Why are CMUs ( Concrete masonry unit, aka block wall ) bad for basments?
steve2021_5a
| Posted in Energy Efficiency and Durability on
I see lots of references as to ICF being superior to CMU basements, but not a lot on *why* that is so. My potential builder is much more familiar with CMU, it would be his first ICF basement. Is there an article that goes over the actual downsides of a CMU basement, and mitigations of those downsides?
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My 2 cents.
CMU is not my favourite because there are a million joints. I've seen mortar in CMU joints erode, causing water ingress, or enough join failure that the wall fails altogether.
I suppose with modern waterproofing, this is less of a concern as hydrostatic pressure 'shouldn't' be present to erode the joints.
The best way to do them is to have lots of rebar and fill them solid. To which point, you might as well form and pour concrete. To which point, you might as well use ICF, kill two birds with one stone by saving the insulation step.
Also ICF tends to use less concrete than a solid poured wall. I think because they tend to spec more rebar.
ICF is scary to most because it's whacky. But it's an interesting product in its concept. I just wish it was cheaper or didn't use styrofoam altogether .
In rural areas that predate building code compliance or locations where indifference to any code, along with lack of engineering foresight and inspection remain problematic, CMU basements and crawls are the norm.
I think the higher rate of failure of a CMU foundation compared to a poured wall is mostly in the prep work and execution of workmanship, or lack there of.
Two things come to mind, you can't put horizontal rebar in a block wall and there's plenty of room to stuff empty cans of beer, as I have demo'd plenty to find pencils, screw drivers and little bottles of booze entombed without a trace of concrete to fill the voids.
If you can find a sober mason, I think a CMU wall would be fine.
The way I see it is comparing ICF and CMU is like comparing apples to oranges.
CMUs are fast low cost and DIY friendly but they also make for weak walls that tend to leak easily and are recognized by the public as inferior product that is seen as a mark of poor workmanship. Maybe that is a regional thing.
Seems to my you are overlooking the most common option of the poured concrete wall.
The ICF has been around for some time and it looks like the market has decided that it high costs exceed its preserved benefits and seem unlikely to become the new standard wall.
The fact that the builder of your choice likes CMU so much is disconcerting.
Walta
I had not asked about a poured concrete wall, had only see that ICF was the bee's knees. I will ask about a poured wall. Anticipating a 10 foot deep walkout ( and all drains to daylight ) basement based on lot slope.
It is in a rural area, where CMU seems to be the norm
Steve,
I agree with Walta. The apt comparisons are probably CMU vs poured concrete, and ICF vs applied foam board.
I can see why your contractor would be reticent to form a wall with ICFs if they had no experience doing so. What is their alternative plan for insulating the foundation if you did go with CMUs? What are the apples to apples choices you are being offered?
the plan in a brief conversation was a CMU wall with sheet foam inside and out, as well as waterproofing. Not sure of soil expansiveness, but a test hole with a backhoe to check for deep rock ( did not hit any ) was red clay near the surface then more yellowish clay deeper
I think lot of it depends on where you are at. If you have expansive soil, block wall would be a bad idea.
If not the big downside is water leaks which is easy to deal with if you go with proper exterior water proofing (dimple mat plus peel and stick membrane).
Another option for your builder that is close to block wall is composite ICF such as Faswall. These don't have any of the foam issues you can have with EPS ICF.
I'm in the imagineering stage of my backyard garage/studio build and keep going back and forth between the two. I can't get a concrete truck back there so regular foundation is off the table which at least make some of the choices easier.
why not just use a pump?
About 400' which is possible but I have not priced it out. Block foundation is common for these here, so that is probably what it will end up anyways.
From what I've seen, ICF walls are hard to brace well and are prone to blowouts during a pour. I'm sure there are ways to address both of those but they aren't standard practice for masons or concrete contractors.
Concrete pumps are expensive and you need a lot of people on site during the pour.
Setting CMUs is something a single person can do over time, little equipment is required, zero risk of blowout, no stress of managing multiple people and worrying whether everyone is going to show up at the right time.
The downsides of CMUs are that they are not inherently as strong as cast-in-place concrete or as water resistant, but those can both be easily addressed with additional products.
An advantage of CMUs is that some are available with low-global-warming-potential concrete, which in theory can also be available from batch plants but it's very rare.