vapor barrier recommendations

Sukhoi29SU

Spent that last couple hours looking for under slab vapor barrier reviews and recs. This is for a radiant heat application. Some will be under Foamular 250 2"xps, and some of the other might be under Barrier X5 insulation. I've been looking at Stego wrap, Perminator, or Americover polyolefin vapor barriers. Question is: do I need to spend the extra bucks for this polyolefin stuff if it's going under insulation? My understanding is that the Barrier X5 has a built in vapor barrier so I most likely wouldn't need it for that, but a portion of the radiant application is in a garage that will be driven on so I needed to go with the higher compression foam for this, and figured I'd need a decent vapor barrier under that? Any brand recommendations? Thanks in advance

CBRob

I've been seeing guys use closed cell spray foam under slabs
The price can end up better than dow board and it's a vapor barrier too.

Pechan

I would place the vapor barrier over your insulation, not under it.
If you place it under the insulation you have potential for the water from your concrete to pool in between the insulation and give you problems later when it slowly comes up through your slab.

hot_rod

Vapor barriers go against everything concrete experts tell me about curing concrete. Some of the "waters of convince" go out the bottom of the mix, some evaporates out the top, some consumed in the the hydration process.

Adding a vapor barrier changes the way the concrete cures, usually slowing the cure.

I've had concrete guys take a rake to my vapor barriers and poke holes to allow the water out

Adding to the problem is concrete finishers that add water to the truck when it arrives to ease the pour of the mud..

The mix leaves the yard with the correct proportions, adding water changes everything, weakens the mix, prolongs the cure time before the final trowel.

You might check with the redi-mix plant, tell them of your vapor barrier, they can put in admixes to help get a good cure and you best shot at a crack free finish.

Sukhoi29SU

Thanks for the comments.
I’ve never heard of vapor barrier above the foam. I guess it’d work if I didn’t use staple-to-foam method for pex tubing and zip tied to mesh over the vapor barrier, instead. I planned to tape all the seams for the foam so no water got down in between the foam boards.

I’m spray foaming a lot in this project but wasn’t planning on the floors. This blue board and a decent vapor barrier isn’t cheap, but I can’t imagine it’d be comparable in price to spray foam. I could be wrong...

I’ll get a hold of the concrete company and ask about concrete pour considerations over a vapor barrier.

After doing some more research, seems like a lot of varying opinions and ways to skin the cat. I figured this far along with radiant heat and pouring slabs there would be a clear cut answer.

Thanks again

CBRob

I've been under the impression that it's prefered to have concrete dry slowly.
Like a pour inside of a ICF, it's going to hold the moisture a long time.

nibs

If I caught any one poking holes in my VB they would be ordered off the job. Once you get the "bleed water" off, the longer you can keep the concrete moist the stronger it becomes.
The bleed water rises to the top of the mud, and carries very little cement paste with it, a drain hole in the VB will allow cement paste to wash out.
We keep all of our pours damp for at least 10 Days preferably 30.
Rather than use plastic, we try to find tarps on sale, that are big enough to cover the floor of the room, you can walk on them and generally be active without damage.
It is very simple to research concrete and its best curing methods, there are concrete associations that post information on line about it. There is also a very interesting series on the concrete canoe racing competitions between engineering classes at a number of universities. Cement is a known and well researched material if we are going to involve ourselves with in slab radiant systems, we should educate ourselves.

Sukhoi29SU

https://www.greenbuildingadvisor.com/article/polyethylene-under-concrete-slabs

For those interested, I found this article and particularly the responses to be very informative.

Sounds like for radiant applications, most installers will put the vapor barrier , 6mil poly min, beneath the foam vs on top - so they can staple the pex to foam , or use mesh if desired. But the 2” of xps foam work as a vapor retarder in itself.

Sukhoi29SU

Here’s another opinion.

CBRob

It's probably worth noting that the vapor barrier is there for the house.. not the concrete.

You don't want moisture coming up though the slab and then finding it's way into the house where it can condensate on cold walls and other surfaces

hot_rod

Plenty of different opinions as usual. This article is more about sand layers under slab pours but he does mention perforated vapor barriers.

If you have moisture coming up thru a slab, you have a water issue under the slab, radiant will not get along with that. Condensation from dew point above the slab certainly. But migrating up thru 4" of concrete?? From where?

I'll call NIBS when I have a crew of concrete guerrillas that need to be thrown off a job


Alan Sparkman director of the Tennessee red mix association presented a seminar for the RPA years ago, specific to concrete and radiant. He suggested water reducing agents be added when pouring over a vapor barrier, for the reasons mentioned above.

https://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems

nibs

@hot_rod in my 20 + years in construction, I have had to fire more contractors than I can count. Never fun but when necessary it is the best thing for the client.
Re letting water drain out through the VB - If water drains out it carries cement paste and other fines away from the slab, and leaves a pocket of gravel in the slab, no competent cement worker would do that.
PS: have had to fire at least one sub contractor whose contract was for over a million dollars in the early 1990's. Been retired for 20 years now and have declined offers to go in and correct projects.
I am in awe of the knowledge base here, and hope I can add a little in the way of things related to construction generally.

ratio

I've seen a good number of acres(!) of concrete slab 'water cured' with a vapor barrier underneath & plastic above. Seems to leave a nice smooth finish, although that may have more to do with quality of the finishers rather than the water-cure itself.

hot_rod

nibs said:

@hot_rod in my 20 + years in construction, I have had to fire more contractors than I can count. Never fun but when necessary it is the best thing for the client.
Re letting water drain out through the VB - If water drains out it carries cement paste and other fines away from the slab, and leaves a pocket of gravel in the slab, no competent cement worker would do that.
PS: have had to fire at least one sub contractor whose contract was for over a million dollars in the early 1990's. Been retired for 20 years now and have declined offers to go in and correct projects.
I am in awe of the knowledge base here, and hope I can add a little in the way of things related to construction generally.

Seems to me millions of acres of slabs have been poured without vapor barriers? sidewalks, driveways, roadways, basements, etc.

Wouldn't it need to be a very watered down mix to run all the sand and portland of a properly consolidated batch?

Water rising out the top doesn't seem to pull along the fines?

The articles comment about walking over the vapor barrier with golf shoes?

Sukhoi29SU

@ratio , when you say vapor barrier underneath and plastic above... do you mean vapor barrier directly underneath the concrete ? Or vapor barrier underneath the foam, followed by a plastic sheet above the foam?

It looks as if a way to go would be to compact the gravel, put down the 2” xps pink board, put a good vapor barrier above the pink board, and then secure the pex to the vapor barrier with staples. It seems counterintuitive to staple the vapor barrier, though. You could put mesh over the vapor barrier and zip tie the pex to that, but I read one article stating the concrete has much more chances of cracking when utilizing mesh. And, per Hot Rod’s advice, get some water reducing agents into the mix

nibs

Definitely use super plasticizers to cut water use pour as dry as you can manage, mop up bleed water if you have too much of it.
XPS will take on water over the years so best to VB beneath the foam.
@hot_rod Concrete is porous so it wicks water by capillary action, sidewalks and roadways allow water to evaporate so it is not a problem. Inside a building you need to keep moisture from entering the slab and evaporating inside the building.

CBRob

Here is a article from Martin Holliday, pretty sharp guy who knows his building science and often writes for the journal of light construction.
The idea of putting the poly below (I meant to say above) the insulation is also in line with the recommendations of Joseph Lstuberick
Of buildingscience.com

https://www.greenbuildingadvisor.com/article/polyethylene-under-concrete-slabs

Sukhoi29SU

@CBRob , that’s the article I posted earlier in the thread. I need to sign up for an account to read the entire article, but found the comments / responses interesting. Joseph Lstiberick disagrees with this article, then? Because this article advocates putting the vapor barrier on top of the foam, correct?

CBRob

Dr Lstuberick and building science.com agree

Notice that the sheet polyethylene “vapor barrier” is located on the top of the rigid insulation between the rigid insulation and the bottom surface of the concrete – in direct contact with the concrete. Do not, and I repeat, do not locate the polyethylene under the insulation as it will keep the insulation wet.

I misquoted my comment about the poly under the installation.
Apparently insulation gets wet and stays wet.

nibs

Wet insulation has a very low insulating value. VB under the foam.
I have one area of foam with no concrete over, it stays dry as a bone, If I spill water there it dries up overnight.
If the insulation is wet it may well grow mold and start stinkin' up the joint.

TAG

This seems to come up often -- everybody has a different idea. GBA seems to feel the small holes placed in the VB when attaching tubing through the foam is not an issue. There are many installations where the foam is taped and treated as the barrier. Many of the plastic board/ insulation products tell you they are the barrier even thought they don't recommend sealing the edges ....

My preferred method is to place heavy 10/5mil VB down over the compacted base and lay the 2" foam on top .... we tape the foam and usually use wire -- but, I have done the staple method. I prefer the wire. The VB comes up along the walls

With a proper base the foam does not move when you walk on it -- I have placed test panels down and had people walk over them for a week .. just to see if the VB gets damaged. It does not.

I'm in a radon area -- so I want that VB barrier. If you have water next to the slab -- that's a different problem. Also -- todays tape is tenacious .. at least for the short time it has to do anything. I really don't see how much or any liquid from the pour is getting under the taped foam.

Sukhoi29SU

Stego wrap makes a ‘true’ 15 mil vapor barrier (vs retarder) that I could use for the above plan. I could lay the pink board down and tape with 4”wide tape, and for extra measure potentially use some waterproof tape on top of that- gorilla tape? To make sure no water gets down in between the foam boards. This is going against all the ‘science’ explained in the articles, but appears to be a valid method from some experienced people both here and on other forums.

I wish that there was an obvious way to do it - but I’ve realized throughout this entire project that isn’t always the case.

TAG

Sukhoi29Su -- you will find all manner of opinions. Many that contradict the same writer when they speak of other parts of construction/ insulation. In one place foam will be fine along the exterior of a foundation -- or when doing an insulted European slab type construction ,,, but ... don't have exposed foam under a slab? We have ICF construction in some articles ... it's foam.

My problem with exposed foam is ants ... have seen this occur.

hot_rod

TAG said:

Sukhoi29Su -- you will find all manner of opinions. Many that contradict the same writer when they speak of other parts of construction/ insulation. In one place foam will be fine along the exterior of a foundation -- or when doing an insulted European slab type construction ,,, but ... don't have exposed foam under a slab? We have ICF construction in some articles ... it's foam.

My problem with exposed foam is ants ... have seen this occur.

I have seen blueboard used on the exterior, vertical walls turn to swiss cheese over about a 15 year span, ants I suspect. I can see some around my edge slab insulation also.

Seems most ICF are more like a beadboard material, not as waterproof as the extruded type foam?

Some of the ICF manufacturers ask for a waterproofing layer over the forms and in some areas a stainless mesh is added for bug issues.

Sukhoi29SU

Fortunately (depending on how you look at it) I’m building on limestone - this might help with the critter aspect. At least I would hope it would.

hot_rod

Semantics I suppose, is a vapor barrier also an air barrier, water barrier the same as a moisture barrier?

Same quandary come up with house wrap, should it be air, water, vapor and moisture tight like tar paper, or breathe like some of the poly house wrap products?

Building Science Joe's opinion seems to be "it depends"

I'm not advocating perforating the vapor barrier, just to be clear. I suspect poly film on gravel ends up with some holes, and the nub type insulation panels with its moulded vapor barrier has a lot of seams that don't seal 100%.

It comes down to what you are comfortable with given all the opinions.

Sukhoi29SU

Thanks again for the comments.

I guess I could spend the $ on a 15 mil true vapor barrier (stego wrap) under the foam, the lay down the pink board, tape the seams, and top with a thin 6 mil poly. Staple pex to the 6mil poly ‘vapor retarder’. Might be overkill but with the amount of money being spent on this , if a 6mil poly will help keep the xps from getting wet and eliminate any risk of water getting down in between the boards...

Gordy

Use the stego vapor barrier. Much more durable. Easy to roll out stays in place. Use the stego tape on joints.

@hot_rod makes very good points.

You must also look at the Code for radon mitigation requirements in your location. Northern Illinois is a hot spot, especially when you get down to rocky, and sandy soils. In that aspect you want 100% vapor protection. NO HOLES in the vapor barrier.

As for the curing process. Finishers want to get the pitch fork, because they will be there longer. It also buys them time on a hot day, or a hot load. The water has no where to go but up,

Sukhoi29SU

Thanks @Gordy

nibs

@Sukhoi29SU Ask your engineer for his recommendations.
I read @hot_rod's, article about the golf shoes a while ago, and thought HMM that is interesting, then continued to make my VB as tight as possible.
PS Fiber is probably the least expensive addition for adding strength to the mud.