GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

Community and Q&A

Polyiso for basement

Sr Kod | Posted in Green Building Techniques on

Morning all

I am about to embark on the basement insulation. I have a question about polyiso. After much reading I was so sure about putting xps 2″ rigid foam against walls and rim joist.

I read the gba, several other forums and as I understand, the steps are – clean the basement walls, clean rim joist, place rigid foam, seal with great stuff, place xps with foam glue on walls, tape the seams, seal the edges with great stuff, leave a inch gap and frame with stud walls.

I have a few questions and will greatly appreciate anyone guiding me through this. Thanks a lot in advance for your time and patience.

1. Can I use polyiso faced with aluminum – if so will the foil go facing with concrete or towards the interior?

2. Must I leave the 1″ gap between foam and studwall?

3. Will placing a perimeter drain with a gradient along the entire basement and letting it drain out in one corner help if moisture accumulates – or am I overcomplicating it in a newbee fashion?

4. One more question – if I am going to leave a little space between stud wall and the rigid foam, is there any mileage in stuffing the stud bays with unfaced fiberglass?
Thanks

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    S.R. Kod,
    Q. "Can I use polyiso faced with aluminum?"

    A. Yes.

    Q. "If so will the foil go facing with concrete or towards the interior?"

    A. Either way, although you will get a slightly higher R-value if the foil faces an air space. That would usually mean that it's better for the foil to face the interior.

    Q. "Must I leave the 1" gap between foam and stud wall?"

    A. No.

    Q. "Will placing a perimeter drain with a gradient along the entire basement and letting it drain out in one corner help if moisture accumulates - or am I overcomplicating it in a newbee fashion?"

    A. Yes, it will help, but it usually isn't necessary unless your basement shows signs of water entry.

    Q. "If I am going to leave a little space between stud wall and the rigid foam, is there any mileage in stuffing the stud bays with unfaced fiberglass?"

    A. You can do it either way, although I think it makes more sense to keep fiberglass insulation out of your basement.

    For more information on all of these questions, see How to Insulate a Basement Wall.

  2. Sr Kod | | #2

    Thanks a lot for your answers. I will keep the fiberglass out of basement. I have some of the basement completed and unfortunately they did [previous owners] what everyone tells not to do! Bare concrete, studwall with fiberglass - faced, face facing the sheetrock. Since this part is already done I am leaving it as it is and finishing the rest of the unfinished part in the current method - foam on concrete and empty stud bays. Hope that does not cause toomuch of a problem.

    Will putting a small, low energy dehumidifier with it's intake sealed in to a box, then running intake pipes from the current finished part with fiberglass in to it's intake manifold so the dehumidifier gets it's intake from behind the fiberglass sound like a reasonable idea or am I going at it wrong way?

    Thanks a lot for your patience and your site.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    S.R. Kod,,
    Your dehumidifier ducting plan is not good. If you pull air from the fiberglass-insulated stud bays, you will be encouraging that air to be replaced by warm, humid interior air. This constant air flow will increase, not decrease, the likelihood of condensation.

  4. Sr Kod | | #4

    Oh ok - then I will stick to what everyone else does. Thanks for the advise.

  5. Charlie Sullivan | | #5

    I suppose you could take the output of the dehumidifier and duct it into the fiberglass-insulated stud bays.

  6. User avatar GBA Editor
    Martin Holladay | | #6

    Charlie,
    Better -- but still not advised, in my opinion.

  7. Sr Kod | | #7

    As a first timer I appreciate all the help here. I think I will keep it to minimum complexity.

    Two more questions. I found a supplier for the 2 in polyiso near me.

    Can I use the same glue and the sealing tape as the xps?

    is there a specific pattern for the glue - horizontal lines or vertical lines?

    The black and Decker basement finishing book suggests not to seal the bottom of the foam where it meets the slab. Is that accurate?

    OK, three questions :-)

    Thanks a lot

  8. User avatar GBA Editor
    Martin Holladay | | #8

    S.R. Kod,
    Q. "Can I use the same glue and the sealing tape as the XPS?"

    A. For information on glue (adhesive) compatibility, read the label on the adhesive tube. Foil-faced polyiso is the easiest type of rigid foam to tape; for more information on tapes, see Return to the Backyard Tape Test.

    Q. "Is there a specific pattern for the glue - horizontal lines or vertical lines?"

    A. Either way will work. Make sure that you come up with a way to apply pressure while the glue sets.

    Q. "The Black and Decker basement finishing book suggests not to seal the bottom of the foam where it meets the slab. Is that accurate?"

    A. No. For a good air seal, you want to seal the perimeter of each piece of rigid foam.

  9. Sr Kod | | #9

    Thank you.

  10. Sr Kod | | #10

    Hi

    It turns out that the polyiso I found was not faced with foil. Can that still be used? Reading about permeance I am guessing it won't offer much of a barrier. So now I am planning to use xps.

    Our local lowes and home Depot don't have 2 in poly iso. Can xps be used to insulate rim joist as well?

    Thanks and happy new year.

  11. User avatar GBA Editor
    Martin Holladay | | #11

    S.R. Kod,
    It's true that polyiso with foil facing makes more sense than polyiso with a vapor-permeable facing in this application. Two-inch-thick polyiso without foil facing probably has a rating of 10 to 15 perms.

    I suggest that you use either foil-faced polyiso or EPS.

    You can use XPS, but XPS isn't as environmentally friendly (because XPS is manufactered with blowing agents with a very high global warming potential) as polyiso or EPS.

    Here is a link to an article with information on insulating rim joists: Insulating Rim Joists.

  12. Sr Kod | | #12

    Oh OK. So the eps is better for the environment. How about r value, permeance and fire safety. Can eps be the same as xps for these purposes? I will check out the local stores and see what they have in terms of eps.

    Thanks

  13. User avatar GBA Editor
    Martin Holladay | | #14

    S.R. Kod,
    The EPS link you provided is one brand of EPS. There are other brands, of course.

    And yes -- EPS is usually cheaper than XPS.

  14. Sr Kod | | #15

    In terms of permeance and fire safety is it as good as xps? If so I owe youa virtual beer :-)

  15. User avatar GBA Editor
    Martin Holladay | | #16

    S.R. Kod,
    Neither XPS nor EPS can be left exposed. Both types of foam have to be protected by a layer of 1/2-inch gypsum drywall for fire protection.

    EPS is more vapor-permeable than XPS.

    1-inch thick EPS has a vapor permeance of 2.0 to 5.8 perms.

    1-inch thick XPS has a vapor permeance of 0.4 to 1.6 perm.

  16. D Dorsett | | #17

    EPS is more vapor permeable than XPS at any given density, but that's not to say it's super-vapor-permeable. Type-II EPS (1.5lbs per cubic foot nominal density) is usually rated at 3 perms max @ 1" thickness, compared to 1.5lb XPS at 1.2-1.5 perms. At 2" Type-II EPS would then be less than 1.5 perms. The Home Depot goods in the link is Type-I (1lb density), which would be no more than 2.5perms @2" thickness if unfaced, but if you click on "specifications" and read the spec you'll note that it has a facer (on one side or both), which is likely to be vinyl, polyolefin, polyethylene, or foil, any of which will bring the permeance under 1 perm (probably under 0.5 perms).

    But even 2.5 perms would be fine. As long as the foam is well under 5 perms any ground moisture coming through the foam will still dry to the interior without a problem, as long as you don't put anything more vapor-tight than latex paint on the finished interior.

    Unfaced polyiso is somewhat rare outside of high-temp roofing or special applications. Unfaced polyiso is usually at LEAST 2lbs density (can be as high as 4-5lbs) and it's vapor permeance than the 10-15 perms suggested by Martin, it is typically 2-4 perms @ 1", half that at 2". (The specs vary depending on actual density & manufacturer.) Polyiso with fiberglass or asphalted paper facers is usually less than 1 perm (at any density), due to the vapor permeance of the facer. The typical 1lb density polyiso with foil facers would be north of 5 perms @ 1", but probably not as high as 10 perms @ 2".

    From a fire safety point of view EPS & XPS are identical. Polyiso is somewhat safer, since it has a much higher kindling point, and even when fully burning it chars in place rather than melting into flaming pool of liquid polymer spreading out on the floor, as is the case with polystyrene insulation (EPS/XPS). In this application it's a somewhat academic distinction however, since if any part of your basement is hot enough to light off the foam, odds are pretty good the rest of the house is already engulfed. It's worth thinking about for upper floors or roof though, since gravity and flaming liquids could spread fire from say, a lightning strike high on the building turning into a burning stream of goo sliding down the side of the house.

  17. Sr Kod | | #18

    It is for the interior of the basement. In that case will it be possible to add a 2in eps on the wall and put another 1in unfaced eps on top with out overlapping the seams for extra r value? I think I will go for eps then since it is cheaper and better for the environment. What but second layer of rigid for also being faced? Will this trap the moisture? Thanks

  18. User avatar GBA Editor
    Martin Holladay | | #19

    S.R. Kod,
    Faced or unfaced -- either way, it doesn't matter much if you are installing 3 inches of foam. It's always better to overlap the seams rather than line up the seams.

  19. Sr Kod | | #20

    That is great. I am sure I will have more questions as my project moves forward. Thanks a lot for all your help.

  20. Sr Kod | | #21

    Hi again

    I started to insulate the rim joist and am proceeding with walls. As I was reading I came up with a couple of questions. With regards to insulating the floor. I read the threads on here. My understanding is

    - poly ethylene layer on the concrete
    - then 1" xps foam or eps foam sealed with tape
    - 0.5" layer of marine ply fixed to floor with concrete screws
    - another 0.5" layer of marine ply
    then the floor coverings as desired.

    Is that correct and for this purpose is EPS still ok? I am hoping to put a pool table on this surface, can the EPS support the weight?

    Another question is does the stud wall go over the floor insulation or does it go over the bare concrete and the floor insulation stops where it butts in to the stud wall foot plate?

    Thanks

  21. User avatar GBA Editor
    Martin Holladay | | #22

    S.R. Kod,
    Q. "For this purpose is EPS still OK? I am hoping to put a pool table on this surface. Can the EPS support the weight?"

    A. EPS comes in different grades; some grades have greater compressive strength than others. Any rigid foam with a compressive strength of 25 psi or more should work fine for this application. (Commonly available brands of XPS have a compressive strength of 25 psi.) If you are purchasing EPS from a reputable source, you should be able to get compressive strength information from the EPS distributor or manufacturer.

    Q. "Does the stud wall go over the floor insulation or does it go over the bare concrete and the floor insulation stops where it butts in to the stud wall foot plate?"

    A. It depends on whether it is a bearing wall. If you have a bearing wall in the center of your basement, the bottom plate of the wall should bear on concrete. If you are installing a new stud wall at the perimeter of your basement, the bottom plate of this wall should be installed on top of the horizontal rigid foam that covers the concrete slab. This type of wall is not structural; its purpose is simply to provide a chase for electrical wiring and a convenient way to attach drywall. (You should also have a vertical layer of rigid foam between the concrete wall and the stud wall.)

  22. Sr Kod | | #23

    Hi

    If by bearing wall, you mean load bearing wall, then it is not. I am putting this one at the perimeter just to hang the sheet rock. So it goes on top of the rigid foam on the concrete slab.

    I am also putting the EPS 2" with the facing, on the concrete walls.

    EPS is from Lowes.

    Thanks

  23. Sr Kod | | #24

    Is there a reason for polyiso when insulating the rim joist?can it be faced eps or xps? Following your suggestion about faced eps I did the walls with it and have air right seals there but now am at the rim joist post.got a ton of eps! Can I not use eps to insulate the rim joist? I don't have a problem, just a waste to not use the eps I already have.

    Thanks.

    Ps: did the floor as well as I read here- poly, 1in xps, then 1in osb.

  24. User avatar GBA Editor
    Martin Holladay | | #25

    S.R. Kod,
    Yes, you can use EPS to insulate a rim joist. Here is a link to an article with more information: Insulating Rim Joists.

  25. User avatar
    Dana Dorsett | | #26

    With 3/4-1" OSB to distribute the weight the floor foam could have been EPS too. Standard subflooring is designed to be able to handle the weight at center span even when supported by joists 16" apart. With any density foam on top of a concrete slab the support is continuous- nothing to span, with much LESS deflection than a floor supported by joists.

    In some respects it's better to put the poly between the foam & subfloor, since any air gaps/depressions that might form a liquid reservoir will have less volume than a typical uneven floor. In the event of minor flooding/spillage the water doesn't pool up and dries more quickly that way. But it's still not a big deal if the poly is between the foam & concrete.

  26. Sr Kod | | #27

    Thank you both. I guess it is too late for the poly! I got a 0.5in osb on top of the 1in xps. Do I still need to put another layer of osb or should that be enough?

    Got the link about the rim joist insulation. Went to HD and got some polyiso and flame retardant canned foam so thati don't have to put a dry wall behind the dry wall! Does the code really mandate putting dry wall over the foam board while behind a stud wall that will again be covered with dry wall!

    Thanks again

  27. User avatar GBA Editor
    Martin Holladay | | #28

    S.R. Kod,
    Q. "Does the code really mandate putting drywall over the foam board while behind a stud wall that will again be covered with drywall?"

    A. Your question is confusing -- I'm not sure what you are driving at. The basic answer is that one layer of drywall is enough.

  28. Sr Kod | | #29

    Sorry about the confusion. I was reading a few articles and one of the articles I misread suggested that you put sheet rock over the insulation of rim joist as well as over the walls. Obviously that is what happens when you put the sheet rock but I misread that you have to double it up. Too much reading!

    Again thanks

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |