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EPS or polyiso? Layered, taped seams?

Nicholas C | Posted in Green Products and Materials on

I have found a supplier for EPS (plain EPS, no coatings) or Polyiso (foil faced on one side, white non-reflective on the other side)

1.) The polyiso is double the price. However, it is effectively better at insulating to my knowledge. I assume it should be my pick?

2.) I don’t want to use more than 2″ of foam on the exterior. With either product, I believe it will be advantageous to layer two 1″ thick pieces with staggered joints. If I don’t have the time I will simply buy the 2″ thick and call that good.

3.) Should I tape, caulk, or spray foam the joints of either product? I was reading that all foam shrinks and the tape will do no good after a few years, more so on the EPS since tape doesn’t stick as well to foam particles compared to a foil facing. I also was reading that it is good to leave the gaps uncovered so any moisture can escape. Manufacture of the Polyiso seems to suggest taping joints. The EPS has no instruction.

4.) I plan on using Jeld-Wen or Anderson windows. They will be double or triple paned with Argon. I do not have money to buy the best windows available. Should I build a plywood box with an extra 2″ extension for these windows, or just use longer screws to go through the foam and into the jack stud? I was wondering if the plywood box had many benefits since I am only planning 2″ rather than 6 or 8″ of foam.

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  1. Nate G | | #1

    Polyiso should be assumed to be about R-5.5 long-term. Cut it down to 4.5 if you live in a cold climate, since polyiso performs worse when it's cold; EPS is the opposite--it gets better in the cold. It's about R-4, and maybe R-4.5 in the cold. For these reasons, EPS is probably more cost-effective, especially if the polyiso you found is double the cost. It's definitely not going to give you double the performance. If you skip the polyiso, you can have thicker EPS for the same price. 2" isn't a ton, but it's definitely better than nothing. 2" of EPS will be about R-8, which is code-compliant for 2x4 walls up to climate zone 6 and 2x6 walls up to zone 5. See

    If you're planning to use the foam as your WRB, you absolutely need to tape the seams, and probably fill them with caulk too. If you're planning on installing housewrap over or under it, this is less critical. Don't leave the gaps "so moisture can escape." Plan to prevent moisture entry in the first place using a ventilated rainscreen gap and good WRB detailing around the windows and doors and such.

    Also, your window budget may go farther than you think, especially if you avoid Anderson. Jeld-Wen's offerings I would avoid too unless you get their very highest-tier products; the low-end stuff is not great IMHO. Look around for local firms. I was recently able to get very nice U-0.26 double-pane vinyl windows for about $150 each ($300 for big ones) from a local firm.

  2. Nicholas C | | #2

    Okay, I will consider the EPS instead. I live in zone 5.

    When using EPS, what is the best method to seal the seams? Tape seems like it would lose the adhesive ability after a winter or two, especially if foam contracts and expands.

    I am planning a double tyvek house wrap *under* the foam layer.

    I was leaning towards the Jeld-Wen windows. Local resalers offer Jeld Wen, Crestline, Anderson, Pella, and some Menard's house brands similar to Crestline. Jeld Wen had the better pricing and seemed to maintain quality. Pella and Anderson are both pricey.

  3. Nate G | | #3

    With Tyvek under the foam, there's no huge reason to seal the joints, especially with a ventilated rainscreen gap (which you should do). But if you want to, I'd caulk the seams and tape them with Tyvek tape at the minimum, which is what I've used, or some exotic European tape (the Siga Wigluv gets good marks here; never used it though). No need for double Tyvek.

    If your Tyvek is below the foam, that's where your windows should go, too. The windows' flanges need to be integrated with the Tyvek, so the discussion about plywood boxes is moot. In this install, the windows will be in the middle of the wall.

    Jeld-Wen is decent if you use their Premium Vinyl line. You could do a lot worse than Crestline, though. Some pretty good specs on them.

  4. Nicholas C | | #4

    I would like to have window installs be as easy as possible. Installing them in the middle of the wall sounds...complex unless I can simply install the windows before the foam. (And what happens when it's time to replace the window...?) I'm not sure how flashing the foam would work out though. I was looking at this article : and it looked like I could just use flashing tape to seal the foam to the house wrap before inserting the window.

    Wasn't aware Crestline was 'pretty good' in specs. Better than Jeld-Wen? Because they seem to be cheaper priced compared to Jeld Wen.

  5. Nate G | | #5

    That article concerns using the foam itself as the water-resistive barrier. The point is that your window flanges need to be integrated with the WRB--whatever it is. If you use Tyvek as the WRB, that means the windows need to be integrated with the Tyvek. If the Tyvek is below the foam, that means you will first install the windows and integrate it with the Tyvek, and then install foam over both of them. You're right that this will complicate future window replacement.

    If you want to have the windows flush with the vinyl siding, and with the flanges more accessible to facilitate future replacement, you need to install them over the foam. This means you need to either use the foam itself as the WRB, or apply Tyvek over the foam. Since you're choosing unfaced EPS, which is somewhat permeable to water, if you decide to do this, I would recommend putting Tyvek over the EPS foam rather than trying to use the EPS foam as your WRB. Since you're only planning to have 2" foam, there's no need to build plywood boxes. You should be able to screw the window flanges right into the rough opening through the foam without any problem.

    Keep in mind that vinyl windows will last longer the more deeply recessed they are in the wall, as they'll be more protected from sunlight, which will eventually yellow and embrittle them, depending on the quality of the vinyl itself and the additives used.

  6. Nicholas C | | #6

    This will be planned as my forever house. Only God knows if that will ring true. Either way, I am trying to design for future changes if and when needed. Such as window changes, new siding, new trim, etc.

    I'm worried about using Tyvek outside the foam board, and I honestly do not know why! I just am not comfortable knowing my OSB Sheating is up against a fairly impermeable foam if water were to ever get behind the Tyvek and EPS. For example, nail holes can never be 100% waterproof, especially around a product that may expand and condense in different climate. Besides the extra cost, is there any reason I could not install Tyvek before the foam and the another layer of wrap over the foam? This would be similar to using two layers of foil faced foam because foil is not very permeable.

    I feel like the Tyvek would be easier to install over OSB with the plastic washer staples from the staple gun, since those would not really stick into the foam.

  7. GBA Editor
    Martin Holladay | | #7

    Q. "Besides the extra cost, is there any reason I could not install Tyvek before the foam and the another layer of wrap over the foam?"

    A. No, there is no reason. You could do that.

  8. Nicholas C | | #8

    I have looked online and there seem to be 'cap nails' that are used to fasten the tyvek to the foam. It seems they are just a ring shank nail with a 1" plastic cap/washer. Is that the method of choice to fasten Tyvek *over* the 2" of EPS Foam? Seems tedious, assuming it is done manually compared to a tack hammer which do not have the ability to drive that long of nail/staple.

    If I go back to my original plan on installing windows before the rigid foam, and then house wrap over OSB, what method would a DIY guy be able to use to address the exposed ends of the foam around the window opening? All I can think of is having a company with a sheet metal brake come in and flash the openings with something seamless.

  9. GBA Editor
    Martin Holladay | | #9

    Yes, the way to secure housewrap is with cap nails. Most housewrap manufacturers, including Tyvek, do not recommend the use of a tack hammer or stapler for securing housewrap.

    The two most common materials used for the exterior jamb extensions and the sill of an innie window are (a) Pre-painted sheet metal and (b) Plastic trim like Azek.

  10. Nicholas C | | #10

    Martin, do you think it is a penny wise and pound foolish to implement Tyvek before *and* after the EPS foam? I realize it is not necessary and so far, I do not find it happening. I'm thinking it would be an added security and put my mind at ease with the window issue.

  11. GBA Editor
    Martin Holladay | | #11

    You wrote, "I'm thinking it would be an added security and put my mind at ease with the window issue."

    So do it. You'll end up with a great wall.

  12. Nicholas C | | #12

    I just wanted to make sure that sandwiching the EPS with Tyvek would not cause any sort of mold issue. :) Thank you.

  13. Joe Suhrada | | #13

    Nicholas, how did you make out with this wall design? I am doing something similar. I was also wondering why no one mentioned the possibility of using BOTH the EPS AND THE POLYISO, with the 1" EPS on top of the 1" Polyiso foam. It seems like the combo might be a better stack up than the 2" of EPS alone. No? Anyhow did you end up deciding whether to use innie or outtie windows? I think the double layer of house wrap is a great plan, too. I think you can use a cheaper "house" brand on the outside too, rather than the pricier Tyvek brand or the higher drawing "Drainwrap" product, which I would personally place on the inside. I currently have a supplier for reclaimed Polyiso and EPS, but the EPS is 2.5". I would love to put an inch of the Polyiso under the 2.5 inches of the EPS. I figure I may end up with an R16 or so outside the wall envelope. We will see.

  14. Keith H | | #14

    Speaking as homeowner with exterior foam insulation, if you going innie windows (at the sheathing plane) I strongly recommend block frame if you think you'll be the person replacing them again. Also look into Proclima Contega Exo (sold by 475 building supply). This tape is especially nice for block frame and foam insulation.

    Last, I don't know what you did on windows but I'm hoping I can convince you to spend more. I've installed JeldWen 1550, 2500 and 4500. I don't bother with less than the 4500. I really think the Pella Impervia, especially for large windows, is worth the upcharge. Their Advanced Comfort Low-E with argon and foam insulation offers 0.24-0.27 U factor at what I thought was tolerable pricing. Installing fiberglass windows in nice true openings is a dream compared to cheap vinyl, I know folks here advocate much nicer windows but 1500+ an opening is not in most budgets.

  15. Joe Suhrada | | #15

    Keith, I am unsure what you mean by "block frame" regarding innie windows. Can you explain? That is part of my own plan but I am not sure what "block frame" means. I intended to use the house wrap the WRB and do the sealing and flashing there, create a removable type of exterior extension jamb that can be removed with long trim screws and then one would be able to get at the window nailing flange. Maybe you have a better option?

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