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Old 05-29-2020, 03:00 PM   #1
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XPS, EPS, or Polyiso foam board?


I need some foam boards for my rim joists, basement cinder block walls, and second floor overhang. Based on my research:

EPS:
Lowest R value
Lowest cost
Doesnt lose R value over time
Breathable, doesnt trap moisture

XPS:
Better R value
Mid-range cost
Loses gases/R value over time
Not breathable, traps moisture

Polyiso:
Best R value
Highest cost
Doesnt lose R value over time
Doesn't do well with moisture

Which foam boards would you guys go with? I will be purchasing from Home Depot, so my brand choices are: R-tech Insulfoam(EPS), Owens Corning Formular 150(XPS), or DOW Tuff-R(Polyiso). I am leaning towards EPS.
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Old 05-29-2020, 03:10 PM   #2
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Re: XPS, EPS, or Polyiso foam board?


One of the objectives of placing rigid foam against an exterior cold surface is to ensure warm inside air doesn't reach that sold surface. EPS has a higher permeability and can allow moisture vapor to pass through to the rim. XPS is far less permeable but not a perfect vapor barrier so blocks the warm air and allows a minimal amount of drying to the inside if moisture gets to the rim from any source.

But XPS may need to be covered with a fire rated barrier like drywall. By using the foil faced polyiso you may be able to sneak by without the drywall. Dow Thermax is the only foil faced one I know of that has been tested and approved to remain exposed.

That will get you started, I'll watch for more questions.

Bud
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Old 05-29-2020, 04:40 PM   #3
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Re: XPS, EPS, or Polyiso foam board?


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Originally Posted by Bud9051 View Post
One of the objectives of placing rigid foam against an exterior cold surface is to ensure warm inside air doesn't reach that sold surface. EPS has a higher permeability and can allow moisture vapor to pass through to the rim. XPS is far less permeable but not a perfect vapor barrier so blocks the warm air and allows a minimal amount of drying to the inside if moisture gets to the rim from any source.

But XPS may need to be covered with a fire rated barrier like drywall. By using the foil faced polyiso you may be able to sneak by without the drywall. Dow Thermax is the only foil faced one I know of that has been tested and approved to remain exposed.

That will get you started, I'll watch for more questions.

Bud

I figure cinder blocks allow some moisture in, so EPS would be the better option. I also do not like that XPS actually loses R value over time.

The R-tech EPS has a foil face, but fire rating isnt all that important in my case, the foam will always accompanied with rock wool and then drywall in the cinder block area.

So, I guess it comes down to Polyiso VS EPS
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Old 05-29-2020, 05:09 PM   #4
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Re: XPS, EPS, or Polyiso foam board?


IMO, whichever you choose will be a huge improvement. Picture frame each cavity with caulking to be sure all gaps and cracks are air sealed. Also seal the sill plate to the top of the foundation. The top block should have a solid top and not open cavities.

If you are going to finish those walls they need to be absolutely free of any moisture issues.

Bud
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Old 05-29-2020, 05:34 PM   #5
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Re: XPS, EPS, or Polyiso foam board?


Polyiso will also lose R value over time, is what I had read. Not sure I would consider the loss for either XPS or Polyiso very significant, however.

I used XPS with snug fit and caulked all around.

Thats great if you put Rockwool in front of the foam.
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Old 05-29-2020, 08:30 PM   #6
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Re: XPS, EPS, or Polyiso foam board?


I would still want to see any of the foams covered with fibrous insulation.
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Old 05-30-2020, 11:45 AM   #7
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Re: XPS, EPS, or Polyiso foam board?


https://www.buildingscience.com/docu...sheathing/view


I would pick xps. But that depends on your exterior finish. Vapor barriers or none. Although eps seems to have vapor permeability, depends on how much vapor from outside. As above article says, eps has slow air and vapor transmission, meaning not drying quickly enough if too much moisture. (For my sake of trying to keep things simple, I would not consider, again for myself, eps will help with keeping materials dry - no definitive sentences here, so depends on your understanding) Along the rim, xps for mostly air seal and moisture will dry to outside or absorbed by lumber. Along the foundation wall, also xps for slower permeability or 2" xps for a barrier. My thinking is if you seal the foam board to the foundation, you trap any moisture between the foundation and the board. If there's moisture, eps may absorb and hold it or leak it through.
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Old 05-30-2020, 11:54 AM   #8
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Re: XPS, EPS, or Polyiso foam board?


Just a quick comment for carp, Foundations don't have a problem with moisture and are exposed 24/7/365 from above grade to the soil beside them and below. If there is a water problem as opposed to a moisture vapor issue that should have been resolved before the wall was built.

Moisture between the foam board and the concrete can also dry to above grade or to the concrete floor below.

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Old 05-30-2020, 11:59 AM   #9
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Re: XPS, EPS, or Polyiso foam board?


Quote:
Originally Posted by carpdad View Post
https://www.buildingscience.com/docu...sheathing/view


I would pick xps. But that depends on your exterior finish. Vapor barriers or none. Although eps seems to have vapor permeability, depends on how much vapor from outside. As above article says, eps has slow air and vapor transmission, meaning not drying quickly enough if too much moisture. (For my sake of trying to keep things simple, I would not consider, again for myself, eps will help with keeping materials dry - no definitive sentences here, so depends on your understanding) Along the rim, xps for mostly air seal and moisture will dry to outside or absorbed by lumber. Along the foundation wall, also xps for slower permeability or 2" xps for a barrier. My thinking is if you seal the foam board to the foundation, you trap any moisture between the foundation and the board. If there's moisture, eps may absorb and hold it or leak it through.

I am using it for 3 applications: rim joists, basement cinder block walls, and second floor overhang. All applications will be accompanied with rock wool.

I have no water problems in any area.

I see it like this, the little bit of moisture that may transfer through the block or from the outside naturally, will be absorbed into the EPS, then hopefully stopped at the foil facing, and finally will be able to dry out quickly due to the nature of EPS. XPS and PolyIso will retain any moisture, basically forever. XPS and PolyIso also lose R value, where EPS will remain the same for the life of the product.

Am I wrong?

..also why spend more money, if I am going to lose R value of the more expensive products anyway..


EDIT: EPS also seems to be used in insulated concrete forms, so they must be doing something right

Last edited by mac1990; 05-30-2020 at 12:03 PM.
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Old 05-30-2020, 12:45 PM   #10
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Re: XPS, EPS, or Polyiso foam board?


3 applications require 3 answers - not necessarily the same. My previous comments were directed towards rim pockets.

Nothing wrong with your conclusion, in my opinion. But some of your thought process I don't agree with. With respect to the basement walls, you are concerned with moisture coming through the walls. Any half modern home would be code required to have a damp-proofing on the outside. The moisture path I would be more concerned with would be warm, humid interior air condensing on cold concrete blocks in the winter. But then again, if the EPS is foil or polymer faced, its going to be pretty close to a vapor barrier anyways.

You really didn't give enough information on the overhang application for me to make an intelligent comment on, one way or another. I don't think you posted location either, which commonly can effect the answers.

Keep in mind also, that although you don't have water intrusion problems now, that can change in the future. CMU basements are notorious for leaks as the structure ages. Countless threads on this forum. Not much you can do about it now, with insulation. but just don't expect a leak to dry up through the EPS.
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Old 05-30-2020, 12:53 PM   #11
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Re: XPS, EPS, or Polyiso foam board?


Quick comment. When rigid foam is used below grade it benefits from a huge amount of insulation value from the surrounding soil, thus any loss of r-value will have virtually no effect, this was mentioned before (?).

We can run the cost of that reduced r-value but I will need a bunch of information. My guess would be $5 a year. Wild guess.
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Old 05-30-2020, 03:18 PM   #12
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Re: XPS, EPS, or Polyiso foam board?


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Originally Posted by SPS-1 View Post
3 applications require 3 answers - not necessarily the same. My previous comments were directed towards rim pockets.

Nothing wrong with your conclusion, in my opinion. But some of your thought process I don't agree with. With respect to the basement walls, you are concerned with moisture coming through the walls. Any half modern home would be code required to have a damp-proofing on the outside. The moisture path I would be more concerned with would be warm, humid interior air condensing on cold concrete blocks in the winter. But then again, if the EPS is foil or polymer faced, its going to be pretty close to a vapor barrier anyways.

You really didn't give enough information on the overhang application for me to make an intelligent comment on, one way or another. I don't think you posted location either, which commonly can effect the answers.

Keep in mind also, that although you don't have water intrusion problems now, that can change in the future. CMU basements are notorious for leaks as the structure ages. Countless threads on this forum. Not much you can do about it now, with insulation. but just don't expect a leak to dry up through the EPS.

Sorry, I am in PA, and I have added that to my profile.

Here is a picture of that "house overhang I was talking about: https://imgur.com/a/KeuKaYS

My house was built in 1978. The only thing they put on the exterior cinder block below grade was tar.

As for water problems down the road, should be good to go, I have installed and am currently installing a significant french drain system.
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