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What would you do? XPS or EPS or ISO?

TrQhFS6ysA | Posted in General Questions on

I am building a 3000 sq foot home in northern ontario (zone 8 or 9) with about 9000 HHD.

I am weighing my options on this wall contruction type.
It will be a 2×6 frame with R24 blow in batt in wall cavity and exterior foam insualtion, 1×4 strapping and siding

After reading and calculating foam thicknesses(thanks Martin) I would like to get to R20 if possible for the foam giving me a total wall R40+ rating, will be detailing like a REMOTE wall (window bucks)

Assume all of these will be taped:
I have been able to find EPS 5″ thick(R20) non faced – plain white stuff
OR 2 layers of 1.5″ ISO (foil faced) for a total of 3″ (R18)
OR 2 layers of 2″ xps for a total of 4″ (R20)

Now the $$$ part
EPS is about $3500
ISO is $9500
XPS is $11,500

If i go with EPS, would i need to cover it with a WRB?

THere is a Labour saving as well with EPS as it’s only 1 layer, but no offsetting joints – just taped
– the other 2 options would have offsetting joints, is that a concern?

What would you do?

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  1. GBA Editor
    Martin Holladay | | #1

    The EPS is clearly the best deal. However, unfaced EPS can't be taped.

    I would probably choose the EPS and cover it with taped housewrap or a taped European membrane that combines the functions of a WRB and an air barrier.

  2. MJDesigns | | #2

    Hi Martin,
    No concerns with the EPS being only 1 layer? Does the thicker EPS product exhibit the same type of shrinkage that Dr. Joe reported about in a recent FHB article? He indicated a preference for applying either XPS or Polyiso which didn't have that tendency. Have formulations improved where that's no longer a concern? Is the price differential between the 3 options shown above that great on average?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "No concerns with the EPS being only 1 layer?"

    A. Of course there is a concern. One layer of rigid foam will not perform as well as two layers with staggered seams. But the title of your question is, "What would you do?" I'm cheap.

    Q. "Does the thicker EPS product exhibit the same type of shrinkage that Dr. Joe reported about in a recent FHB article?"

    A. Probably. But I have researched and documented cases of XPS shrinkage and polyiso shrinkage, so I don't think that any type of rigid foam product is immune from this problem.

    Q. "Is the price differential between the 3 options shown above that great on average?"

    A. All prices are local. The way to compare prices is to ask local suppliers what their prices are.

  4. homedesign | | #4

    Brian, What is your air control layer?
    I think that taped exterior foam(staggered or not) and or taped Housewrap(European or not) is a not-so-great Air Barrier

    I have noticed that even BSC is starting to include Airtight Sheathing(taped plywood).

  5. TrQhFS6ysA | | #5

    How would you affix the WRB to the EPS foam if it cannot be taped?
    Around the window bucks I see that I will be using a "Resisto" red waterproofing membrane, but how do I get the lenghts between windows to hold up?

  6. TrQhFS6ysA | | #6

    As much as I have read against poly behind the drywall, that's were it has to be, my local building inspector will have it no other way.
    What options would you recommend?

  7. GBA Editor
    Martin Holladay | | #7

    Q. "How would you affix the WRB to the EPS foam if it cannot be taped?"

    A. The WRB is usually held in place by vertical furring strips screwed through the foam to the underlying studs.

  8. GBA Editor
    Martin Holladay | | #8

    I agree with you that taped sheathing makes an excellent air barrier. However, the taped housewrap would reduce wind-washing at the EPS seams.

  9. GBA Editor
    Martin Holladay | | #9

    You you mention "poly behind the drywall" because you plan to make the polyethylene your air barrier? That's possible, but you'll need to be sure that all poly overlaps are at framing members, that the overlaps are sealed with Tremco acoustical sealant, and that you have used airtight electrical boxes. Be sure to come up with a method to maintain the air barrier at your rim joists.

  10. TrQhFS6ysA | | #10

    What other choices do I have for air barrier?

    If i covered the EPS with a product like this...would it be ok?

    I figured if I use this product right over the EPS and the window bucks, it would give me a pretty good air seal...

    What do you think?

  11. trinker | | #11

    I am anxious to hear your final solution. I did this in a more moderate zone and used bib in 2x6 walls, sheathing, 1" xps, TYVEK, strips and wood siding. I effectively ended up with a double layer air barrier since the foam was tongue and groove. I did supply whole house ventilation. In your zone they would want the vapor retarder on the inside. I spent hours researching this topic and concluded that you have to be careful to not vapor barrier both the inside and outside of the wall. ISO is impermeable so I avoided it in my case. Plain XPS is permeable but enough will form a pretty good barrier. I think EPS is fairly permeable. In conjunction with the TYVEK, it should work. There is another brand of wrap that works like TYVEK but the brand name escapes me. Most house wraps are perforated and are not like Tyvek.
    Regardless of the wrap, make sure it is installed carefully and that all openings are correctly flashed. Although the wall is somewhat permeable to the outside, you don't want to introduce any water behind the foam from either direction. The folks at Pella have some fine techniques for flashing windows illustrated on their website.
    Even the extra inch of foam and the furring strips I used introduced a ton of window and door problems because I bought windows with jambs. After you pass that 6" wall, prices or work increase significantly.
    imo, bib insulation is tight and provides no airflow paths if installed correctly. Foam is tighter but $ for $, bib looks like it is hard to beat. The biggest advantage of the external foam is that it covers and insulates the building materials. R40 sounds pretty impressive but doors, windows and even a high efficiency fireplace will give you plenty of places to fight airflow and cold.
    btw, I am a rank amateur and some of the pros on this site may deem me crazy. In my research, I discovered diverse opinions on many of these topics, a lot of really wrong information from 'expert' sites and decided you really have to do your own research and hope you are right. You won't find any 100 year old homes with 5" of foam and blown in bat to prove your point.

  12. GBA Editor
    Martin Holladay | | #12

    Q. "What other choices do I have for air barrier?"

    A. For covering the outside of the rigid foam layer, I would stick with housewrap, since housewrap is designed to be vapor-permeable. I would NOT choose an impermeable product (a vapor barrier) like the one you linked to.

  13. GBA Editor
    Martin Holladay | | #13

    I disagree with your conclusion that "they would want the vapor retarder on the inside" of a house with exterior rigid foam.

    Once you install exterior rigid foam, the design principles change. Walls with exterior rigid foam are designed to dry to the interior, not the exterior. Here is more information on the topic: Calculating the Minimum Thickness of Rigid Foam Sheathing.

  14. TrQhFS6ysA | | #14

    I'm confused...
    - If I were to use the ISO and tape the seams, it would be a fairly tight air barrier that would be impermeable
    So if i used the EPS and the RESISTO waterproof covering, am I not doing the same thing? impermeable barrier?

  15. GBA Editor
    Martin Holladay | | #15

    True enough. But the normal location for a PERSIST-style peel-and-stick membrane like you are proposing is between the rigid foam and the sheathing, not on the exterior side of the foam. If any water gets between the seams of your rigid foam (due to flashing errors), you want the moisture to dry outward.

    The point of the proposed housewrap is to limit windwashing in the cracks between your EPS sheets. If you are willing to invest in an expensive peel-and-stick membrane to cover the entire exterior surface of your walls, then put it where it belongs -- between the rigid foam and the sheathing -- and be sure that you include enough R-value on the exterior side of the vapor barrier to keep your wall out of the risky zone.

  16. TrQhFS6ysA | | #16

    Thanks for the clarity...

    One last question...Is there any issues with using the ISO, EPS or XPS in conjuction with each other?
    ex: XPS covered by iso board? or EPS covered by XPS? etc....

  17. GBA Editor
    Martin Holladay | | #17

    Q. "Are there any issues with using the ISO, EPS or XPS in conjunction with each other?"

    A. No.

  18. sTEf7aH5Zq | | #18

    SIP's (Structural Insulated Panel's) using SIP's you can achieve R-54 with the 6" wall system, also works very well for the roof & flooring. Typical construction cost is about $1.25 per square foot more compared to stick framing. They can even be ordered pre-finished with a complete interior surface (sheetrock, vinyl,etc.) and finished exterior (stucco, plywood, metal). When built on an ICF (insulated concrete foundation) the rewards of this construction in energy savings will be as high as you can get. For example, your HVAC unit will only have to be half the size, your insurance is cheaper (fire-resistant, storm resistant) and the construction time is faster.

  19. user-1066984 | | #19

    OK...I have been able to purchase used 2.5" XPS ( a little dirty) they are 2X4 with lap joints
    I have decided to use new 1.5" ISO in conjunction with it. It comes in 4X8 sheets and no lap joints however is silver faced on both sides.

    WHat order would I install this?

    Do i put the new the foil face against my 2X6 wall first, tape all seams, then cover with the used XPS?
    I assume this would give me a cleaner face for the blow in in the 2x6 stud cavity?

  20. GBA Editor
    Martin Holladay | | #20

    Q. "What order would I install this [one layer of XPS rigid foam sheathing and one layer of foil-faced polyisocyanurate sheathing]?"

    A. I don't think it matters. My choice would probably be to put the new foil-faced polyiso on the exterior, because once it is taped, it makes a fairly good barrier to liquid water. However, don't worry too much. Either way will work.

  21. user-1066984 | | #21

    SHould I be concerned that the XPS is used and a little dirty?
    When it comes time to blow in the BIBS in my 2x6 wall...the blow in against a little dirt?

  22. GBA Editor
    Martin Holladay | | #22

    Q. "Should I be concerned that the XPS is used and a little dirty?"

    A. No, but you should be sure it's dry. It wouldn't hurt to use a brush or broom to remove as much dirt as you can.

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