insulation on exterior


Old 03-23-06, 07:44 AM
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insulation on exterior

I am remodeling a house built of uninsulated 4x8x16 CMU with plaster applied directly to the inside surface of the exterior walls. I have added metal studs 16in o.c. on the EXTERIOR of the building.
The studs will be covered with 1in foam lath, tar paper as an exterior moisture barrier to shed rain, then wire and stucco.

I plan to insulate the cavity with fiberglass batts -primarily to reduce heat transmission from the outside (this is Phoenix). My question is about the vapor barrier. Normally, the vapor barrier is to the inside at the boundary between the warm heated space and the insulation. If I install the insulation with the face in, it will be up against a relatively cold surface. I'm guessing that during our short winter, the CMU will be rather cold due to it's porosity and the air space inside (in spite of the 3.5 in of fiberglass on the outside.) So I believe that any moisture passing from the inside of the building will condense inside the CMU just as it does now.

My guess is that is OK to put the vapor barrier IN or to use unfaced batts (if i can find them in 16inx3.5 - I don't think that size is stocked at the home centers).

Does my reasoning seem correct? Thanks for the help.
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Old 03-23-06, 08:34 AM
Join Date: Nov 2001
Location: USA
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This site discusses vapor barriers and it recommends for your area vapor barriers should be omitted. In other words, you should not install a vapor barrier either to the inside or exterior.
Old 03-23-06, 05:01 PM
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this sentence at the DOE site seems to cover me. Phoenix is around 1,000 Heating Degree
Days. thanks. now i just need to find 3.5x16 unfaced insulation.

>In climates with less than 4,000 Heating Degree Days, materials like painted gypsum wallboard and >plaster wall coatings impede moisture diffusion to acceptable levels. Usually, no further vapor diffusion >retarder is needed.
Old 03-23-06, 07:30 PM
Join Date: Mar 2005
Location: USA
Posts: 6,126
insulation on exterior

If you are in Phoenix, you probably have a wall built our of 8x4x16 CMUs and not 4x8x16 CMUs. I assume your wall is nominally 8" thick, built out of 4" high units.

You are falling into the "fluffy" R value game promoted by steady state insulation concepts that do not exist. R-value tests are conducted in a guarded hot-box tests that are simple and cheap and easy to advertise and promote.

You do not have a lightweight structure, but you have a home that possesses thermal interia. This mediates temperature variation is the same way the fluff slows the rate of heat loss. Heavy walls provide a mass factor that is recognized in climates like yours to improve the theoretical "R-value".

Your concept of keeping the mass inside the insulation is good because it gives you the moderating and comfort benefits of masonry. Despite the nit-picking, you do not have a climate that requires great concerns about vapor barriers that exist in other more variable and humid climates.

You may be over-rating the value of the steel stud/fluff concept. Steel studs provide a thermal "short circuit" that can reduce the over-rated insulation values of loose or batt insulation between steel studs. Steel studs can reduce the real "insulating" R-value by 10 - 40% depending on the stud spacing and insulation thickness. - Steel studs can reduce the R19 to about R11. Rigid extruded polystyrene spans over the studs and it not affected appreciably by the thermal short-circuiting.

If you really feel you can justify the construction cost in the interest of lower utility bills, you may want to consider applying rigid extruded polystyrene directly over the masonry wall. In this method, you could use either a conventional stucco or even a synthetic stucco that is nor recommende over wood/steel studs and in-fill insulation.


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