Basement Walls and Concrete-Free Slab
Hello,
Please feel free to skip ahead: Just a few quick questions below, re Concrete-Free Slab, and also re Basement Walls.
And if you have time, here’s some background info:
I really enjoyed the great article by Josh Salinger, “Inside the Concrete-Free Slab.”
https://www.finehomebuilding.com/project-guides/insulation/concrete-free-slab
We are building a cabin out in the Columbia Gorge (near Carson, WA), and we would like to do a similar floor in our crawlspace/basement. Please see my replies below, for Section Drawings and Details. The cabin is in climate zone 5B.
I talked to a Structural Engineer at work, and he suggested the retaining wall details shown on A2, A7, and A7.9, based on some earlier feedback from Malcolm Taylor. This should make the North stem wall and footing more stable during any seismic activity. I think the Engineer also suggested that lower drain shown on A7 and A7.9. BTW, I added A2 to the end of the pdf set, since we’re mostly talking about the sections.
Last Fall, we finished getting the cabin dried in, so several of the details are existing.
I’m not showing all of the same layers as are shown in the article, because we’d like to maintain the crawlspace headroom that’s shown. The crawlspace is not a living space, but we’ll do our best to get the flooring level. I’m hoping to get by with a 2″ layer of ¼” Minus gravel, as the bottom layer.
I plan to switch from a vented crawlspace to unvented, using one of the methods described on the last page of this great article, by Justin Fink:
https://www.finehomebuilding.com/project-guides/insulation/crawlspaces-that-work
And to have the first floor and the crawlspace be part of the same conditioned space. The amount of rigid foam that I’m showing might be overkill, but with the amount of insulation on the first floor, I’m assuming that I don’t want to skimp on the crawlspace insulation. Does that sound right?
Please disregard the 2” EPS Rigid Foam on the outside of the basement walls. It’s there, but I know that it won’t contribute to the R-value, since it doesn’t go all the way up.
The Vapor Barrier shown in the two photos below will be removed, and placed later. So that I can get it in between the proper layers, as shown in my drawings.
A Few Questions:
-Will the flooring work, with only the layers that I’m showing? ¼” Minus Gravel, Rigid Foam, Vapor Barrier, Plywood.
-If the headroom in the basement will only allow for a 2″ layer of . . . something, is it better to do a 2″ layer of 3/4″ minus gravel (with no fines), to create a capillary break, or a 2″ layer of 1/4″ minus gravel, because it’s easier to screed?
-For the walls, I would like to use ½” OSB over the Rigid Foam. With a Vapor Barrier in between, of course. OSB anchored all the way through the foam, and into the concrete stem wall. Do you see any issue with that?
-Am I showing my basement Air Barrier/Vapor Barrier in the correct place? It’s shown in Red.
-Do you see any details that I need to revise?
Thanks,
Miles
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I will also post the pdf pages as jpegs, since those can be easier to see. Here are the JPEGs for Sheets A7 and A7.1.
JPEGs for Sheets A7.3 and A7.4.
JPEGs for Sheets A7.9 and A2.
Deleted
Miles,
- The stack-up will work fine. I would be inclined to use the 3/4" gravel. It will tamp down pretty flat.
- On the back wall I would install the poly against the concrete, not over the foam.
- On the pt wood ledge I would omit the interior poly. That may allow some small amount of drying.
Hey, great. Thanks for your reply.
- when you say poly up against the concrete, you mean on every wall, correct?
-and when you’re talking about omitting the interior poly on the wood ledge, you’re talking about detail 1 on A7.9?
Thanks,
Miles
Miles,
When you have interior foam on concrete walls you already have a vapour-barrier so the poly is optional. If you decide to include it the place to stop the vapour drive is against the concrete, not the interior face of the foam.
The place for the poly on both the ledge (1 7-A9) and the concrete-free slab is against the ground (fill) to stop moisture migrating in towards the interior. The foam acts as an interior vapour-barrier without any additional poly.
Gotcha. Thanks again, Malcolm.
Miles
Miles,
It is really difficult to see the drawings clearly. I tried zooming in on them, but the labels are too fuzzy to make out.
A few things in general. If you are using foam against the basement walls, you won't need the poly layer. The foam, ideally a low GWP XPS which would be a low Class 2 semi impermeable vapor control will throttle down any meaningful vapor drive. Tape the seams for the air barrier or use the concrete as the air barrier. I can't see the drawings well enough to see your strategy here.
Also, I would definitely not use just the 1/4- as this will contain fines which will wick water via capillarity. I would use 3/4" clean gravel with no fines. I would also do 4" as this is also your soil gas depressurization field.
I would not put the fill or soil directly against the wood framing, PT or not. Why not just use concrete there? If you were going for a wood foundation to avoid the concrete, why not do it everywhere? If you put the poly on the interior of the PT framed wall, the wood will reach parity with the RH of the soil, which is about 100%. It could cause rot over time. I would put the vapor barrier on the exterior in this condition, add a dimple mat and then fill it with gravel and a footing drain. One really needs to reduce the hydrostatic pressure drive in these sub grade locations.
For the floor, do a 4" layer of 3/4 clean gravel, then a layer of landscape fabric to keep the fines from filtering down, then 2" of 1/4- to level it out. Then the foam, then the vapor barrier, then the plywood subfloor.
Hi Josh. Thanks very much for your reply. Sorry about the jpegs. I have attached the PDFs here, and to my original post.
-Walls: OK, no poly necessary. I was thinking I would use Polyiso on the walls since I have some left over from the walls of the cabin. Unless that's a bad idea. Tape the seams of the rigid foam - got it.
-3/4" clean gravel with no fines - got it. I don't think there's enough headroom to go 4" thick, though.
-For that upper bench shown on A7 and 1/A7.9, I think I'm following you. Dimple mat between the PT plywood and where I'm currently showing soil? Feel free to mark up my PDF drawings, if you have time.
-Backfilling with gravel is a good idea - thanks.
-When you say For the Floor, you're talking only about the floor in that Upper Bench area, correct? Like I say, not much headroom, but I know you got that, based on your reply above.
-So, do the floor in that area as per your article. The only trick is that the Engineer was wanting to tie that all together: All the way from the Cabin footing, through the wood framing, and into the 8" diameter footings. To keep it more stable during any seismic activity.
Thanks again,
Miles