Insulating Basement with Poly Over Sill Plate
I’m in the process of finishing my basement, and trying to figure out if I should remove poly sheeting before adding insulation. I currently have 6mill poly sheeting running from sill plate down to a drain tile in the floor around the entire basement, similar to the attached photo except no stud wall built yet. Purpose of the poly is to channel water down wall and into drain tile. I’m planning to insulate according to the https://www.buildingscience.com/documents/information-sheets/basement-insulation article, but it makes clear that “No interior vapor barrier” should be used to allow for drying to the inside.
I’ve got 2″ xps that’s ready to install over the poly, but thinking I should remove the poly sheeting first. Will I have issues if I leave the poly up? Seems like the interior surface of the XPS should be warm enough to avoid condensation. I’m in NY climate zone 5.
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That poly between the block and the back of your foam insulation isn’t an “interior vapor barrier” in your application. What would be a problem here is if your wall was block, foam, batts, poly, drywall, in that order. The poly is now on the interior side of the stack up, and could potentially trap moisture in the wall.
There is no need for the masonry foundation wall to dry. Your rigid foam would count as a vapor barrier there anyway. I would leave the poly in place, but keep in mind that with it pressed against the wall, it won’t do much to help with drainage. You would need a dimple mat here to help with drainage if that’s important.
Bill
Short answer: assuming that the polyethylene on the wall is doing its job correctly, and draining water into the below-slab gravel, leave it in place, cover it with closed-cell spray foam (or XPS, and don't forget to air seal and tape the seams), and call it done.
Longer answer: I just responded to comments saying that "we can't use a vapor barrier in foundation insulation; keep it 1 perm or higher"--check out that discussion here:
https://www.greenbuildingadvisor.com/question/basement-vapor-barrier#comment-212813
But what you have is basically an interior drained system: it intrinsically involves a Class I (0.1 perm or less) drainage layer, to direct bulk water into the below-slab gravel. These types are assemblies are shown in the links below:
BSI-041: Rubble Foundations
https://www.buildingscience.com/documents/insights/bsi-041-rubble-foundations
BA-1108: Hybrid Foundation Insulation Retrofits: Measure Guidelines
https://www.buildingscience.com/documents/bareports/ba-1108-hybrid-foundations-retrofits-measure-guideline/view
Also, what you're showing is similar to the AVID system shown in BA-1108 (see 3.3 Building Envelope Services, Inc. (2008) AVID System, page 19).
Thanks for your insightful replies! Your comments and the linked documents support my design and give me some assurance I'll be successful. In the 10 years I've lived in this house, I've only had small amounts water coming through the walls (weeping) twice and both were due to clogged gutters in a downpour. I'm now better maintain the gutters, but as a precaution I installed a drain tile and the poly as part of finishing the space since I was replacing the basement floor anyway.
I've attached my wall assembly plan below and some in-process build pics in case others are looking for a similar solution. All XPS joints will get taped or caulked, and the interior wall cavities will not be insulated.
thanks for this detailed explanation of what you did. How is it working out? How did you attach the foam board to the wall? I'm worried about comprimisng the poly? Did you put the foam board in front of the dimple board?
We’ve been enjoying the finished basement for a couple years now and very happy with the results. The space stays warm in winter and cool in summer, and doesn’t smell like a basement or have moisture or humidity issues.
The foam board insulation was attached to the wall using Christmas tree fasteners. Not sure if that is correct name but a google search of that term will show you what they look like. You drill through foam, poly, and into block wall, then lightly hammer the fastener in which pins the foam to the wall. Was concerned that this punctures the poly, but hasn’t been an issue. Wanted to glue the foam up but can’t do that with poly under it.
I did not use dimple board behind the foam. Just block wall, poly, foam. I don’t expect to get bulk water, and the foam could also help in that case, but the poly is like belt and suspenders. Poly is cheap and easy to install, so I figured why not.
Here’s a pic showing the fastener in place. Tried to minimize the amount used. All seams taped or filled w spray can foam.
Plasti-grip makes anchors like that ("Christmas tree anchors"), as does Hilti. I like them too -- they go in quick and easy, and tend to be pretty reliable.
Bill
Hey there! I’m in a similar situation right now, but it’s an old house remodel and I have block walls and the framing already up which makes things a bit tougher. Some history of water issues but have most of it fixed with grading and gutters. How would you proceed? A layer of thinner board to squeeze behind the framing and then a second layer of thicker stuff between studs? Do you think the poly sheeting is necessary? I was going to possibly Killz the concrete blocks instead and then glue XPS to it
You should be able to glue rigid foam directly to the block without needing to prime the block first. If you do decide to prime it, I wouldn't use Kilz, I'd use a block fill paint or a sealer like Drylok. Note that you get very low coverage painting like this, maybe only 50-100 sq ft per gallon instead of the usual 400, because the block will suck up a lot of paint. Plan accordingly if you decide to go this route.
Ideally you'd put the rigid foam up first before framing out any walls. It is NOT necassary to frame regular structural walls in this application, either, you can save space by using 2x3s "on the flat", which will provide you with a 1.5" deep cavity for electricals. Use 4" square boxes and mudrings for devices and you can work normally in terms of wiring but without wasting space on studwalls. You don't need or want to use batts here, either, it's much better for ALL of your R value to be in the rigid foam.
Your last option would be spray foam against the wall, burying the back of the studs a bit. That's the least labor, but it's expensive and it uses spray foam. What I'd probably do if this were my project is to knock out every third stud or so to let me slide full sheets of rigid foam behind the studs. You probably won't need glue, since the studs should support the foam. Fill the gap between the studs and masonry wall with the thickest rigid foam that will fit. I'd tape the seams and seal the top of the foam to the masonry too. Replace the studs you knocked out after you finish installing the foam.
Bill
Thanks for the advice. I’ll probably try and slide the boards behind the framing without removing, even if that means more cutting. I know, tedious, and probably not the best for an airtight seal but I’m a little too far down the road to remove framing with my electric and plumbing roughed in.
My only remaining question is how would you deal with the bottom near the sill plate and the top where the framing meets the rim joists? Cut the xps all the way down to the drain tile dimple board so it’s touching? Or leave a tiny gap? And spray foam the top to create a seal with the xps board in the joists?
If you are expecting future problems with bulk water, then leave about a 1" gap between the bottom of the foam board and the floor. This way the foam board is never sitting directly in water. This is more of an issue with polyiso than with XPS though, but it's still good insurance.
If the "bottom near the sill plate" you mentioned is that pic with the staircase arrangement of three pieces of XPS, the easiest way to do it is probably to use foam board glue between the pieces to form a good seal. You could also get some really wide tape and tape over the entire assembly. It would probably be tricky to do a neat job with super wide tape though in what I suspect is a pretty confined space. Glue is probably the easiest way to go. Loctite makes some foam board glue that works pretty well, but be careful when you first squeeze it out of the tube: the first bit is often very runny and watery. You want to get rid of that and get to the stuff that is more like blue toothpaste, at which point it works pretty well.
Bill
Replying to this one because it won’t let me reply to latest - the pic I’m referring to is the one here, with the dimple board from the drain tile coming out of the concrete and sill plate/framing in front of it. Should I just run xps to sill plate and tuck the little section of dimple board behind the XPs? Or should I stop right at the top of the dimple board and leave a gap? Worried about water seepage from above if a hard rain event or gutter failure
I'm assuming that dimple mat is going down around the outer edge of the slab there, and tying into a footing drain beneath the slab. That dimple mat is intended to drain any seepage coming through the wall. Ideally you'd extend that dimple mat up the wall, sealing it at the top, and put the XPS over that. That way the dimple mat provides a drainage plane for the wall that can drain all the way down to the footing drain. I would tape the lower edge of the new dimple mat to seal it to the existing strip at the edge of the slab.
If you don't want to do that, and you don't expect any seepage, then you'd want to put the XPS over the interior-side edge of the dimple mat so that the wall could still drain behind the XPS down to that existing dimple mat at the edge of the slab. That's not idea, but it's better than putting the XPS above the existing dimple mat.
Bill
You could save some money here and use the cheaper foamular 150 version of XPS. The higher compressive strength of the 250 version is not needed here.
Bill