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Add insulation when changing siding?

konowl | Posted in General Questions on

We are getting our windows done on our 40 year old over the next month (yes they are the original windows LOL). We currently have 2×4 walls with full insulation, zone 6 climate. Has anyone ever increased the insulation R value by adding styrofoam? We are residing the house within the next couple of years.
Thinking about adding 2 inch foam around 3/4 of the house (front wall is brick) just not sure if the cost recovery will be worth it (not to mention the difficulty in matching the wall that meets the brick wall due to the extra 2 inches). It looks like a 48×96 sheet will run $60. We are in Ottawa, so fairly cold winters.
Our walls don’t get THAT cold in the winter, and heating/cooling costs aren’t horrible. However, if we think it’s worth it, the time to do it is now, as I can get the windows with the extra two inches added.
Has anyone added extra insulation in the way of styrofoam when residing the house? Worth it? Or more beneficial to put more money into the attack insulation.

The only thing I’m really worried about is the vapour barrier – I know if it’s not there it’s not a big deal, but even if it IS there, it was put up 40 years ago so I doubt it’s overly thick. Hell, the headers in the basement doesn’t even have a vapour barrier, just insulation.

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  1. GBA Editor
    Martin Holladay | | #1

    You need to begin reading articles on the GBA web site to get up to speed on these issues. Start with these articles:

    How to Install Rigid Foam Sheathing

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    Roofing and Siding Jobs Are Energy-Retrofit Opportunities

    Once you have read those articles, feel free to post any followup question here.

  2. konowl | | #2

    I read the information provided yes; that's where I came to the conclusion that 2 inches would be sufficient for my zone. My main responses I suppose is the ROI; would it be more beneficial to add the extra insulation in the attic instead, for instance. I took a quick look at the vapour barrier used and it's quite thin compared to modern standards, so I don't believe I'm in danger of condensation on the inside of the 2 inches of foam (even though I'm always worried about it LOL)

  3. GBA Editor
    Martin Holladay | | #3

    For an accurate return-on-investment calculation, you would need to use energy modeling software to compare a couple of scenarios.

    That said, the answer will always be subjective, because it will be based on several guesses. The most uncertain factors are (a) how long you will live in the house, and (b) your best estimate of future energy prices. Another variable: How much you care about lowering your energy bill. (Some people love to see their energy bills drop. Others just shrug.)

    My guess is that the payback calculation will show that this investment is barely worth it. However, your house will be more comfortable, and you will be protected (to some extent) from the economic battering that occurs from swings in energy prices.

    Here is a link to an article with more information on the topic: Payback Calculations for Energy-Efficiency Improvements.

  4. charlie_sullivan | | #4

    ROI is definitely better adding insulation in the attic--not because insulation does anything different there, but just because adding insulation in the attic is very easy--you just blow it in and you don't have to change anything about the windows. But you might want to do both. I am doing both on similar age, similar construction, similar climate house.

  5. konowl | | #5

    Yeah I think I'm going to go ahead and do the 2 inches but with the Roxell Comfortboard - that will alleviate my (probably silly) concerns about condensation.

  6. Chaubenee | | #6

    Gary, I think you will be very pleased. The house will be warmer, less drafty (assuming you use the opportunity to do a little caulking) and QUIETER than ever.

  7. christopher_peck | | #7

    We are about 3/4 of the way through adding 2" of rigid foam insulation to the exterior of our house here in Sonoma County (zone 3). We're upgrading a bunch of things as we go: windows, adding exterior doors and windows in some rooms, new siding, new paint job (obviously), improved earthquake reinforcements. It costs us about $1500 for each 10 ft. linear section of wall, including replacing windows. That's with my dad and I and one highly skilled handyman working on it. As is apparently the case around the country, we're all learning as we go, innovating on some pieces, following the lead on most others. We are getting faster and are doing it more cost effectively.

    All in I figure it'll cost about $50,000 to do the whole house, including wall upgrades, attic, crawlspace, etc. In our easy climate there's no way that will save me enough energy over time to make it make sense, but we are still doing it. In zone 6 certainly the ROI will be better.

    I wrote an article about it here:

  8. Chaubenee | | #8

    The first time I did a house over was in the very early nineties and my Dad and I did 1/2 foam over the wood siding on the 1957 ranch homestead (I currently live there today again) and blown in cellulose and new vinyl wondows and siding. I recall the immense improvement in comfort and noise reduction. In 2002 I bought another small ranch and used one inch of exterior foam and blown in cellulose in the 2x4 walls and the attic. I caulked the seams that time on the exterior XPS I used.The house was very
    Comfy after that. Now I am building a new house and am using 2.5" of ext. reclaimed EPS, air sealing the OSB, taping the foam, and installing dense pack cellulose in the 24oc 2x6 walls. The attic will be flash foamed to R14 with R50 spray foam over the top plates and will have R65 ish continuous above the flashing. The evolution of the techniques has brought me here. I really recommend that people employ the best methods they can. I am in zone 5 in upstate NY. Anyone in a colder zone would be even happier I assume.

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