Under slab insulation question...
Last Post 01 Sep 2015 09:16 PM by ronmar. 7 Replies.
Printer Friendly
Sort:
PrevPrev NextNext
You are not authorized to post a reply.
Author Messages
ururkUser is Offline
New Member
New Member
Send Private Message
Posts:30

--
28 Aug 2015 12:02 PM
I've found a source of used 2" XPS, 25psi. I'd like to use it under my slab (4") and on the perimeter of the foundation (2"). I'll be putting plastic sheeting below the foam, and some amount (6"?) of crushed aggregate below the sheeting to provide a capillary break. However, I did the unthinkable and started googling EPS vs XPS. I'm now starting to rethink my decision, but it is too late. Will water absorption be a huge problem with XPS? I'm having a hard time separating actual studies with marketing material.
ronmarUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:479

--
28 Aug 2015 02:46 PM
If the plastic is sealed, and there is a good path to drain thru that capillary break under the plastic to a drain, where is the moisture going to come from? If isolated from ground by a good lapped and sealed membrane, I would think the foam and the concrete should ultimately settle near the rooms moisture level...

What is your soil like under the slab?
ururkUser is Offline
New Member
New Member
Send Private Message
Posts:30

--
28 Aug 2015 03:43 PM
I've been thinking about running a drain out from under the slab (slab level will be a few feet above where the drain will exit), but at the same time I'm wondering if it is even necessary?
ronmarUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:479

--
28 Aug 2015 04:55 PM
Even the best sealed membrane can develop leaks. The only way this is really a problem if there is water built up against it so it can develop enough pressure to inject itself past the membrane. The capillary break below the pad will only be effective if there is a way for any moisture that makes it up into the gravel to drain away at the edges, such as into a perimeter drain. It dosn't take much, just a path of least resistance that keeps any water from building pressure up against the plastic.

The drain for that gravel may or may not be necessary, depending on your soil and water conditions under the build site. Either being close to the water table or having an impermeable layer(clay) close to the surface that could possibly duct water from heavy rains up under the pad. It is of course simple enough to put in a drain system and ifyou can guarantee it a drain path, you should never have to worry about water intruding from below...
ururkUser is Offline
New Member
New Member
Send Private Message
Posts:30

--
29 Aug 2015 10:31 AM
Ok. I'll make sure I include a drain then - it will be on top of clay. I'm not building the foundation, but want to better understand how I should have this built.
Bob IUser is Offline
Veteran Member
Veteran Member
Send Private Message
Posts:1435

--
31 Aug 2015 08:27 AM
You might not really know if it is necessary until the house is done and you are living it; then it will cost you far more to install the drains than if you do it when the excavation is being done. Why risk it?
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
jonrUser is Offline
Senior Member
Senior Member
Send Private Message
Posts:5341

--
01 Sep 2015 01:52 PM
if you can guarantee it a drain path


I agree with Ronmar - but be aware that the gravel acts as a filter and unless cleaned, all filters will eventually clog. So preventing surface water from getting underneath the slab is preferred over letting it get there and then draining it away. The latter is a good backup.
ronmarUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:479

--
01 Sep 2015 09:16 PM
Yep, A perimeter drain should never allow it to get that bad under the slab, and will take the brunt of the ground water flow. That flow should be less if you pay carefull attention to your surface grading around the house and make sure your gutter drains, like the grading, lead well away from the foundation. The perimeter drain is the more likley to buildup debris IMO, but it is also the easiest to repair if the need arises... A drain for the capillary break under the slab is cheap insurance for a clogged perimeter drain especially if you are setting on a layer of clay...

What does the topography of your property look like? Any slope at all so you have someplace to send that water from the drains...
You are not authorized to post a reply.

Active Forums 4.1
Membership Membership: Latest New User Latest: hudson2000 New Today New Today: 0 New Yesterday New Yesterday: 2 User Count Overall: 34707
People Online People Online: Visitors Visitors: 179 Members Members: 0 Total Total: 179
Copyright 2011 by BuildCentral, Inc.   Terms Of Use  Privacy Statement