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  1. #1
    Join Date
    Oct 2012
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    Default insulating a solid brick wall

    I live in a solid brick 1920's home in Montreal Canada...and there is no insulation any exterior wall. One room is particularly cold (my kids room). So i ripped off the plaster and am planning on building a 2x4 cavity for bat insulation in(R14). My questions are as follows..
    1- do i build the 2x4 structure up against the bricks or do i leave a one inch cavity for air circulation?
    2- i'm thinking of a plastic vapor barrier behind the new drywall and ashalt paper facing the bricks for an air barrier...is this ok?

    Thanks!

  2. #2
    Join Date
    Dec 2007
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    Tennessee
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    Default Re: insulating a solid brick wall

    It appears that the wall is already water tight so unless you got some condensation on it, it must normally stay dry. When you put up the insulated wall, there is the danger of moisture migrating into the areas between the inner and outer wall and being trapped.

    I would suggest that you have a small air gap between the studs and the bricks, any gap will do, it doesn't have to be a full inch. I don't think you need the tar paper on the brick side but you do need the vapor barrier under the sheetrock.

    What you will need is ventilation for the gap between the bricks and the insulation. To do that, you need weep holes in the bricks about every 3 feet at or below the floor level. It should slant downward as it goes outside so that water can't go into the wall, it should always drain outward. You need openings at the top, ideally to the attic or to the soffets.

    Or, you could apply foam boards directly to the bricks with a special adhesive and then apply sheetrock to the foam boards with the same adhesive.

  3. #3
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    Oct 2012
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    2

    Default Re: insulating a solid brick wall

    Keith, I appreciate your help. The rigid foam boards directly on the bricks was my first choice but I was under the impression they do not insulate as well as the batt insulation. Also I thought the rigid boards are usually used in cellar walls and floors for insulation.

    Using the rigid boards would be a huge time and space saver for me...any thoughts?

    Gefstra

  4. #4
    Join Date
    Dec 2007
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    Tennessee
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    Default Re: insulating a solid brick wall

    Rigid boards, like any other insulation is only as good as the installation. The foam boards have a higher insulating value (R factor) per inch thickness than fiberglass batts, but are often not as thick. People are sometimes under the impression that they can use a 3/4" thick foam board in place of a 3.5" thick batt, but it doesn't work that way. For practical purposes, you should consider foam at R-4 per inch thickness, regardless of the rating on the board.

    Some foams use an inert gas, such as argon or nitrogen in the foam to give it a higher R value, but that gas will eventually gas out and the R-value will drop to about 4 per inch. If you use 3" thick foam boards, you are looking at an R-12 with no thermal bridges caused by the wood studs. You need to seal all around the edges with a spray foam.

    Something to consider is putting the foam boards on the outside of the brick wall and covering it with stucco. Then the brick wall will add thermal mass to the interior, slowing down the heat cycles, that is retaining heat from the day to be released at night. This really only works if you have either very warm days and cold nights as in the southwestern states or you have a high solar gain during the day. Also, the brick wall has to be isolated from the earth or it will want to maintain ground temperature, and that will be a little too cool for comfort.

    You should also consider some type of furring strips, which could be 2x3 instead of 2x4 and oriented flat side to the room and wall, but about 2" away from the wall and then have the whole thing filled in with a closed cell spray foam. That would give you about an R-14 with little thermal bridges to the bricks. Since it is not load bearing, the studs can be on 24" centers. This gives you a very efficient wall with no worries about moisture problems, no drilling weep holes and vent tubes.

  5. #5
    Join Date
    Sep 2012
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    11

    Default Re: insulating a solid brick wall

    I would leave a small gap for air, you never know when wetness can sneek up. Leaving a gap for ventalation on both sides will not hurt you.

  6. #6
    Join Date
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    Tennessee
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    Default Re: insulating a solid brick wall

    Quote Originally Posted by brianz999 View Post
    I would leave a small gap for air, you never know when wetness can sneek up. Leaving a gap for ventalation on both sides will not hurt you.
    I have to conditionally disagree here. If you are using batt insulation, then you do need the gap and ventilation to the outside only. If you use a closed cell foam, you do not want a gap or ventilation. You do not have moisture coming from the outside right now and you do not want moisture from the inside air getting next to the brick and condensing.

    You can try to use a vapor barrier on the inside of the foam insulation, but the best way is to simply not have a place for warm moist inside air to go to get nest to the wall. That is why the spray foam would actually be your best option, though probably not the least expensive. With the spray foam, you need some form of framing to hang the sheet rock from, but since the studs are not structural, you could actually use 2x2 with a 2" gap to the wall so that the spray foam is 3.5" thick (R-14), but I think 2x3 placed on the flat instead of the normal orientation would be better. Use 24" centers and that gives you a 2.5" wide nailing surface every 24" which is adequate for sheetrock.

    You should use 2x3 or 2x4 for the top and bottom plates and they will need the normal orientation, but if you use 2x3, that still gives you 1.5" of foam between the plates and the wall, 2x4 only leaves you 1/2" of foam, but that is still a thermal brake from the heat conducting bridge formed by the wood.

  7. #7
    Join Date
    Oct 2012
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    29

    Default Re: insulating a solid brick wall

    You can also consider sprayed polyurethane foam with good air barrier characteristics which can provide you advantages of air tightness and can be applied on irregular surfaces.

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