What would you consider best practice details when using CMUs for “Pretty Good Basement”?
We’re formulating plans for the possible build of a small “Pretty Good House” this coming year. At this point we’re considering a basement. Locally almost all of the basements are laid up CMu’s by experienced block masons. I’m not doubting their skill but wouldn’t be surprised if their knowledge of waterproofing and insulation might be a bit behind the times. So, if you were have a basement constructed this way, I’m interested in thoughts regarding waterproofing, insulation type, quantity and placement, subfooting/basement slab drainage and handing water against the outer wall. To add to the possibilities, there is a Superior wall plant nearby as well as a source of ICF materials.
Thanks for any and all thoughts on the topic.
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sciguyh,
I can’t think of any differences between the way you would insulate and manage moisture with CMUs or a poured concrete basement.
Thanks Malcolm. That being the case, the local masons parge the outer surface of the wall then coat with asphaltic emulsion then attach 2" of extruded polystyrene which is parged above grade. I'm wondering if interior insulation makes more sense or a combination of inside and outside. How about dimpled drainage mat on the exterior?
Section 406.4 of the IRC deals with different ways to waterproof or dampproof depending on the conditions where you are building. https://codes.iccsafe.org/content/IRC2021P2/chapter-4-foundations#IRC2021P2_Pt03_Ch04_SecR406 If only damp proofing is required where you are (which sounds like the case according to how your local masons dampproof typical basement walls but often this can differ from site to site even locally) but you want to be extra cautions with regards to water you could always follow the waterproofing procedures. From my understanding exterior insulation is the preferred method using EPS (I've read EPS is more environmentally friendly and doesn't lose it's R-value over time) in order to avoid possible condensation in the insulation if you insulate on the interior. Chapter 4 of the IECC will give you options of different ways to insulate based on your climate zone. https://codes.iccsafe.org/content/IECC2021P1/chapter-4-re-residential-energy-efficiency Typically if you follow all of the codes you'll end up with a pretty good building. But if you can do better, why not.
You'll also want to consider whether or not you need a radon gas system whether passive or active. If you have the slope to get foundation drainage to daylight I would do that whether your site has water issues or not. If I didn't have the slope I would drain it to a sump. Personally I would water proof the basement walls as well in lieu of damp proofing but These are areas I've seen issues so I prefer to be cautious.
sciguyh,
There are pros and cons to both insulation locations. I would not do a hybrid approach with some in and some out, as that comes with the complications and costs of each with no real advantage over either. Because remediation of water infiltration is so difficult after the fact, I would use a dimple matt over just damp proofing.
https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall
https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall
We use block foundations for a lot of additions and the early ones we did often cracked at the mortar joints causing leaks, but that was because the masons did not rebar and grout the cores, they just dumped all their block cuts and chips into the cores. Now we have the masons drop a 1/2" or 3/8" rebar into every other core and then grout the cores solid. And no dropping their block cuts and chips in the cores either. Its been a struggle to get them to do it this way even when its in the written subcontract and I tell them on site, but no more cracked foundations. There are many options for exterior insulation, drainage planes and water proofing, but we parge the outside and then have it water proofed and insulated by a subcontractor that specializes in that.
I would vote for the exterior insulation option especially if the norm. Make sure they carry it up all the way to the bottom of your siding not just to grade. You should not have any exposed CMU visible anywhere.
If you can get the exterior insulation thick enough to meet code, you won't need to do anything on the inside and finishing the space is simply painting the CMU. No need for studs or drywall.
One suggestion would be to use a membrane over the CMU as your water barrier, this way if (when) the CMU cracks, it won't leak.
Make sure the EPS is grooved for drainage or installed with trowled on adhesive with vertical grooves. EIFS suppliers a good source for drainable EPS. If not an option dimple mat under the EPS.
Another important detail is your drain should be beside the footing not on top of it. The peel and stick should go all the way down along the footing and over the edge a bit this way water can't get in the joint between the CMU and the footing.
If you run your exterior foundation insulation right up to the siding you are creating a potential path for termites and carpenter ants so you will want to address that with a a termite shield at the top of the exterior insulation. Termites are another reason to fill the block cores solid, in my own house that was built in 1951 with hollow core block, termites got inside the block below grade and moved up to the plate and into the center beam supporting the first floor joists, we had to replace a 20' section of triple 2x10 due to the termite damage.