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How to prevent moisture/condensation in building prohibited from using insulation?

bsgsjkbn23450 | Posted in General Questions on

I am building a small cottage that by code is prohibited from using any insulation (so as to prevent its use as a full time dwelling). It is a simple wood framed structure with drywall interiors that has A/C but not heat and will be used only in the summers (northeast US climate).

Clearly the lack of insulation will make cooling with A/C a very inefficient proposition. My bigger concern is that cool air on the inside during the hot summer months will lead to condensation in/on the walls. What are my options (short of insulation…which I CANNOT have..)? Would a vapor retarder in the wall (on the the outside wall) help prevent moisture from coming in from the hot humid air outside or at least keep it to the outer wall and away from the interior drywall?

Since the obvious answer here involves insulation but that is prohibited, everyone I ask (contractors, architects) is at a bit of a loss as to what to do. Thanks so much for any help.

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  1. GBA Editor
    Martin Holladay | | #1

    B.S. [Can you please tell us your name?],

    This is an odd one, for sure.

    My first thought is that lots of building materials have R-value. How do you distinguish between "insulation" and 4 inches of solid wood? Four inches of solid wood has R-value.

    You could try the old trick of building a series of one-inch-deep air spaces separated by radiant barriers. Each air space has an R-value of about R-2.

    It's clearly a matter of judgment to determine what type of material is "insulation."

    How about heavy curtains lining the walls from the ceiling to the floor? Is that insulation?

  2. GBA Editor
    Martin Holladay | | #2

    If all materials in your wall are vapor-permeable, I don't think you will get condensation. Summertime condensation problems -- the classic example being problems related to inward solar vapor drive -- generally only happen if there is a vapor-impermeable layer like interior polyethylene -- in other words, a condensing surface.

  3. bsgsjkbn23450 | | #3

    Thanks Martin. There is no real definition of insulation in the code - totally up to code enforcement to decide when they inspect (which they do) and my experience has unfortunately been that they are unpredictable and not inclined to cut you a break. We could probably get away with using wood to create smaller air spaces.

    I guess the underlying question is what problems - if any - am I likely to encounter in this situation? The exterior walls are basically plywood + tar paper and then clad in cedar. Interior is drywall + paint. That's it. I assume my A/C bill (for the 20-30 days a year it gets used) will be much higher than otherwise and can live with that. What I don't want is a structure (really just a small room and bathroom...maybe 250 sq ft) that has condensation or moisture problems or won't cool at all. It sounds like from your second reply that I'm overly concerned about vapor and might be best off just doing nothing?

    Many thanks for the advice, been going in circles trying to figure this out.


  4. GBA Editor
    Martin Holladay | | #4

    Don't worry. If there is no interior polyethylene, you'll be fine.

    You might get mold or condensation behind mirrors glued to the drywall. So don't glue any mirrors to your drywall.

  5. bsgsjkbn23450 | | #5

    Thanks, I'll avoid gluing the mirror in the bathroom. Speaking of which, I neglected to mention that the bathroom walls are all tiled (and there's a shower)....any concern I should have?

  6. GBA Editor
    Martin Holladay | | #6

    If there is tile on an exterior wall, that's the most worrisome wall for possible condensation. I'm not sure the best way to minimize the risk, frankly.

  7. Jon_R | | #7

    Perhaps you could convince them that drainable EIFS is siding not insulation. Most AC partition moisture problems are caused by air infiltration - pressurize the building slightly (but not during showers) and the chance of a problem goes way down.

  8. Expert Member
    Dana Dorsett | | #8

    How far along are you with this project?

    Painting the underside of the roof deck with a silvery low-E "radiant barrier" paint (no magic-mouse-milk ceramic nanosphere scams, please, something bright & metallic such as LoMit-II MAX or similar) followed by perforated aluminized fabric type radiant barrier stapled onto the underside of the rafters makes a big dent in it. Perforated radiant barrier runs about 5 perms, and is not a vapor barrier or moisture trap. Even at box store pricing a single 4' x 125' roll (more than enough for your application) is under $70.

    It's hard to find radiant barrier paints in gallon cans (5 gallon buckets are more common), but you only need about a gallon for the roof. But if you buy 5 gallons (~$250-375) the rest could be applied to the interior side of the wall sheathing once it's dried-in and air sealed.

    Low SGHC dual pane windows (or heat reflecting window films on single pane storm windows) can cut the direct solar gain quite a bit, but exterior shades/shutters are even better. Minimizing the size or eliminating west facing windows lowers the peak cooling loads, since west windows have a lower sun angle, and are difficult to shade with overhang, taking their solar gain late in the day when it's already heated up.

    Reducing can reduce the humidity load of any air conditioning, making it easier to stay comfortable. With RB in the attic, low-E windows, and a reasonably air tight building envelope a single 5000 BTU/hr window-shaker can provide substantial comfort for a 250' cottage in New England during peak heat & humidity days, and will usually cover the entire load despite the lack of insulation.

    If the inspectors will allow it, a half-ton point terminal heat pump is nice luxury upgrade, since it provides fairly efficient heating during shoulder-season visits or colder summer nights. Most of the new ones have scroll compressors too, which is quieter than typical window AC units. The unit price is typically $700-750, plus another $100-20 for the wall-sleeve & grilles etc. Total installed cost DIY should still be under a grand, and it's a lot nicer to deal with than window AC. Whether it ever "pays back" the cost difference in lower power bills is dubious at 30 days/year usage, but I'm throwing it out there as the "deluxe package", if you're interested. They're out there: LG LP070HED, Amana PTH073G35AXXX, Frigidaire FRP77PTV2R, GE AZ65H07DAB, Gree ETAC-07HP265V20A-A It's hard to spend more than a grand, or under $600 for the basic unit.

    If instead of plywood the wall sheathing was 3/4" asphalted fiberboard (may be difficult to find locally) it would add about R1.5 to the wall's stackup, and is both moisture tolerant and fairly vapor open. If you're willing to take a road trip to Quebec with a truck, SonoClimat ECO4 fiberboard sheathing runs about CDN$30-35/sheet (comparable to Huber ZIP sheathing), and is good for R4, see: (This stuffs hard to find even in Quebec- call them to find out the distributor closest to you that has it in stock if contemplating this.) It would roughly double the thermal performance of empty-cavity 2x4 walls sheathed with CDX or OSB, and would allow the shingles to dry toward the interior due to it's high vapor permeance.

  9. Anon3 | | #9

    Where in the world are you OP? Canada? I heard they banned wood burning stoves to force you to be dependent on the grid too.

  10. Jon_R | | #10

    No attic insulation (at least at inspection time :-)) may make powered (and pressure balanced) attic ventilation cost effective.

  11. Expert Member
    MALCOLM TAYLOR | | #11

    Where do you get this stuff?

  12. bsgsjkbn23450 | | #12

    Thanks all for the responses. I should have specified that the structure itself is already essentially complete (inc siding, windows and doors). However the interior walls (and ceiling, there is no attic) have not been closed up so there is a window to do something in the wall cavity if it makes sense. Let's just say the rules on insulation were in flux and that's how I find myself in this bizarre situation. (This is in Long Island NY).

    Martin, re your point on tile, I could scale back tile to only be in the shower so that we're trapping less condensation if that would help.


  13. Expert Member
    Dana Dorsett | | #13

    So, what are the specs on the already-installed windows?

    Any of those windows facing west?

    RB paint on both wall & roof sheathing are still a possibility, as is perforated RB mounted to the underside of the rafters. The RB on the rafters is still "worth it" even if it will be covered by gypsum board.

    If it's not too late, a CRRC rated "cool roof" shingle on the exterior with a 3 year SRI north of 40 will also reject a significant amount of cooling load.

    Using 3/4" fiberboard (low density, not MDF) instead of or under the wallboard adds about R2. Installing perforated RB behind the interior finish wall in conjunction with RB paint on the sheathing will help too.

    Perhaps a bit exotic & expensive (I've never priced it out, don't know where to find it near you), phase change wallboard can add a lot of "apparent thermal mass" at room temperatures near 73F, which is probably going to make a summertime comfort difference in Long Island climate, since typical overnight outdoor lows are enough below 73F to re-charge the phase change material a bit. See:

    A small whole-house fan mounted at a gable or a large window fan would allow you to comfortably use a night-time ventilation strategy for cooling & recharging the phase change drywall on nights when the outdoor dew points are under 60F. Pay attention to sound specifications when contemplating this approach- it's hard to sleep in the engine compartment of a light plane. :-)

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