GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Basement wall insulation in Climate Zone 5

Arthur Sperry | Posted in Energy Efficiency and Durability on

Hello,

I am hoping someone can help me determine the correct way to insulate my basement. After reading many forums, completing a lot of research, and even contacting the builder of my home I am confused more than ever.

I’m located in central Illinois zone 5. The front of basement is fully below grade with poured concrete foundation. The back of the basement has daylight windows with standard 2×6 framing on top of a 3′ foot foundation wall. The 2×6 framing is currently filled with unfaced fiberglass and poly on the interior side. The home was purchased new and was built late 2011.

I contacted the builder and verified that the foundation is not insulated; however he did say they used a vapor barrier on the exterior of the foundation walls. He advised me that when they finish basements in homes almost exactly like mine they do not use foam on the concrete, frame 2×4 walls in front of the concrete and fill them with fiberglass but do not allow the fg to contact the concrete. The explanation for no foam on the concrete was to prevent moisture from getting trapped between the foam and the concrete which could cause mold issues. This advice seems to go against everything I have read so far.

My plan based on much research is to use 2” EPS or XPS adhered to the concrete and continued into the box sills, seal all seams, frame a 2×4 wall tight to the foam, add fg or roxul in the stud bays, no poly, and finish with 1/2 drywall.

Is it the general consensus to use this approach rather than my builders suggestion?

Can I use regular 2” EPS instead of 2” XPS? The EPS is going for $10/sheet vs. $25/sheet for XPS at the local big box store. I’ve read that EPS is actually more environmentally friendly and will not lose R-Value as opposed to XPS, but is that all EPS? Are there different types? The cost savings of EPS over the XPS seems like a no brainer to me to use EPS. The guy at the box store even questioned my approach with using EPS and suggested ‘the pink stuff’ (XPS).

As for the above grade section of the basement I plan on removing the fg and poly. Filling those stud bays with the 2” EPS foam sprayed around the edges, puting the fg back in front of the EPS, no poly, and same 1/2 drywall. Does this sound correct? If I frame the wall in front of the short foundation wall all the way up to the ceiling to avoid the ‘ledge’ on the wall how should I fill the void created between the 2×6 framing and the framed 2×4 wall from floor to ceiling? The edge on top of the short foundation wall is about 4 inches.

Sorry for so many questions. I’ve just ran into a wall (pun intended) on this and really want to get this right and start building asap.

Thanks in advance for any help.

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. GBA Editor
    Martin Holladay | | #1

    Arthur,
    First of all, if you haven't seen it yet, I suggest that you read this article: How to Insulate a Basement Wall.

    Q. "My plan based on much research is to use 2 in. EPS or XPS adhered to the concrete and continued into the box sills, seal all seams, frame a 2x4 wall tight to the foam, add fiberglass or Roxul in the stud bays, no poly, and finish with 1/2 in. drywall. Is it the general consensus to use this approach?"

    A. Yes, although the layer of fiberglass or Roxul is optional. Some people use 100% rigid foam and leave the stud bays empty (my preferred approach).

    Q "Can I use regular 2 in. EPS instead of 2 in. XPS?"

    A. Yes.

    Q. "I've read that EPS is actually more environmentally friendly and will not lose R-Value as opposed to XPS, but is that all EPS?"

    A. Yes, it's all EPS.

    Q. "Are there different types?"

    A. Yes. For more information on this issue, see Choosing Rigid Foam.

    Q. "As for the above-grade section of the basement, I plan on removing the fiberglass and poly. Filling those stud bays with the 2 in. EPS foam sprayed around the edges, putting the fiberglass back in front of the EPS, no poly, and same 1/2 in. drywall. Does this sound correct?"

    A That will work. It's a variation on the cut-and-cobble method. For more information, see Cut-and-Cobble Insulation.

    Q. "If I frame the wall in front of the short foundation wall all the way up to the ceiling to avoid the 'ledge' on the wall, how should I fill the void created between the 2x6 framing and the framed 2x4 wall from floor to ceiling?"

    A. You can fill the void with fiberglass or mineral wool batts, as long as you pay attention to the ratio between the R-value of the rigid foam layer (the cut-and-cobble layer) and the fluffy insulation layer. For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.

  2. Arthur Sperry | | #2

    Thank you for such a quick response.

    I had read the insulating a basement article, which is what I was basing my approach off of. I guess I got confused after my builder's advice and mostly about EPS vs. XPS. I thought possibly the EPS at my local box store might be the wrong type. The difference in price between XPS and EPS makes me wonder why anyone would use XPS.

    I'll need to add the fg/roxul to obtain at least R-15. Is 2'' thick EPS the minimum I should use? I wouldn't be better off with say for example 1'' XPS which is around same price as 2'' EPS would I?

    Is the builders caution about moisture between the cement and foam a concern?

    Again thank you so much for your responses.

  3. GBA Editor
    Martin Holladay | | #3

    Arthur,
    Q. "Is 2-inch-thick EPS the minimum I should use? I wouldn't be better off with say for example 1-inch XPS, which is around same price as 2-inch EPS, would I?"

    A. A conservative approach is to follow the guidelines in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing. If you live in Climate Zone 5, and you plan to install a 2x4 wall on the interior side of the rigid foam, then the rigid foam layer needs to have a minimum R-value of R-5. You can do that with EPS that is at least 1.5 inch thick, or with XPS that is at least 1 inch thick.

    As I wrote in my first response, thicker fluffy insulation requires thicker rigid foam. The ratios are explained in the article titled Combining Exterior Rigid Foam With Fluffy Insulation.

    Q. "Is the builder's caution about moisture between the cement and foam a concern?"

    A. The answer can be found in another article that I urged you to read, How to Insulate a Basement Wall. The basic story is that you can't add insulation to a concrete wall until you are sure that the wall is dry enough to proceed. In my article, I wrote:

    "If you want to insulate an existing basement, you’ll probably be working from the interior. Before installing a layer of foam insulation on an existing wall, the first step is to verify that the basement wall doesn’t have a water-entry problem.

    "Diagnosing and fixing water-entry problems in existing basements is a big topic in its own right, and is beyond the scope of this article. Suffice it to say that if your basement walls get wet every spring or every time you get a heavy rain, the walls should not be insulated until the water-entry problem is solved.

    "Among the possible solutions to this problem:
    Adjusting the grade around your house so that the soil slopes away from the building on all four sides;
    Installing roof gutters connected to conductor pipes that convey the roof water away from the foundation;
    Excavating the exterior of your foundation and installing new footing drains leading to daylight;
    Installing an interior French drain around the perimeter of your basement and connecting the drain to a sump equipped with a sump pump; and
    Installing a layer of dimple mat against the basement walls before insulating.

    "For more information on these issues, see Fixing a Wet Basement."

    In the article mentioned at the end of this section (Fixing a Wet Basement), I provide the following advice:

    "Maybe you are ready to insulate your basement walls, but you just aren’t sure if they are dry enough for you to proceed. Is there any way to test them?

    "The traditional test is simple: tape a piece of polyethylene measuring about 12 inches square to your concrete wall or concrete slab. (If you’re testing your walls, it’s good to test them in multiple locations.) After 24 hours, if you see water drops under the plastic, that indicates that your walls are too wet to insulate. If you see water drops on the interior side of the plastic, it means that your indoor air is humid, and the concrete is cold. (Fortunately, that problem won’t prevent you from insulating. After all, a layer of rigid foam will eliminate the cold surface. And the high indoor humidity can be addressed by air sealing your home’s thermal envelope, or — in a pinch — with a dehumidifier.)"

  4. Expert Member
    Dana Dorsett | | #4

    A few comments:

    *If the sill gasket between the top of the foundation and the 2x6 studwall is something better than the cheap foamy stuff the concrete doesn't need to dry toward the interior.

    *If the above-grade exposure of the foundation doesn't have a moisture barrier on it, the below grade portion doesn't need to dry toward the interior.

    *If the footing is dry, or there is a capillary break between the footing and stemwall, the foundation doesn't need to dry toward the interior.

    If I had to guess, I'd hazard that the sill gasket is a cheap foamy, there is no capillary break between the footing and stemwall, but that there is sufficient exterior exposure on the concrete that it will stay dry enough to protect the foundation sill, even with foil-faced foam on the interior face of the foundation.

    Assuming there isn't an adequate capillary break at the foundation sill, such as an EPDM sill gasket, and you're still concerned about keeping it drier, use only UNFACED foam on the foundation (or at worst fiber-faced roofing polyiso) which would lower the average moisture level at the foundation sill relative to foam with true vapor barrier type facers.

    Even though 1" of XPS would be sufficient from a labeled R point of view, from a long term performance point of view assume it's actually R4.2/inch, not R5.

    Cheaper than box-store foam, try digging up some reclaimed roofing foam. eg: These folks in St. Louis are sitting on 26 sheets of used 3.5" fiber faced polyiso for $25/sheet (which is on the high side for used-foam- 3.5" reclaimed polyiso regularly trades for under $20/sheet in my neighborhood, often under $15/sheet), enough to do 100 linear feet of foundation perimeter 8' high.

    http://stlouis.craigslist.org/mat/5718259366.html

    Even derated to R5/inch that would meet code, and it could be held in place with 1x strapping through-screwed to the foundation with masonry screws, and you wouldn't incur the expense of an insulated studwall. Even with the furring it would only take up 4.25" of space between the drywall & foundation, which is thinner than a moisture-safe foam + insulated studwall solution.

    If you have time to wait for the right deal, these come up regularly. There aren't currently any listed in the Peoria or Springfield craigslist pages. If it's time-critical nationwidefoam.com has midwestern depots, and could quote you on whatever they have in immediate stock + shipping to your site, usually for well under box-store pricing (usually ~25% of retail for the foam, plus shipping).

    If polyiso, put an inch of EPS between the cut bottom edge of the foam and the slab (or leave a 1/2" air gap) as a capillary break, to prevent any ground moisture present in the slab from wicking into the foam.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |