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Insulation question

rockets1 | Posted in Green Building Techniques on

I have an apartment want to add insulation from the exterior.
It is a block building 1950’s vintage. Walls in the inside are stripped with 3/4 furring strip. Insulated with 3/4″ fiberglass with vapor barrier on inside. Finished inside with plastered walls.
I am thinking of adding foam sheet insulation to the outside of the building. I do not want to trap in moisture and cause mold problems. .
What is the proper way to do this. I do my own work. If anyone can guide me in the right direction I would appreciate it.

Thank You,

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  1. GBA Editor
    Martin Holladay | | #1

    The best way to insulate this type of wall is by installing a continuous layer (or two layers) of rigid foam on the exterior. The wall can be clad with synthetic stucco (a system known as EIFS), or you can install vertical furring strips, 16 inches o.c., in preparation for almost any kind of siding.

    Most building codes classify this type of wall as a "mass wall." Minimum R-value requirements for this type of wall range from R-3 in Climate Zone 1 to R-19 in Climate Zones 7 and 8.

  2. Expert Member
    Dana Dorsett | | #2

    With the vapor barrier (I'm assuming this is polyethylene sheeting?) on the interior side of the interior furring you have to be more careful about what you put on the outside, in which case many EIFS products are too vapor retardent.

    If the vapor barrier is kraft paper or rosin paper, that's not a problem, but if it's polyethylene or foil it's an issue. If the vapor barrier were between the block and furring it would also not be a problem. But it sounds like you have:

    Exterior paint | concrete block | furring & fiberglass | vapor barrier | plaster layup | interior paint

    To preserve at least some outward drying capacity you can go with up to 3" of unfaced Type-II EPS (R12.5-ish) behind rainscreened conventional siding. At 3" that would give you about 1 perm vapor retardency between the interior furring & exterior, which is about the limit of how low you can safely go.

    With XPS you'd have to stop at about 1-1.5".

    No foam with foil or plastic facers would be advisable.

  3. rockets1 | | #3

    I would like to thank you gents for the information you have both provided.
    One last question I should have included in my question. If I go with sheets of foam can I use a dense foam or should I use something on the line of the shall I call it a more open cell foam (The white sheets of foam which tends to crumble into small pieces when cut with a knife.)

  4. Dana1 | | #4

    The really crumbly EPS is Type-I (1.0lbs per cubic foot nominal density). It is more vapor open, but not very rugged. It's often sold with facers to enhance it's handling tolerance, but the facers reduce the vapor retardency to near zero. The right compromize of ruggedness to vapor openness is Type-II EPS (1.5lbs nominal density). You can break it, but it doesn't fall apart easily. Most insulated concrete forms are made fromType-II EPS- it can take quite a bit of beating, unlike it's lighter-duty cousins.

    BASF NeoPor is a higher-performance grahpite loaded Type-II EPS, but it's hard to find, and it's not super-cheap. There are many EPS vendors large & small, and you can usually find a local vendor or manufacturer dealing in Type-II sheet goods if you look for it. You won't find it at box-store home centers- try distributors who deal primarily with the construction contractors, and be specific about the density, thickness and un-faced aspects.

    Even though the macroscopic bead structure of EPS gives it some interstitial spaces that give it a higher vapor permeance, it is still a closed cell product at the microscopic level, even at lower density.

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