Assistance with dew point calculations to determine basement insulation requirements
I’m looking for help to determine if I need to insulate my basement slab to combat condensation at any time during the year, and the minimum thickness foam board wall insulation I can utilize. I am not set on flooring yet. Deciding between 1) polished concrete, 2) LVP, 3) insulated slab w/ T&G subfloor if required.
Climate Zone 5A, 21 year old poured concrete foundation. 8ft ceilings, 1ft above grade. I don’t know if there is sub slab insulation, but I suspect not. I believe there is a vapor barrier under the slab (no slab moisture issues using tape down plastic test). The space is not heated or cooled aside from duct leakage. I mechanically dehumidify with a high efficiency dehumidifier to sub 50% year round. The air temperature in winter hovers between 66-69F, and in summer 70-72F.
I have been using a laser thermometer to measure slab and foundation temperatures throughout the winter and summer. In winter the temps range from 56-61F (coldest part being the foundation wall right around the window well), and in summer 70-72F.
I’ve been trying to use this calculator http://www.dpcalc.org/index.php to assist, but I’m not sure how to use the information. Is temperature the air temperature, then as long as concrete temp remains below dew point based on the current interior humidity, I will not get condensation? Example, if I put summer air temp 72F, humidity 50%, I get dew point 52F, which is well above the summer concrete temp of 70-72.
I realize that my slab temperatures are probably heavily influenced by the interior air temps because there is no insulation on the interior. But, if I leave the slab uninsulated with a polished floor, or LVP on the bare concrete, the slab temps should remain the same, and I should have no summer condensation issues as long as I continue to mechanical dehumidify, correct?
So questions:
1) Do I need to insulate the slab if I mechanically dehumidify?
2) Can I get away with 1.5″ polyiso on the walls and prevent condensation?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
While you don't think you are heating and cooling the basement, you are, it's connected to the rest of the house and leading to heat and cooling loss. Insulation will reduce that loss. Insulating the concrete will change its temperature. The above-ground portion will tend toward the outside temperature, and the below-ground portion will tend toward the ground temperature. Once you get more than a few feet below the surface the ground tends to stay near the same temperature year-round, at the average temperature for that locale. Where you are that's probably in the low 50's.
Yes I understand. So I assume 1.5-2" of polyiso on the walls will be more than enough to isolate the interior from the foundation walls and provide sufficient thermal and vapor break to prevent condensation since the walls will now be much cooler (low 50s at the base year round). And then if I leave the floor uninsulated and continue to dehumidify to sub 50%, the slab should remain at the current temperatures -- 56-61F winter, 70-72F summer -- which are above the dew points. Is this accurate?
>"So I assume 1.5-2" of polyiso on the walls will be more than enough to isolate the interior from the foundation walls and provide sufficient thermal and vapor break to prevent condensation since the walls will now be much cooler (low 50s at the base year round)."
In most of zone 5A the deep subsoil temperatures are in the low 50sF, but at shallower depths it can be both much warmer and much colder than that on a seasonal basis.
A couple inches of polyiso won't bring the foundation insulation up to current IRC code spec (3" would), but it would be enough to limit summertime moisture on the walls. The surface facing the room and the basement air will be several warmer on a year-round basis, including in the summer.
Insulating under (or on top of) the slab to R3-R4 (and inch of EPS) or better would be enough to manage humidity, moisture accumulation and mold at the floor level. Due to the hygroscopic nature of polyiso it's not advisable to use it for slab insulation but it's fine on walls, as long as the bottom edge isn't resting directly on the slab, where it can take on moisture even from concrete that is not visibly wet. As little as a half inch of air gap is plenty as a capillary break, and provides room for small amounts of potential bulk water seepage at the slab-wall seam. If the basement has a history of flooding, stop the polyiso a few inches above the high-tide mark, or it can become waterlogged in subsequent flooding events, taking forever to dry out.
>"And then if I leave the floor uninsulated and continue to dehumidify to sub 50%, the slab should remain at the current temperatures -- 56-61F winter, 70-72F summer -- which are above the dew points."
In most cases you won't need to dehumidify to 50% @ 65F (dew point =46F ) to keep the "musty basement smell" at bay, but even 50% won't always be enough to protect susceptible objects such as corrugated cardboard boxes from taking on moisture from ground water wicking. If the slab was poured on top of 6" of well drained washed screenings with a vapor barrier between the stone & slab you can probably get away with 60% RH @ 65F (dew point =51F), but if poured directly on clay soil with no vapor barrier all bets are off.
Here is some great advise: https://www.greenbuildingadvisor.com/article/installing-rigid-foam-concrete-slab
Your calculator (which I like, BTW), is giving you parameters for the air and surface. The first temperature on the left is the air temperature, the %RH is the humidity level in the air as a percent relative humidity, and the temperature on the right is the dew point for that air. The dew point is the temperature a SURFACE needs to be to cause moisture to condense out of the air assuming the air is the temperature and %RH you entered.
You need the dew point to always be BELOW the temperature of a surface if you do NOT want condensation to form. That means a dew point of 52F is fine if you have a surface temp of 70F, since 70F is well above 52F, so no condensation will form. I would expect an uninsulated concrete slab in concrete with grade below the frost depth for your area to remain above 50F all the time. That temperature will probably vary a little based on your indoor temperature, but I doubt you'd ever see colder than the low 50s in practice for the slab. Walls are going to be colder, and they'll be colder the higher up the wall you go since you have less ground between the wall and the outdoor air as you go higher up the wall. That's why it's more important to insulate the walls than the floor when the floor is down around frost depth.
1.5" of polyiso is around R9. Don't worry about derating for cold temperatures here, since polyiso on the interior of foundation walls does not see the low temperature extremes that polyiso used on the exterior walls of a house will. R9 is below code, but it will be plenty to prevent condensation. You do want to try to keep the indoor air away from the foundation wall though, so install the polyiso tight to the wall, and ideally tape the seams too.
I don't think you'll have any problems leaving the floor slab uninsulated here.
Bill