Home & Garden

New basement slab - vapour barrier between dirt and gravel?

  • Last Updated:
  • Apr 6th, 2022 2:31 pm
Newbie
Mar 27, 2022
7 posts
5 upvotes
Toronto

New basement slab - vapour barrier between dirt and gravel?

I’m having an interior waterproofing and underpinning job done around the block foundation of my home.

The contractor is preparing for the new concrete floor to be poured and has put dimpled waterproofing membrane over the walls, down to and over the weeping tiles, and then over the dirt floor. The gravel will go over the membrane on the floor, and concrete over the gravel. The new sewage pipes perforate the membrane, disrupting any kind of seal in the membrane sheets.

Everything I’ve found online, and our architectural drawings, show gravel over the dirt and weeping tile, with vapour barrier overtop, and then concrete. My understanding is that the porous gravel allows for water to travel to the weeping tile around the inside perimeter of the basement.

I’m unhappy with this non-standard approach, but my contractor has always done it this way and has never seen an issue. It’s never been an issue with permits and sign-offs. My contractor will not warranty the standard approach if that’s what I ask him to do.

So how much does it matter? If it does matter, how can I support my argument?
19 replies
Sr. Member
Dec 21, 2020
946 posts
822 upvotes
I imagine the dimpled membrane should do the job ok. The point of all this is to keep water getting INTO the basement FROM the ground (and I guess to some extent radon gas). As long as the seal is continuous from the walls to under the slab I imagine it should be fine. I would probably ask them to seal around any penetrations and seams with lots of tuck tape (the blue one not the red one) and call it a day.
Deal Addict
Nov 9, 2008
1852 posts
879 upvotes
Toronto
I have seem dimple membrane OVER a concrete slab, very common for a subfloor. I have never seen/heard of dimpled membrane UNDER a slab.

I don't believe a dimpled membrane qualifies as a vapour barrier, which is what you would want beneath your concrete slab but ontop of the gravel to prevent any moisture/vapour migration through your slab and into your finished living space.

In order:

Soil
3/4'' clear gravel
Vapour barrier
Poured concrete slab

I would want a vapour barrier beneath my slab. Either a continuous poly that qualifies as a vapour barrier, or better yet 2''+ of closed cell foam (XPS with tucktaped seams) to provide both vapour barrier AND insulation.

Good you are asking questions OP!
Deal Addict
Nov 16, 2011
1535 posts
1256 upvotes
Hamilton
presumably you have a building permit . Have you check with the building inspector to see what it code ?
Newbie
Mar 27, 2022
7 posts
5 upvotes
Toronto
I wish my contractor were as enthusiastic about my questions..

I failed to mention that there will be foam over the gravel for a radiant heat system.

Thanks for your feedback!
Deal Guru
Jan 25, 2007
12695 posts
7861 upvotes
Paris
TheeDuke wrote: I wish my contractor were as enthusiastic about my questions..

I failed to mention that there will be foam over the gravel for a radiant heat system.

Thanks for your feedback!
How thick of foam? That could be a vapour barrier.
Deal Fanatic
Nov 17, 2012
5381 posts
4997 upvotes
Toronto
What does foam over the gravel have to do with the radiant heating system?

Are you having tubes embedded in the concrete slab, or are you putting a system on top of the slab?
Newbie
Mar 27, 2022
7 posts
5 upvotes
Toronto
The foam has channels that the radiant heat tubes are laid into. AFAIK it also acts as insulation so I'm not heating the ground below.
Deal Expert
User avatar
Feb 11, 2007
21246 posts
25711 upvotes
GTA
TheeDuke wrote: The foam has channels that the radiant heat tubes are laid into. AFAIK it also acts as insulation so I'm not heating the ground below.
Foam makes sense under the concrete whether you have radiant or not. Usually the radiant tubes are installed on to of the concrete on channels in foam/wood. Under the slab would just waste energy heating the slab before it got to your floor.

Are you sure you got it right? Either you or your contractor are all mixed up. Did he give it to you in writing or engineering drawings?
If the women don't find you handsome, they should at least find you handy.
Sr. Member
Dec 21, 2020
946 posts
822 upvotes
engineered wrote: Foam makes sense under the concrete whether you have radiant or not. Usually the radiant tubes are installed on to of the concrete on channels in foam/wood. Under the slab would just waste energy heating the slab before it got to your floor.

Are you sure you got it right? Either you or your contractor are all mixed up. Did he give it to you in writing or engineering drawings?
If the whole slab is getting replaced then the radiant heat tubes are embeded in the concrete slab with the foam insulation underneath to prevent heating the soil below.
Deal Expert
User avatar
Feb 11, 2007
21246 posts
25711 upvotes
GTA
RCLapCar wrote: If the whole slab is getting replaced then the radiant heat tubes are embeded in the concrete slab with the foam insulation underneath to prevent heating the soil below.
Yea, I think you're right now that I think about it I've heard it done that way.
If the women don't find you handsome, they should at least find you handy.
Deal Addict
Dec 19, 2015
4373 posts
2499 upvotes
Calgary, AB
TheeDuke wrote: I wish my contractor were as enthusiastic about my questions..

I failed to mention that there will be foam over the gravel for a radiant heat system.

Thanks for your feedback!
Generally you want the Vapor barrier between the foam and the concrete. Otherwise the foam just acts as a sponge beneath the concrete.

Gravel>foam>Vapor barrier (10mil poly ideally)> concrete. That’s how I’m getting mine done in the next few months. Under slab should come up and join the Vapor barrier on the interior of the concrete walls, overlapping at least 6 inches and sealed with tape.

Edit: here’s a commentary from people far more knowledgeable than me on the subject.

https://www.greenbuildingadvisor.com/ar ... rete-slabs
Deal Addict
User avatar
Aug 29, 2010
1020 posts
958 upvotes
Toronto
jacquesstrap wrote: I have seem dimple membrane OVER a concrete slab, very common for a subfloor. I have never seen/heard of dimpled membrane UNDER a slab.

I don't believe a dimpled membrane qualifies as a vapour barrier, which is what you would want beneath your concrete slab but ontop of the gravel to prevent any moisture/vapour migration through your slab and into your finished living space.

In order:

Soil
3/4'' clear gravel
Vapour barrier
Poured concrete slab

I would want a vapour barrier beneath my slab. Either a continuous poly that qualifies as a vapour barrier, or better yet 2''+ of closed cell foam (XPS with tucktaped seams) to provide both vapour barrier AND insulation.

Good you are asking questions OP!
Dimpled membrane is a vapour barrier.

It's not common practice, but people do actually put rigid foam insulation under the slab for heat loss.

Concrete is pourous so you're not really protecting it, you're protecting your floor finish. You can have a vapour barrier on either side, it just needs to be on top of your gravel so that it can drain.

Ideally you don't want to over do any water proofing because you might end up trapping water in your slab, so it's best to just use a single layer either above or below the slab.
Newbie
Mar 27, 2022
7 posts
5 upvotes
Toronto
Lemonylol wrote: Dimpled membrane is a vapour barrier.

It's not common practice, but people do actually put rigid foam insulation under the slab for heat loss.

Concrete is pourous so you're not really protecting it, you're protecting your floor finish. You can have a vapour barrier on either side, it just needs to be on top of your gravel so that it can drain.

Ideally you don't want to over do any water proofing because you might end up trapping water in your slab, so it's best to just use a single layer either above or below the slab.
Drainage and trapping water is what I’m worried about, but my contractor won’t hear it. He said he’ll do it my way (i.e. the industry standard) but won’t warranty the work. He’s always done it this way and I have nothing to refer to to change his mind.
Newbie
Mar 27, 2022
7 posts
5 upvotes
Toronto
Andy34 wrote: Generally you want the Vapor barrier between the foam and the concrete. Otherwise the foam just acts as a sponge beneath the concrete.

Gravel>foam>Vapor barrier (10mil poly ideally)> concrete. That’s how I’m getting mine done in the next few months. Under slab should come up and join the Vapor barrier on the interior of the concrete walls, overlapping at least 6 inches and sealed with tape.

Edit: here’s a commentary from people far more knowledgeable than me on the subject.

https://www.greenbuildingadvisor.com/ar ... rete-slabs
I wish that article contained some references to scientific or engineering sources.

Editing - there is a reference to the illustration. But that’s the same kind of drawing our architect made for our reno, and my contractor dismissed it with anecdotes about architects being out-of-date.
Newbie
Mar 27, 2022
7 posts
5 upvotes
Toronto
I managed to find a few resources, though from the US:


Certainly I could send these resources and others to my contractor, but is there a contractor on earth who would accept being told by a client they’re doing their job wrong?

When industries come up with uniform solutions there’s usually a good reason for it. I’m just at a loss here.
Sr. Member
Dec 21, 2020
946 posts
822 upvotes
I think your choices here are:

1. accept what your contractor wants to do and get it done with their warranty (whatever that's worth to you)
2. get them to do it your way and loose any of their warranty
3. find another contractor who will do it your way and give you a warranty

Which you choose will most likely come down to how much you trust this contractor to uphold their warranty and money of course.
Deal Fanatic
Nov 17, 2012
5381 posts
4997 upvotes
Toronto
TheeDuke wrote: Drainage and trapping water is what I’m worried about, but my contractor won’t hear it. He said he’ll do it my way (i.e. the industry standard) but won’t warranty the work. He’s always done it this way and I have nothing to refer to to change his mind.
What 'warranty' is he providing anyhow? Trust me - there's nothing that will go wrong that you'll ever be able to blame on him and get covered by whatever non-binding warranty he claims to be providing.

If you're convinced you want something done, buy the materials and get the contractor to put it in. I wouldn't let some vague promise of a warranty from joe-shmoe contractor keep you up at night for the next 20 years after the work is done.

That being said, I'm sitting in my 100 year old basement on an ancient concrete slab that I guarantee you has no foam or vapour barrier below it. I put some 2x4 sleepers, rigid foam, a layer of poly and plywood/engineered hardwood down 20 years ago and have been enjoying it ever since without much concern.
Member
Mar 1, 2011
458 posts
118 upvotes
Stoney Creek
I would recommend styrofoam not as a vapor barrier but as a great insulation
Deal Addict
May 2, 2011
1380 posts
549 upvotes
Oakville, ON
torontotim wrote: What 'warranty' is he providing anyhow? Trust me - there's nothing that will go wrong that you'll ever be able to blame on him and get covered by whatever non-binding warranty he claims to be providing.

If you're convinced you want something done, buy the materials and get the contractor to put it in. I wouldn't let some vague promise of a warranty from joe-shmoe contractor keep you up at night for the next 20 years after the work is done.

That being said, I'm sitting in my 100 year old basement on an ancient concrete slab that I guarantee you has no foam or vapour barrier below it. I put some 2x4 sleepers, rigid foam, a layer of poly and plywood/engineered hardwood down 20 years ago and have been enjoying it ever since without much concern.
This is the right answer. Their warranty doesn't mean anything. They will fight tooth and nail to repair anything major and none of it will ever be their fault.

I think anyone is sensitive to being told how to do their job from someone outside their line of work, but there are many "old school" guys that are convinced they are doing it the right way, but in fact have been doing it incorrectly their entire lives. Standards and codes change and some people just don't keep up with it. The company that manufactures the radiant heating system should have some sort of guideline on how it should be installed in a basement slab.

Top

Thread Information

There is currently 1 user viewing this thread. (0 members and 1 guest)