Insulating Basement Before Finishing Half of It

8 years ago

Ok, looking for some advice here as I am clearly overthinking this. I am preparing to finish half of my 900 sq ft basement. Right now its a blank slate and I want to do this right the first time so looking for help. My main concern, even though the basement is dry all the time (in that no water pours in when it rains), is that basements are humid and I know that adding a heat source in part of the basement will create moisture. I want to avoid a situation where I create an environment for mold. Some background first:

• the house is 3 years old

• the basement is poured concrete

• we live in central Pennsylvania (temps tonight in the low teens and 0 forecasted this weekend). According to the IECC Climate Zone Map, we are Zone 5 and 4 marine.

• ceiling insulated with batts (R19 I think) between the joists

• the sill plate currently has batts cut to fit and jammed up against

My first task right off the bat is get rid of the fiberglass insulation on the rim joist as I've read that this can create the perfect environment for mold. I was going to cut rigid foam to fit and then fill gaps with Great Stuff.
Then I was going to use the right adhesive to attach 4x8 sheets of rigid foam to the poured concrete walls. Again, taping seams and filling gaps with Great Stuff to create a barrier stopping moisture from getting to the cold concrete wall. For the top of the wall (where the concrete is exposed because it is wider than the 2x6 plate), I was going to cut pieces to fit, almost like a cap. This way, no concrete is left exposed.

So, my questions:

1. Should I use XPS or Polyiso between the joists on the sill plate? I've read that Polyiso actually loses R-value the colder it gets...true?

2. What adhesive is the right adhesive?

3. If xps, is a 2 inch piece enough? In other words, is R10 going to cut it in our zone? Or would you add another inch?

4. What tape is the best to use on xps...if that's the material I should end up using?

5. If I am only finishing half the basement, should I only worry about the foam board on the walls that will be finished? 2 of my finished walls will be up against the concrete foundation, the other 2 studded out in the middle of the room. I am leaving space for storage. With the unfinished part being bare concrete walls, am I being inefficient and leaving myself open for cold getting in?

6. On the 2 walls that aren't against the foundation, are batts in between the studs enough to combat the situation above (unfinished part of basement not being insulated)?

7. Should I still insulate between the studs on the walls where the foam board is installed? Or is R-10 good enough?

8. Would you fill the inch or so space at the bottom of the wall (between the stud wall and the foam board) with Great Stuff to stop air/moisture from getting to the concrete foundation wall?

9. Thoughts on the concrete floor? Is a roll of foam (the stuff you'd put down under a laminate floor) enough for moisture? Or what is best practice there?

There's probably more, but that's it for now I suppose.

Thanks in advance for reading and for any input you can give.

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worthy
8 years ago
last modified: 8 years ago
How appropriate a topic as it's hitting -23˚ C. (-9.4˚ F.) tonight here in the banana belt of Canada!

1) If you choose polyisocyanurate, follow building experts' advice and consider the R Value as R5 per inch and increase accordingly. I always use XPS as it's much cheaper.

2) Adhesives are really designed for block walls, which are perfectly smooth. All the imperfections in a typical poured wall--especially those ridges where the forms were connected--make it virtually impossible to get a tight bond between the foam boards and the walls.

Instead use mechanical anchors designed for the purpose, such as Hilti plastic insulation anchors. Also, if you can find them, consider four foot wide XPS. And always use shiplap boards, which seal well even without tape.

3) R-10 is inadequate in climate Zone 5, where IECC 2012 calls for R15 continuous. You can reduce the cost by using 1-1.5" foam board on the walls followed by fg between the studs. EPS boards are acceptable, too, though they will be thicker to reach the same R Value.

4) Any tape approved for housewrap will work well. I use Tuck Building tape.

5-6) As long as the freestanding walls are airtight and insulated to Code, there's no need to insulate the unfinished walls.

7) See 3 above.

8) I would rest the stud wall on strips of XPS--tight to the foam wall if you wish. Besides acting as a thermal break, it keeps the wall above any minor flooding that might occur.

9) Foam cushion is not an acceptable sub-floor. It will absorb moisture and harbour mold growth. Unless you're using tile or stone, which can be laid directly--though an uncoupling membrane is advisable--consider using: 1) a layer of XPS followed by one or two layers of ply (as per diagram below); 2)Dri-Core or similar competitive products; or 3) DeltaFL or equivalent.
Source: Building Science Corp.
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Insulating Basement Before Finishing Half of It

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