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I recently pulled off old wood paneling in my basement, and would like to insulate it using rigid foam (silverboard or durofoam). Behind the panelling, there is tar paper, and 1x2" studs approximately every 1.5 feet. I see the tar paper as extra protection/moisture barrier, and am inclined to install the rigid foam over top. I have gotten such a range of advice on this...

Option 1) keep tar and studs, fasten rigid board on top leaving a 1-inch air gap between the wall and foam - and build a frame in front of foam for drywall.

Option 2) remove tar paper and all studs, adhere foam directly to cement wall and build out from there.

Option 3) remove studs, adhere foam to tar paper directly.

Option 4) cut foam to fit between existing studs and use tuck tape and expandable foam to fill in spots.

Does anyone have experience with this type of situation? The home is 65 years old, and we live in zone 6 for weather. Spray foam is not an option for us. Thank you!

Dish
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  • Is it really a 1x2 or could it be a 2x4? A 2x4 measures 1.5" x 3.5". A real 1x2 is simply going to be a nailing surface and can be removed to get a continuous insulation barrier. – BMitch Feb 22 '16 at 15:38
  • True size isn't 1x2, but that's what they called them in the store... They're basically furring strips along the tar paper used to hold the paper in place, and as a surface to nail the old wood panelling. – Dish Feb 22 '16 at 15:51

3 Answers3

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Building Science Corporation recommends the following model these days for basement insulation:

  • rigid foam (or spray foam) directly against wall
  • stud wall (I prefer steel...easier to put up, no mold substrate, really lightweight)
  • finished wall (I prefer paperless sheetrock again, to avoid having a place for mold to grow)

Note the absence of any vapor barrier...of which tarpaper could be considered one.

The foam board acts as a vapor retarder so it slows moisture movement, but doesn't completely prevent it in case one side has to dry to the other.

For more details, see my answer on this question: Should I use steel or wood studs for basement exterior walls?

Community
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DA01
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Option 2 is the best or only sound advice. Here's a class on what's been determined. You can skip the "adhering" of the foam boards & have the stud walls hold it in place without issue.

If building the stud walls in place you'd just temporarily tack the wall's top plate in to hold the foam boards for you while you tape &/or spray foam seams & gaps. If prefabricating the stud walls to be tipped up into place, then the foam boards can be sparingly screwed to the back of the stud wall & then seams can be spray foamed & taped from the back.

Iggy
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  • Thanks for your reply - so no value in your opinion to keep the tar paper for any extra insulation value it could add? Would you recommend 1, 1.5 or 2 inch of the board? – Dish Feb 22 '16 at 15:52
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    Yep, it's been determined than any air space is therefore a humidity or mold space. Not having insulation before meant everything could breathe itself dry. The tar paper, though tarred, would still retain moisture & doesn't have any insulation value of note. The more insulation the better, more means less transfer of everything. The walls can then be constructed with 2x3's or even 2x2's. 2x2's can have splitting issues though & it would be better to use them (2x2's) just for the top & bottom plates & go with 2x3's or 2x4's laid flat on the foam. – Iggy Feb 22 '16 at 16:04
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    According to the link you provided, the foam should be sealed to the concrete at top and bottom as well as all vertical seams. – bib Feb 22 '16 at 17:11
  • Yeah, the best video I've run across. That's why I said spray can foam & tape any gaps & seams. It can't be perfect in any situation & even using contact cement won't get any wall 100% air-tight. Doing caulk adhesives will create the most gaps & attaching by screws or anchors into the walls increases all transfers. I actually avoid insulating & finishing basement walls to just do insulated & air-tight ceilings where it won't be living space per se. That's where the most benefits are noticed & you'll never have a mold, rot or flood problem so comfort is never interrupted. – Iggy Feb 22 '16 at 17:26
  • What's your take on something like EPS that has a thermal coating (Silverboard or Durofoam Plus), versus XPS with no thermal coating (like Formular by Owens Corning)? – Dish Feb 23 '16 at 02:19
  • The EPS is quite good & you want the foil facing into the room. I just got screwed at Lowe's with a few sheets of the 2-inch stuff, twice the price of Home Depot & the same cost as HD's much better Poly-iso. But, they completely killed the cold convection breeze completely & the wall's now warm to the touch. XPS I'd stay away from for a basement, it gets & stays quite wet in just that area. The Poly-iso (yellow/tan stuff) with a foil is the best for the money & the most water resistant, the step above the EPS. But, you'll be plenty happy with the 2" EPS at $15 a sheet instead of $30 a sheet. – Iggy Feb 23 '16 at 02:37
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Given that this is just nailing strips for the prior wood paneling, there's no reason to try insulating around them. Pull them off and make a continuous insulation barrier for the best efficiency, or frame up new walls and insulate between them if space is a large concern. The tar paper is keeping moisture off of the wood nailing strips, so there's no need for it with the foam insulation. As DA01 points out in the comments, the foam board is only a vapor retarder, and I wouldn't suggest the only vapor barrier being on the outside wall since this can result in condensation problems. So of all the options presented, it looks like option 2 is the best.

Assuming you are going to frame a new wood wall in front of the insulation, do take care to separate the wood base plate from the concrete floor. This can be done by extending a moisture barrier from up the wall 6" or so, out to the floor and under where your new wall will be located. There are also foam gasket products that you can buy in roles to install under the base plate.

BMitch
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  • I don't know so am asking: would tarpaper be a substrate that mold could grow on? If so, I'd want to remove that before putting up the insulation boards. – DA01 Feb 22 '16 at 16:15
  • Also, I have to downvote for suggesting a vapor barrier. These are not recommended anymore in basements. See this question: http://diy.stackexchange.com/questions/8636/should-i-use-steel-or-wood-studs-for-basement-exterior-walls – DA01 Feb 22 '16 at 16:17
  • The tar paper is simply a moisture barrier, though not a perfect one and I expect some vapor can get through any seams. Foam insulation is also going to be a moisture barrier so you don't need to worry about it like you would fiberglass. If the OP has the option to dig up the foundation, install exterior insulation, a moisture barrier, and a weeping tile system to remove any moisture there, as the other answer suggests, that would be idea, but I don't believe it's practical here. – BMitch Feb 22 '16 at 16:42
  • With the foam board, you'd skip any additional vapor barrier inside, so only a single vapor barrier. My suggestion is to extend that under the bottom plate to avoid having capillary action rot it out from below. – BMitch Feb 22 '16 at 16:43
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    Oh! I see. Minor detail...foam board isn't considered a vapor barrier, but rather a vapor retarder. Semantics, perhaps, but to be clear, there still wouldn't be an actual vapor *barrier* involved. – DA01 Feb 22 '16 at 17:03
  • BMitch - can you explain what you mean by capillary action rot from below? That's not a term I've encountered yet.. Thank you! – Dish Feb 22 '16 at 17:10
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    Concrete is porous, think of it like a sponge, and it will get moisture inside of it despite your best efforts. That moisture can be wicked into any wood in direct contact with it, so you want to separate it somehow. DA01's advice is good, I'm just giving the alternative with wood framing since it's more common. There's lots of options to separate the wood framing from the concrete, including a thin foam gasket that comes in a large role, – BMitch Feb 22 '16 at 17:15
  • Thanks for the help this has cleared up a lot of my concerns. One other question, I was wondering if i should use a EPS that has a thermal coating (like Silverboard), (Durofoam Plus) or a XPS with no thermal coating (like Formular by Owens Corning)? – Dish Feb 22 '16 at 22:33
  • I don't know if there's enough of a benefit for basements to justify any extra cost of the thermal coating. I'd typically think of the silverboard in locations that would have a lot of radiant heat, like attics and or on the opposite side of any heat source like in floor heating. Many insulation decisions come to price per Rs in an inch, so the thermal coating will give you more Rs in the same space giving you a bit more floor space. – BMitch Feb 22 '16 at 23:30