Insulating a raised floor over an old concrete slab
Hi, I have a house in Seattle. The back portion of the house has a 60 year old 600 square foot slab with approximately 2′ deep footers. The ground slopes gently away from the foundation, and the roof overhangs each side by about 2-3′. Let’s assume there is no gravel, sand, plastic, or insulation underneath the slab. There is no plastic or insulation on the exterior perimeter of the foundation either. The foundation perimeter vertically extends approximately 1/2″-1″ above the slab. While the perimeter is level, the concrete slab is uneven, hence the 1/2″-1″ inch variance.
The plan is to hang 2×10 joists (using ledger board and joist hangers) 1.5″ above the foundation perimeter so that the floor will be at the same level as the rest of the house. That will leave approximately 2″-2.5″ of unvented space underneath the joists. I’m thinking to lay 6 mil poly over the old slab, and then 2″ of XPS over the poly, underneath the joists. Then apply spray foam behind where the joists meet the walls, and any exposed concrete between the joists and XPS foam. Will that be sufficient to control moisture issues? Do I need to tape the poly securely? Will any air gaps (due to uneven slab) between the poly and the XPS be an issue? Are the empty spaces between the joist bays or small gaps between joist and XPS an issue?
Thank you!
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Replies
Jo,
Q. "Will the details I described be sufficient to control moisture issues?"
A. Probably -- but the answer depends on whether the slab has ever been damp, and how high the slab is with respect to the exterior grade. (The greater the height above grade, the better.)
Q. "Do I need to tape the poly securely?"
A. Yes, taping the polyethylene seams is a good idea.
Q. "Will any air gaps (due to uneven slab) between the poly and the XPS be an issue?"
A. No, as long as the XPS seams are sealed with high-quality tape.
Q. "Are the empty spaces between the joist bays or small gaps between joist and XPS an issue?"
No.
You didn't ask the question, "Is this the best approach?" but I'll answer it anyway. I'm not really a fan of inaccessible crawl spaces. Instead of installing joists, is there any chance that you could install 8 inches of EPS insulation above the existing slab, followed by a new 4-inch slab on top of the EPS?
Hi Martin, Thank you very much for the speedy reply.
To answer your questions:
This is slab on grade. The top of the perimeter of the foundation is about 6" above grade. So the top of the slab is about 4-5" above grade. We can regrade and increase height by a few inches this if you think it's necessary.
Yes the slab has been damp. We tore down the back portion of the house and the slab has been rained on all winter. Has the slab been damp before tearing down the structure? I'm not sure. The house was damp, but that could be because of the poor grading, and the poor construction - the sill plates were rotted through, the roof was leaking, etc. We actually added the foundation perimeter with 2' deep footers this spring to improve the foundation.
I know this isn't the best approach, but unfortunately, new framing has already been completed (except for the floor), so we can not install the 8 inches of EPS and 4-inch slab as you suggested.
Given the situation is as it is, and the information that I just added, do you think this is a sound plan? And would you suggest anything else along the lines of this approach? Thanks again!
If your plan is 2x10 joists with 3/4" subfloor on top, that's 10", plus at least 2 inches below them, that's a foot, which is exactly the same as Martin's suggestion of 8" of EPS plus a 4-inch slab. So that can work. If you need more room, you could reduce the 8" of EPS to 6" or whatever works. You'd avoid the inaccessible crawl space, get better insulation, and get an assembly that's less prone to moisture.
You could also skip the slab and put a sub-floor directly on top 8" thick or even thicker EPS.
Yes, I see how that could work. However, due to budget restraints it would be difficult to do 8" of EPS and a new slab at this point. I also already had all the lumber for the floor system delivered and cut. If I were to do this again, I probably would have ripped the slab out completely. At this point, I just want to know the best way to prevent moisture issues given that we will be going with the 2x10 joists over old slab.
Jo,
The fact that the slab has been damp is not encouraging. Lowering the exterior grade as much as possible (consistent with maintaining a slope the directs water away from your building) would certainly be advisable.
There are a lot of red flags here. If you really don't want to take my advice to install 8 inches of EPS plus a new slab, you might consider using closed-cell spray foam on the crawl space floor instead of XPS. The closed-cell spray foam will be a more effective air barrier and moisture barrier than the XPS.
Martin, that sounds like a great idea. If I apply 1.5"-2" of closed cell spray foam directly over the old slab (no poly), would you feel comfortable with this approach? I know we are cutting corners here. I will do what I can to lower the exterior grade as much as possible. Thanks again for taking the time.
Look at this article http://buildingscience.com/documents/insights/bsi-003-concrete-floor-problems -
You could apply a liquid vapor barrier directly to the concrete instead of the plastic sheet
Which would be the better option? Liquid vapor barrier + 2" XPS foam, or just 1.5-2" of closed-cell spray foam?
Also, which liquid vapor barriers would you recommend? That article is very informative, but it seems to be very particular in which type of epoxy would work.
Jo,
Q. "Which would be the better option? Liquid vapor barrier + 2" XPS foam, or just 1.5 to 2 inches of closed-cell spray foam?"
A. Between those two options, I would vote for the closed-cell spray foam.
Thank you guys for your advise!
Just an update and one more question. I've scheduled a contractor to do the 2" of spray foam over the concrete slab. Now as far as the exterior insulation goes - should the dirt around the foundation be dug up and 1" of foam board around the exterior perimeter be installed as well? How necessary is this step given the 2" of spray foam on the slab on grade? Would this be redundant?
Jo,
I advise you to have the spray foam contractor spray the crawl space walls (including the rim joists) at the same time that the foam is being sprayed on the slab. Consult your local building department or fire marshal to determine whether the cured spray foam needs to be protected by an ignition barrier (for example, an intumescent coating).
Sprayed 2lb foam on top of a slab seems like a waste, and it's pretty expensive and un-green compared to an EPDM vapor & moisture barrier + 2" of sheet EPS (not XPS). Using 2" spray polyurethane to insulated & seal the short amount of wall & band joist to the sill has merit though.
When it's in a dead space like this the subfloor above the joists serves as the ignition/thermal barrier for meeting fire code. If your inspector disagrees (unlikely), 3" of rock wool installed between the foam and presumed conditioned space makes it.
With 600' of 2" thick 2lb polyurethane you're looking at something between USD$1000-1500. With 2" of EPS it's more like $500, and the difference in price will pay for both the EPDM vapor barrier and most of the band joist/foundation sill sealing. At 2" you're looking at a bit better than R8 performance, which for a slab in your climate/location would be pretty much at the limits of financial rationality. The funds for more than R8 for slab foam would be better applied to rooftop PV (even in foggy-dew Seattle, if not Hoquiam or Kalaloch).
Both XPS and 2lb polyurethane are still pretty un-green in terms of the HFC blowing agents (with less than a handful of exceptions for 2lb spray foam). EPS is blown with low-impact pentane, most of which is recovered a the factory. As the HFCs leak out to do their environmental damage, over time (a few decades at most) performance drifts inexorably downward to roughly the same level as EPS of equal density and thickness. In 50 years XPS has effectively no performance advantage, 2lb polyurethane maybe a 10-15% advantage, not more.