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Final countdown: Will 2 inches of rigid foam be worth it for me?

CTSNicholas | Posted in Energy Efficiency and Durability on

Hello all!

I have been asking questions and researching energy efficiency for the past two years. Finally I am getting around to building my own home and will be installing, or not installing, rigid foam in the next couple weeks!

My house build is financed mostly by a construction loan.

  • I have calculated $2,380 in the cost of rigid foam
  • $160 in material to bump out window and door openings 2″
  • $80-$160 in Aluminum
  • $300-$500 for labor to get Aluminum fabricated for the bottom
  • $40 in cap nails
  • $300 in 1×4 furring strips
  • $140 in 4″ exterior screws screws for fastening furring strips
  • $40 in tape for foam seams

which accumulates up to about $3,550. Then additional math included for the loan interest on that money which I figure $400 per 1k @ 5% interest on 15 year note. This brings the grand total of the rigid foam on exterior of my house to $4,750. This does not include the labor or time I will have in installation.

I was pretty set on doing this foam install until talking with local builders who told me I was silly and to spend money elsewhere. My brother suggested spending the money on foam instead on ceiling insulation as he said the majority of heat/cool is lost through the ceiling & windows and not the walls.

I was hoping I could mull this over a bit more with some of you more familiar with this data and building technique.

I must add that I am in Zone 5B along Nebraska/Kansas border. I will have my main source of heat be an electric heat pump. I dream of a wood burning stove for additional comfort. No gas furnace in my house due to no gas line near by.

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  1. CTSNicholas | | #1

    I forgot to add I have 2x6 walls spaced 16" O.C.

  2. DIYJester | | #2

    Nicholas, you can rent an aluminum brake for $66 plus tax from a local big box. Use a test piece to get the bends down and you can easily bend and cut a few hundred feet of coil roll in a day. This would drop the price a good bit.

    As far as is it worth it? You would have to perform energy models and calculate the difference it would make. I myself did a model and decided that spending the money was worth 8t for my piece of mind and electric bills. My calculations say I'll have paid off the cost in just under 10 years an I won't have to worry anymore about my wife freezing in the morning if the woods tone goes out while I'm in a hotel for work.

  3. CTSNicholas | | #3

    Unfortunately I do not know what to expect in terms of energy costs because I have never had a heat pump and not sure what seer rating the HVAC system will be. To me it sounds like I may not be making the best investment with rigid foam and should maybe focus more on proper attic insulation, and stud wall insulation perhaps...?

  4. user-4053553 | | #4

    I think its time for a simple article on insulation for the beginner (i assume its probably already written).
    I'm guessing Martin will show up and write a reply here that will blow away everything we say, but in a nutshell, you won't regret more insulation, its never cheaper to add more insulation then it is at the building stage, don't go overboard (but do spend some money) on good windows, put a couple inches of foam under the slab, thermal bridging is worth fighting with foam, modelling the energy usage of your building is not hard and very worthwhile, and most people will tell you what they know well and resist new approaches.

  5. GBA Editor
    Martin Holladay | | #5

    Considering how much time you have spent making a detailed cost estimate -- and I commend you for doing the math -- I think it's worth the time it will take to do some simple energy modeling.

    You should do some energy modeling two different ways: once with R-16 walls (or whatever you think the whole-wall R-value of your framed walls will be) and once with R-26 walls (or whatever you think your whole-wall R-value will be with the added foam). Compare the results to get an idea of your annual (or monthly) energy savings.

    Here is a link to an article about payback calculations: Payback Calculations for Energy-Efficiency Improvements.

    If your construction estimate is accurate, and if I understand your calculations properly, the rigid foam will cost you an extra $26 per month in mortgage payments for the life of your mortgage. If you want the investment to be "cash flow positive," then you want to determine whether the added insulation will save you $26 per month in energy bills.

    Of course, the rigid foam insulation will last much longer than your 15-year mortgage -- so it could be argued that the foam is worth it, even if it doesn't save you $26 per month.

    My guess is the biggest factor in your decision will be your assumptions about future energy costs -- a factor which is unknowable. Some homeowners are comforted by the idea that they have invested in insulation to cushion them somewhat from possible future energy price increases.

  6. CTSNicholas | | #6


    I was able to calculate heat loss to the best of my ability. It appears the difference with just the walls will be a reduction of 2,370 BTU with the extra R-10 added. I realize now that my basement walkout wall that will have no dirt around it would be more of a concern to insulate probably than adding extra insulation to an already insulated wall (main floor). I have 6684 Heating Days, a winter design temp of -4, and cost of about $0.13 for KWH. It looks like I would be saving anywhere from $150 to $300 a year perhaps with the added R10 to the walls. I will have to check into the efficiency of the heat pump to get a better idea.

  7. Expert Member
    Dana Dorsett | | #7

    I'm assuming by "It appears the difference with just the walls will be a reduction of 2,370 BTU with the extra R-10 added." you really mean it saves 2,370 BTU per HOUR at the -4F outside design temp?

    Assuming a 6684 base-65F heating degree days, implying a 65F heating/cooling balance point (though it's probably closer to 60F after the foam), that means you're looking at saving 2,370 BTU/hr at (65F- -4F= ) 69F heating degrees, or (2,370/69=) 34.35 BTU per degree-hour.

    x 24 hour in a day that becomes (24 x 34.35=) 824.4 BTU per degree-day.

    x 6684 HDD/year= 5,510,290 BTU per year.

    With resistance heating at 3412 BTU/kwh that would be (5,510,290/3412=)1615 kwh/year, which at 13 cents / kwh becomes $210/year.

    A pretty-good air source heat pump with resistance strips for backup heat would average a COP of about 2.0 in that climate if sized right. Look at the HSPF efficiency, not the SEER number. The HSPF needs to be derated it for the colder climate- the HSPF model presumes a climate with a +17F design temp. It's really more like saving $105/year with a better-class 2-speed heat pump, more for a single speed, if the heat pump is undersized, or unsuited for the colder temps on it's own. There are several ductless mini-splits out there and even a few ducted versions that would deliver an average COP of better than 3 in your climate, as well as fully modulating ducted air source heat pumps (which are a bit on the expensive side, but more comfortable than 1 & 2 stage heat pumps) that might hit a seasonal COP of 3 if sized correctly.

    For building the financial model, don't forget that mortgage interest costs are tax deductible, whereas the power bill is paid in after-tax dollars. The higher up the economic food-chain you are the more important that is, but it's still a real difference for anybody paying income taxes.

    If you want a more precise estimate you really need to look up the base 60F HDD, not 65F. Even IRC code min houses have balance points below 65F these days. Base-65F made more sense when looking at semi-leaky 2x4 / R11 homes without foundation insulation, and clear storm windows over single panes.

    Beyond the straightforward net present value of future energy cost savings, there is also hedge value on future energy price volatility, moisture resilience value lowering the risk of future mold-remediation, as well as comfort value in the higher average room radiation temps in winter to an R10 foam-over.

    Almost all envelope improvements such as wall insulation beyond IRC 2015 code min are going to have a fairly long "payback" at zero energy price inflation, but from a lifecycle point of view there is no question that the "extra" R10 will do better than break-even in your climate. If you're only looking at a 10 or 15 year time frame it might be hard to make the case for 2x6 / R20 (current code min) over and above 2x4 / R13 in your climate and current energy costs. But a 2x4 R13 wall wouldn't meet code-min in your climate zone (not even under IRC 2003).

    For improvements like insulating sheathing breaking-even over a lifecycle of 25-50 years makes sense, but if it takes a century you're probably better off spending some of that elsewhere. A good starting point for the lifecycle financial rationality discussion would be Table 2 p10 of this document:

    You'll note that those are "whole assembly-R" , not center-cavity R values, and the presumption is average construction cost & average energy pricing. In cheap construction or high energy price markets going higher can still make sense, in cheap energy expensive construction markets those number might not make sense. The table suggests R30 for zone 5 as a potential limit. Your 16" o.c 2x6 wall w/ R10 foam over comes in at about R24, and it's likely that going a bit higher would still be financially rational on a long term basis, but maybe not. Take a look at some of the other columns in the table, and compare it to your design.

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