam panels, but we'll make the housewrap work. As I wrote at the top, I think most anything I do will be a big improvement.

    I haven't heard of Knez; I'll have to check them out.

    Thanks for the input.


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