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Community and Q&A

Is external polyiso worth the cost?

wfprUFkYpT | Posted in Green Building Techniques on

First, let me say, that this community is fantastic. It is good to see so many people invloved and really dedicated to making building more efficient, and thus, playing a part in saving our planet and environment. I am always pleasantly surprised at how helpful everyone is, most of all, you Advisors. Thanks for the time and effort.

Now to the details. I am doing a renovation, that will basically be a new home in a Mixed Humid climate here in NJ. The only thing I am SURE of is that we are using 2×6 construction and building a 3500 sq foot colonial, using Andersen 400 windows and plan on sealing this baby tight. That being said, I have been trying to really weigh the cost/benefits of everything else when it comes to insulation and construction ie: spray foam, rigid exterior insulation on the roof and walls etc…

So I am hoping that some Advisors and other green gurus with experience can help me out. Should I doing rigid foam around the whole house? Is this a $1000 upcharge or $5000 (I know size of the house matters, but just trying to get a basic idea)? How does this affect the depth of the windows I am purchasing, will I need custom jambs now since I have added an extra inch over the plywood sheathing? If I do use rigid foam, I can see added costs in the following: The foam boards, their installation, the added effort of installing windows against the foam/installing, the added jambs. Then repeat that whole process for the roof…

So, is it worth it? I plan on installing spray foam throughout, inclusing directly to underside of the roof. Am I overestimating the amount of additional work//cost?

I know I have thrown out a loaded question here and I understand the benefits of a tight, strongly insulated house, I just cannot get a good grip on what the upcharge is for this…

Thanks in advance for all the helpfull advice and comments I will get!

Keep up the greening!

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  1. GBA Editor
    Martin Holladay | | #1

    If you are not familiar with the steps required to produce a construction cost estimate (or if you are not the GC for the project), then your question needs to be directed to a contractor.

    Who's the designer? Your designer should be able to provide some information on cost estimates; this can be helpful if your contractor has not yet been chosen.

    Whether any energy-efficiency measure is "worth it" depends on the price you pay for energy and your assumptions about future energy prices. There is no single answer to your question.

    You may wish to read the following articles:
    'Innie' Windows or 'Outie' Windows?

    Roofing and Siding Jobs Are Energy Retrofit Opportunities

    Thinking About Net Zero Energy

  2. GBA Editor
    Martin Holladay | | #2

    One more point -- a personal opinion: I would never consider building a new building with a wall construction type that fails to address thermal bridging. That means that walls need either exterior foam sheathing or two rows of staggered studs.

  3. wfprUFkYpT | | #3

    Ahh Martin, you are saying what I thought you might, but hoped you wouldn't! I am the homeowner and managing most of the construction, although I do have a construction manager helping me out. that being said, he is not a green builder by any stretch so I am going at this info on my own. It is amazing how going green is like and start to do a couple little things, that lead to bigger and better things (and more costly) ie...I started with the thought of using spray foam...which led to 2x6 construction, which led to learning about thermal bridging which I was not accounting for, which leads to exterior foam etc...BUT, I do understand this house is an investment for he next 75 + years so I want it done right the first time....

    So Martin, I know you have problably said it 100 times in other articles and posts, but if I were to have (inside to outside) drywall, closed cell spray foam, 2x6 studs, Plywood sheathing, then 1 inch of foam, furring strips and then fiber cement siding, am I addressing the thermal bridging enough in a mixed humid climate of NJ? Am I looking for trouble in mixing the external rigid foam and internal closed cell?

    Thanks much for all your insight and expertise!

  4. GBA Editor
    Martin Holladay | | #4

    Spray foam is quite expensive. If you are convinced of the value of exterior foam sheathing, that's good. You can pay for the exterior foam by choosing a less expensive insulation to fill your studs. I would recommend cellulose.

    Assuming you are in Climate Zone 5, your exterior foam sheathing (on a 2x6 wall) needs a minimum R-value of R-7.5. Meeting this minimum R-value is important if you are using cellulose; unimportant if you are using spray foam between your studs.

    If you go the cellulose route, you'll still need to design your air barrier. Actually, you still need to design your air barrier even if you go the spray-foam route. There are many locations in your thermal envelope that require careful planning for air sealing.

  5. Daniel Ernst | | #5

    Kurt - Here's some good resources to help answer your last question (looking for trouble?):

    NJ heating seasons vary a lot. Near the coast they generally have less than 5,000 HDD. Some areas to the northwest go as high as 7,000 HDD. The answer probably depends on your exact location and heating requirements. It also depends on a lot of other details (like your air barrier and interior humidity controls).

    It would be money and time well spent to read Lstibureck's building guide. Find it here:

    In the mixed humid guide you'll find a discussion and graphs of dewpoint temperatures and exterior sheathing. You'll also see how foam board insulation (different thicknesses) change that dewpoint.

    For a REALLY rough cost estimate, call your local lumber yard. Find out the cost of polyiso (in my area Home Depot sells it for ~ $13 per 1" x 4' x 8' sheet, but we're talking NJ here!). Now you can take your wall and roof area, divide it out and at least have an estimate on your material cost. Your labor cost all depends on your GC - and how comfortable he is with the installation. Only he can answer that question . . .

    I'm going through a cost / benefit analysis for an upcoming build - much the way you are. It's a time consuming process. Hopefully you aren't rushed to make decisions.

    One more point. If you're costing this out yourself you'll probably find that exterior polyiso board insulation is CHEAP compared to spray foam - especially a 2 x 6 stud cavity filled with closed cell foam. You might consider other options, like dense packed cellulose for your wall cavity.

  6. Avi Elbaz - Westmount Fine Homes | | #6

    Hello Kurt,
    I'm a builder located on the Jersey Shore. I have installed exterior polyiso sheathing and will be installing some coming up on a new home that we are framing now. Please feel free to contact me if you'd like to come by and see how we address your concerns.

  7. Riversong | | #7


    There is vast latitude in what designers and builders consider "green" building. Energy efficiency is only one part of that definition, though the one that dominates the discussion. A better term for what we must be focused on might be "sustainable" building.

    "Is it worth the cost?" is a useful question only if we consider what our economy has historically failed to consider: the costs to the earth and all its living creatures, which includes future generations of our own species.

    With that broader question in mind, it's reasonable to answer that a 3500 SF single family residence is not "efficient" or "green" or "worth the cost". It may also be rational to answer that the use of petrochemical plastics is not worth the cost to our grandchildren (or perhaps even to ourselves if we consider the associated health costs).

    It's also logical to question whether our narrow-focused concern about present and near-term energy (utility) costs is fostering an approach that creates many more costs in terms of unintended consequences or ignored externalities.

    A perfect example of this blindered thinking is seen in the leading "environmentalists" who are so frightened by the rapid onset of global warming that they morphed from anti-nukers to nuclear energy supporters, in spite of the obvious, extreme and long-lasting dangers of that technology (and the fact that it's anything but carbon neutral if you consider the entire fuel cycle).

    If we are to leave a habitable world for our grandchildren, and all the millions of other species upon which they will depend for co-existence, we have to take off the blinders and ask much broader and deeper versions of "Is it worth the cost?".

  8. jklingel | | #8

    Kurt: Having read here for several months now, I've done a 95 degree turn. CELLULOSE. If you don't want to blow in dense pack, get the batts and double wall it. I built similarly w/ fiberglass in 1980; 2x4 w/ 'glass, 5 1/2" 'glass batt to batt, then another 2x4 full of glass. Working very, very well; cellulose would be doing better. I don't see why that would not work w/ cellulose or cotton batts. S^&%$ the plastic. For the same R your wall will be a tad thicker, but you won't be polluting and it may be cheaper than the iso outside. Good luck w/ whatever you do. john

  9. wfprUFkYpT | | #9

    Well, I just read about a new product from owens corning, it is a spray then blown in fiberglass. I am including a link here, would love to hear if anyone here has looked into it, and since most of you have a greater deal of expertise than I have, what your worries would be about it. Thanks for all the helpful comments...Permits come in next week so going to get started soon!

  10. Kopper37 | | #10

    Kurt - Have you received an estimate on this system?

    The EnergyComplete system from Owens Corning uses a highly vapor permeable foam (40 perm dry cup) to provide an air barrier. It's latex based, applied in a thin layer, used to seal stud cavities and rim joists. It's a lot like the flash-and-batt system used by some closed cell spray foam companies, except the foam itself doesn't provide hardly any insulating value.

    A couple of notes:

    1) Like spray foam, it improves airtightness, but it doesn't hit some parts of the structure (i.e. where the bottom plate meets your floor, built-up columns, etc.). You'll still need to focus on those details.

    2) It doesn't solve your thermal bridging problem. To do that you need to use exterior foam insulation OR one of the various light-frame systems (larsen truss, double-stud wall, I-beam).

    3) Condensation on exterior sheathing is still a possibility. It's a risk. And there are a lot of factors that go into this equation. Airtightness. Interior humidity control. OSB vs. plywood. Heating Degree Days. How much risk do you want to take on?

    The EnergyComplete system is proprietary. I doubt you could get more than one bid in your area. Just a thought. Call around.

    One final point, if you took a poll on this site, CELLULOSE vs. FIBERGLASS, you would probably find a large majority favoring cellulose. So why fiberglass?

  11. schoeb93 | | #11

    I have a related question.

    Doing 2x6 with internal batts and planned 1.5" polyiso in NJ 4/5 climate. Doing cedar shingle which will require furring. Prefer not to do vertical and horizontal furring. Is just horizontal ok? Is this a rookie move? I am a total rookie at this. Where to put wrb and vapor barrier and how to integrate into window flashing? I am the homeowner, not the GC. The great GC is older and not up with green building so I am charged with figuring it out.

    Any insight would be much appreciated.

  12. charlie_sullivan | | #12

    I think the best answer is Martin's: Substitute cellulose for spray foam, and use the savings to pay for the exterior polyiso.

    There's something of a philosophical rift here between people who think petrochemical-based insulation can't be a good thing and those who put efficiency first. Personally, I don't mind petrochemicals, especially when you aren't burning them ... but some foams, specifically north american XPS and almost all spray foams are produced with blowing agents (the gas the makes the bubbles) that have more than 1000X the global warming impact of CO2. You can argue about whether the energy savings outweigh that impact, but other options such as EPS, polyiso, and cellulose don't have that impact, so at that point it's hard think of a reasonable justification for using XPS or spray foam.

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