Do *I* need basement slab insulation, zone 5?
Basement finishing question in regards to insulating my slab floor. 23 year old concrete foundation basement in zone 5 (Central Indiana), zero insulation inside the basement and do not think there is insulation under the slab. I believe my subsoil temps are around 53F (based on Google search).
I have an IR thermometer that I’ve used to measure slab temperatures during winter and summer. Summer interior basement air temps are usually around 68-70, with slab temps 64-70. In winter air temp is 62-64, with slab temps between 57-63.x.
Right now for example in late Jan, I have interior air temp 63.2, slab temp 57.8 near the base of a large egress window, ~60 near most edges of the slab, and 62-63.1 on more central areas of the slab.
Using this dew point calculator http://www.dpcalc.org/ I see in the winter there is no risk of condensation, as with a reasonable 68F air temp, evaporative humidification to 50% RH, dew would be only 49F, well below the minimum slab temp of 57-58 I see in winter.
Summer may be a different story. Currently I do have to mechanically dehumidify the space using a whole house style unit, otherwise humidity reaches upwards of 80+%; however, I would expect this to drop significantly once I insulate the walls with sealed rigid foam, meaning I could potentially stop using the dehumidifier. So, even if I had summer air temps of 68F, RH of 60%, dew point comes out to 54, which is well below the slab temps I see in summer, and even below the temps I see in winter!
What am I missing here? I realize the interior slab temp is influenced by the interior air temp, and the subsoil temp doesn’t change very much, but seems like my numbers point to not needing insulation.
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Replies
We built basements for hundreds of years without insulation so no it is no a necessary unless required by your local code.
Will insulation lower the risk of mold? YES.
Will insulation lower your fuel bills? YES.
Will you live long enough for the fuel saving to recover the cost of a new slab on top of new insulation? NO!!!
Walta
Basement slab insulation is mostly about comfort. An insulated slab with flooring above is way more comfortable especially to bare feet. The heat loss through the slab even in colder climates is pretty minimal.
Code requires R15 on your basement walls. Code doesn't seem (see all the articles about the somewhat ambigious wording) to require insulation under the floor slab if the slab is down deep at basement floor level.
If your local building people require some amount of floor insulation, then I'd put whatever they require. If they don't require anything, then it's really up to you. If you want your finished basement to have that cozy "indoor" feel though, then some amount of slab insulation is a good idea for comfort. Even 1" will make a noticeable difference.
Bill
Thanks for the responses everyone. I don't want to insulate the floor in order to maintain ceiling height, not have to deal with out of compliance stair riser height on the bottom step, cost, etc. I would only do so if the numbers point to the need for it based on a likelihood to generate condensation due to cool slab temps in the summer. Local code doesn't require it. Actually appears to only require R10 continuous insulation in basements, or R13 for batt for walls, per 2020 updates. Seems strange!
For climate zones 5 and above, you need R15 continuous insulation on the basement walls, or R19 with framing. I would go with the continuous insulation, which is better in a basement anyway. You're probably looking at an older code revision if you're seeing the lower numbers. There is a good table showing the insulation requirements over a number of code cycles here:
https://thehtrc.com/2020/insulation-requirements-2021-iecc
If you're in an area that is still on a very old code revision, then they might have a lower R value requirement.
Bill
The latest building code appears to be the 2020 Indiana Residential Building code, which still lists R10/R13 for basement walls, R-20/R-13 + 5 for above grade wood construction walls
The 2021 code shows that as OK for CZ4. Does Indiana treat the entire state that way, or do they have different requirements for the part of their state that is CZ4 vs the part that is CZ5? In the national code, the CZ4/CZ5 line is the place the requirement changes from R10 to R15 continuous insulation for basement and crawlspace walls.
There was a Minnesota study that pretty much showed that above R10 continuous insulation on walls, you're well into the point of diminishing returns, so it would make some sense if a state kept R10 as their requirement.
Bill
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