Basement insulation / wall framing with limited space
Hi. I’m working on a basement in a 1960s ranch in Minneapolis, all below grade, with a concrete block foundation. I’ve read pretty much all of the articles here about basement insulation, and I was set on doing the following: 2 inches of XPS glued to the block, then a 2×4 framed wall, and then either XPS or spray foam in the stud bays. HOWEVER…
One day I would like to turn this finished space into a bedroom. Thanks to the already-finished part of the basement (done by the previous owner in the 1970s), I am finishing a small space. Building code requires a bedroom to have a minimum size of 7 feet in any direction. Now, if I were to frame the wall like I had planned, my room would be about 3 inches too narrow. And since part of the basement is already finished, I can’t do much about it except re-think how to insulate my block wall. I have a total of 3 inches to work with for insulation and framing, but excluding drywall. Here’s some ideas that I was thinking about:
1. 0.5” XPS on block wall, with 2×3 framed wall, and 2” XPS filling stud cavities
2. 1” XPS on block wall, with 2x2s anchored to concrete, and then either 0.5” XPS or spray foam in stud cavities
3. 2” XPS on block wall, with 1×3 or 1×4 strapping anchored to concrete, drywall attached to strapping
I don’t need to run electrical or anything else through this wall, as I will have 3 other sides with regular 2×4 walls to do that. Are any of these options worthwhile?
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Of the three options you list, #3 is the best.
If you wanted to squeeze out a little more room, you can use one of the rigid foam products with routed channels designed for inset furring strips. Two examples are Owens Corning InsulPink and ABT Foam PreFur.
It's also possible to glue drywall directly to rigid foam, if you can come up with a way to apply even pressure to the drywall until the glue dries. (Use scraps of plywood and lengths of 2x lumber that extend to the opposite wall.)
That said, you have a bigger problem than figuring out how to insulate your wall. If your basement is "all below grade," as you explain, then a basement bedroom is illegal unless you can find a way to install a properly sized window for emergency egress. That type of window requires a very big window well, usually with masonry or concrete steps.
Hi Martin. Thanks for your response. In regards to the basement window, luckily there is already a small basement window in the space that I am finishing. Thus, I will enlarge it and make it a code-approved egress window in order to consider is a bedroom.
I've attached a drawing of what I'm planning to do. Yes, it will be a small room...but that's okay. I'm trying to minimize the tear-down of the already-finished portion while increasing the finished space in my house.
Martin, after thinking about this some more, I realized that I will have to go the route of gluing drywall to the foam itself. I can't acquire InsulPink (won't meet minimum order), and if I use 1x4 furring, then I won't be able to meet the code-minimum for basement wall insulation in my zone. I need R-15, which would be three inches of XPS. I only have three inches of room for the insulation/wall combo, so it will all be used up by the insulation. The only other option is using polyiso...but I have already purchased and brought home the XPS, so I kind of feel stuck with it.
I don't work for them, but I'm a very satisfied customer. I would take a close look at insofast and see if it meets your needs. Foam board with integrated plastic studs that glue to the wall. Key advantage is that the integrated plastic studs negate the need for a separate framed wall. You can either attach another layer of blue board for more insulation, or screw drywall right into the plastic studs. Easy do it yourself product.
Minnesota has some state-code specific restrictions on how foundations can be insulated, including requiring that a minimum of R10 be on the exterior side, extending to the footing or 10' below grade, whichever is less:
R402.2.8 Basement walls.
Walls associated with conditioned basements shall be insulated from the top of the basement wall down to 10 feet (3048 mm) below grade or to the top of the footing, whichever is less. Foundation insulation shall be installed according to the manufacturer’s installation instructions. Walls associated with unconditioned basements shall meet the requirements of this section unless the floor overhead is insulated in accordance with Sections R402.l.1 and R402.2.7 and the following requirements:
a. R-15 insulation for concrete and masonry foundations shall be installed according to R402.l.l.1 to R402.l.l.8 and a minimum of a R-10 shall be installed on the exterior of the wall. Interior insulation, other than closed cell spray foam, shall not exceed R-11. Foundations shall be waterproofed in accordance with the applicable provisions of the International Residential Code (IRC).
Exception: R-10 continuous insulation on the exterior of each foundation wall shall be permitted to comply with this code if the tested air leakage rate required in Section R402.4.1.2 does not exceed 2.6 air changes per hour and the total square feet between the finished grade and the top of each foundation wall does not exceed 1.5 multiplied by the total lineal feet of each foundation wall that encloses conditioned space. Interior insulation, other than closed cell spray foam, shall not exceed R-11. See footnote c to Table R402.2.l.
b. Minimum R-19 cavity insulation is required in wood foundation walls. See footnote 1 to Table R402.2.l.
Note that if there is any interior side insulation on CMU foundation, EVERY core must be set up to drain to an interior side drain system, per section R402.1.1.4.
None of the proposed insulating methods meet code. If you're going to do it any differently it's important to get sign off from the local code inspectors first, or you may be forced to rip it out and start over.
Digging down to the footing on the exterior may or may not be difficult or expensive. The below grade portion can be insulated with 4" of EPS for less money than 3" XPS, and you might step that back to 2" of XPS on the above grade part, with 1" polyiso on the interior side down to 5' below grade. That would allow the bottom of the CMU wall to drain & dry toward the interior, and should limit any code-officials concerns about frost heaving the footing. With 5' of concrete between the above grade portion and the uninsulated bottom portion it's effectively ~R5, and not a very big heat loss.
Code on a retrofit can be a gray area. They may accept an improvement even if it doesn't bring it all the way up to code. A conversation with your local official would be good to have sooner rather than later. You might learn that you have more leeway than you thought, or you might learn that you have less. Then if you come up against a specific challenge, that could be a good thing to discuss here.
Thanks for all of the advice. I did speak with my local building inspector this afternoon, and what he had to say is actually going to make things easier for me. Apparently, Minnesota does not enforce any energy code requirements for existing homes when it comes to finishing basements. He said that he could help point towards best practices, but that's it. No minimum R-value, no vapor barriers, no exterior grade insulation.
With that in mind, I still will want to design the room to have a 7-ft width in case I want to convert it to a bedroom (by adding egress window) in the future. My plan will be to use 2'' XPS on the block wall with 1x4 furring strips and drywall attached to the furring. Since this is only on one wall, I'm not worried about electrical.
For the slab floor, based on what I have read here, I will use EPS (less $$) and plywood (https://www.greenbuildingadvisor.com/content/eps-insulation-under-basement-floor).
Glad to hear that it won't be so difficult. I'd suggest other options vs. the XPS, but if you already have it ordered, it will work well there.
Hi Charlie, what would you recommend? I enjoy learning new things, so I'm just curious.
I'm guessing that Charlie's comment has to do with the fact that XPS is manufactured with a blowing agent that has a very high global warming potential. That's why green builders try to avoid the use of XPS. From an environmental perspective, either EPS or polyiso is preferable to XPS.
What Martin said. XPS is the least green foam currently on the market, and as it loses it's blowing agent over time doing it's climate damage it loses performance too, eventually settling in to the same R-value as EPS of the same thickness and density.
Even if you don't dig down to the footing on the exterior, with 2" of EPS or polyiso on the interior, fattening up the R on the exterior with 1.5-2" of EPS finished off with a cementicious EIFS such as QuiKrete Foam Coat down to at least a foot or two below grade will be worth it. The above-grade portion has by far the highest heat loss, and is well worth the bringing the above grade portion up to code min levels.
At 2" Type-II EPS runs R8.4, at 1.5" it's R6.3. So with 2" of interior side EPS and 1.5" of exterior above-grade EPS the above grade portion that counts the most would be R14.7, nearly R15 from a labeled perspective, but since EPS gains performance as it's tempearture drops, it would actually exceed R15 during the winter months.
At 2" polyiso would be labeled R12-R13, but it loses some performance at lower temps, and would have mid-winter performance on the above-grade portion of about R9 if it were the only insulation in the stackup But with as little as 1.5" of EPS on the exterior side the polyiso performance would average R10-R11, being kept a bit warmer, and closer to it's higher performance temperature range.
With 2" + 2" EPS you get a modest benefit from the thermal mass of the CMU, and it would be above code-min R even without counting the thermal mass.
Another option is to attach 1.5" XPS to wall with 1" strapping (0.75" thick) and then fill in between the strapping with a layer of 3/4" XPS. Drywall on top of that -- in effect a 2.25" foam layer with inset strapping. Plus maybe some extra benefit from the staggered overlapping seams on the foam layers.
I appreciate everyone's advice. I think that I have the wall insulation/framing figured out. I didn't want to start another thread, but I did want to ask about egress windows when it comes to this bedroom.
Currently there is the standard 30'' wide basement window. I understand the code requirements for egress windows and wells, so I was planning on just lengthening the current window opening in the concrete block foundation. By doing so, I will meet the requirements for the finished sill above the floor as well as the well height below the outside grade.
My question is regarding drainage. I have no interior sump pump or drain. I am assuming that I have no exterior drain as well since my house was built in the 1960s. I do have good gutters, a decent roof overhang (2 ft), and good exterior grading away from the house. Even if I wanted to run a drain pipe out of the window well, I don't think I would get very far due to about 4 very old silver maple trees with deep roots. Is having a well drain a "must"? If not, what precautions could I take to make sure water stays out of my basement?