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Community and Q&A

Is 4″ of foam worth it?

DIYJester | Posted in Energy Efficiency and Durability on

I am building an addition onto my house in climate Zone 4 (central Missouri). Part of the plans of the addition are trying to incorporate having 2×6 construction of the addition next to a 2×4 framed original construction.

I have decided to allow the 2″ difference in framing to stick out to the exterior of the south side of house with the intention of having 2″ of foam on the 2×6 wall (which appears to be enough to prevent condensation) and adding 4″ of foam to the 2×4 walls.

The north side of the home is brick with a brick ledge. The length of the wall is ~60′ with ~R 9.72 (effective).

Is it even worth my time and money to try and insulate the original portion of the home which requires removing the brick, re-pouring an wider brick ledge, and re-bricking? Or should I just leave the front of the addition flush and not worry about re-insulating beneath the brick and no foam for the front of the addition?

From my calculations this would save me nearly $200 a year in heating costs just for that wall alone and I would spend about $1200 on foam, plus tape, brick ties, mortar (re-using old brick), and the most costly part would be extending the brick ledge out a few inches..

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  1. GBA Editor
    Martin Holladay | | #1

    You've done the math. You know the hassle involved. Only you can decide whether the work is worth it.

    Most homeowners would probably say, "It's not worth it," and would focus on other areas of the house that are easier to improve.

  2. DIYJester | | #2

    The only reason it is even an option at this point is because of the addition planning and the addition. I have to remove the edges of the siding that the addition will touch and same with the brick. I figured if I'm tearing some of it off and already have the house in pieces, I should get it done how it should be now.

    I also didn't take into account cooling loads as I have been unable to find calculations for this. Am I able to just use the same heat load calculation the same way for cooling loads? I know the delta T will obviously change and degree days.

  3. iLikeDirt | | #3

    Looks like a 6-year payback on only the foam part. If you have the skills and are planning to do the concrete and brick work yourself and add another few hundred to the total, maybe it goes up to a 7-9 year payback period. If you're planning to stay in the house that long, it's a no-brainer. Even if you're not, it could still make sense if fuel costs rise. Personally, I'd do it and consider it a gift to my future. I just did a similar thing myself as a matter of fact when I demo'd a wall of brick veneer so I could fit thicker foam there, and while it was difficult work, the sense of pride and accomplishment I feel every time I look at the result of my work makes me feel great.

    Of course if you hire that part out and they're going to charge you $2,500, then maybe it's not so worth it.

  4. DIYJester | | #4

    Well, after doing calculations for the entire house's vertical walls, my ROI is going to be quite a long time for $18,000 (total estimated including refinish). The only equation I have found that even pretends like insulation addition to this existing home is the website. I tried using $1.50 per sq. ft. as the tongue and groove foam is $1.18 per sq. ft. and this said my ROI was 4 years. I'm not sure how that works out when it would take me nearly 16 years to pay back the cost of materials alone let alone my labor.

    Years to Payback = (C(i) × R(1) × R(2) × E) ÷ (C(e) × [R(2) - R(1)] × HDD × 24)

  5. DIYJester | | #5

    To Martin's credit (of course he's an expert), I just changed the number of air changes from .7 an hour to .5 (I assumed the 6 skylights, crap door and window seals, woodstove made it somewhat leaky home) and the cost savings was almost as much as if I spent $18000 (entire house) on foam plus finishing materials, tape, etc.

    I will do all of the work myself. I would not have a problem pinning a 2-3" wall to the existing foundation. I know that it is not more than a few feet below grade to the footings for the majority of the length of home.

    If cooling load calculations are similar the overall cost for doing the entire house and addition would be about 14-18 years I'm guessing after insulation and other materials.

    The reason I find it hard to believe cooling calculations would be the same is because my losses are about 1/3 of what my heating losses are due to the Delta T. I know my 5 ton A/C was way oversized, but I was hoping the addition would load it a bit more.

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