Perimeter insulation/vapor barrier setup variations
Last Post 10 Dec 2015 06:52 PM by sailawayrb. 29 Replies.
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milnerptUser is Offline
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28 Nov 2015 06:30 PM
In creating a walk-out basement hydronically heated floor, it leaves for 2 of the basement walls to be partially above/below grade. In looking at the picture below for 2 variations of setup of the different layers of the setup. Looking to maximize the living space (about 900ft2), these 2 walls are partially below-grade foundation walls and transition to 2x6s above grade (already insulated)--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Option 1) perimeter insulation is tapered at the top, allowing the concrete slab to reach the foundation edge. This would allow for maximum space. A plate could be placed with studs along the foundation wall (both pressure treated wood) with insulation running between the studs. Vapor barrier would protect the insulation and run up to the top of the foundation. Problems: does the tapered edge of the perimeter insulation greatly affect heat loss into the foundation, as not only is it touching, but there is <2" insulation along the top 2 inches of the slab? Is putting fasteners through the plate into the floor going to hold, or will the concrete crack, as its thin there. Or should glue/adhesive be used instead?----------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------ 2) Maintain the perimeter XPS insulation running the entire length of the foundation wall. Fasted a pressure treated plate into the floor, and run regular studs up to a top plate. This would lose 2" of space, but have a better insulated foundation wall (2" rigid plus additional in the stud spaces) as well as allow for a more secure bottom plate fastening, and maintain 2" insulation along the perimeter, all at a loss of 2" of space. Additionally on the subject, how high should the vapor barrier run up along the wall, and should a capillary break be used (as well as how high/low to run this)? ------------------ wow, sorry for the run on. My browser isn't making paragraphs and looks like the worlds longest run-on


FBBPUser is Offline
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28 Nov 2015 07:33 PM
# two is far better. And don't forget to return the foam over the "2 1/2"" on top of the concrete wall. The concrete there will be just as cold as the outside air, so this strip now becomes the condensate panel, running condensate down into the wall.
In theory, you can glue and shoot the foam onto the concrete and then glue the drywall to the foam, saving the 3.5". But remember, less living space is better then big mouldy living space.


jonrUser is Offline
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28 Nov 2015 08:52 PM
I would lap the vapor barrier to the inside of the vertical wall foam - make sure that any water that comes down the concrete wall is directed to gravel. And the vapor barrier should continue under the slab (horizontally).


Edit: not my basement, just one I found to help visualize.


ronmarUser is Offline
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28 Nov 2015 10:16 PM
I like design #2 also except the vapor barrier detailed. I agree with Jon, Especially if you know you get moisture in thru that wall/footing joint. Ideally you would dig it out and seal it/drain it on the outside of the footing, but failing that, Extend the vapor barrier horizontally across under the XPS, tape all seams throughly. You could also tape the XPS joints. If it is a lot of water leakage, you may also consider an interior perimeter drain under that gravel. With a low pressure escape route down thru a drain, any water that passes in between footing and wall will be less likley to force it's way up along the vapor barrier and into your wall structure. Way easier and not a lot of cost to provide this before the concrete is poured, than after...


milnerptUser is Offline
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28 Nov 2015 10:37 PM
FBBP- thanks for the reminder.... for sure insulate along that top lip.... however I likely don't plan on using adhesive foam to foundation and drywall to foam.... Im not sure how well that would hold up.... Id rather have a sturdy studded wall placed.

jonr - I did mistakenly draw the vapor barrier, I meant to place it under the foam horizontally under the slab. What do you use to tape these seams?

As far as having it along the interior of the foam.... I guess taping and sealing the foam would serve as a barrier as well, correct? Would you only run the vapor barrier up a few inches along the foam as you pictured? (I assume above the level of the slab?) Would you 'weave the barrier between the horz/vertical foam panels in my diagram, or would you lay the foam panels opposite of me - (the vertical first, and butt the horizontal against it) and again, 'weave' the barrier between the foam panels to the inside?

I guess I don't see a big difference between placing the barrier on the outside of the foam, unless the barrier would allow for less evaporation or something.... both would be sealed up along the side of the wall to basically the top of the foundation wall.

ronmar - The water is extremely minimal, however I do think I will lay the French drain as a backup.... as its easy to do now, and impossible later. Exterior perimeter drain is already in place, the concrete is impregnated with some waterproofing agent.... however the foundation and footings were poured separately, and allow for some 'leaking'.... which is ultimately dampness, although no physical puddling is ever seen.


milnerptUser is Offline
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29 Nov 2015 12:19 AM
hard time adding pictures.... is this the idea? or run it all the way up along the interior of the XPS and attach it to the 2x6 at the top of foundation?

Attachment: thumb_1.jpg

jonrUser is Offline
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29 Nov 2015 12:26 AM
Water could come down the wall/foam intersection and end up on either side of the plastic. With the plastic running horizontally, it may have to dry through the concrete. Lap/weave it and it's clear which side it will be on.


milnerptUser is Offline
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29 Nov 2015 05:52 PM
Jonr - yes, I guess I was only considering water coming at the base of the foundation/footing interface (as that is the only non-waterproof area), and not considering possible condensation that may form on the wall, and trickle down onto the 'wrong' side of the barrier. I would thinking running the barrier all the way to the top of the wall would prevent this, although it would likely be easier (and I plan on) doing it the way you describe, along the inside of the rigid foam. Obviously you would not need to run the barrier very high - it looks as if yours is just a few inches above the level of the concrete pour? I assume you tape that edge to the insulation board with tyvek tape or similar?

Additional consideration -
Given that the foam board is up against the foundation wall, would you want to place anything like 'dimple sheeting' to allow for some air flow and evaporation, or is that really not necessary, as long as you tape all seams as well as seal off along the top of the wall?


jonrUser is Offline
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29 Nov 2015 06:47 PM
IMO:
a) the poly should be above the horizontal XPS (or EPS) and just below the slab - not underneath the foam. You can search for good discussions of this issue.
b) tape for overlapped poly to poly seams under concrete is not critical. Air sealing everything would be more critical if radon is an issue.
c) Extending poly up to grade level: it should reduce dehumidification needs slightly. And radon. Or it might provide a condensing surface if there is any inadvertent air movement from the interior, to the poly and then back into the interior. So I'm undecided.

You will use a floating wall?


milnerptUser is Offline
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29 Nov 2015 09:01 PM
a) Search feature doesn't seem to be working at the moment. I will read up on it, however given that I plan on stapling my pex onto my insulation rather than tying onto rebar, poking a bunch of holes in the poly with staples isn't going to do any good. Ive seen someone voice concern over moisture between the insulation and barrier affecting curing of the slab when poured, however I am working on an enclosed space.

b) Im not as worried about radon, just moisture. Wouldn't you want to tape either the poly or the insulation to assure no water can seep up? (although realistically I don't think Ill get that much water coming up, even in a flood. Also, wouldn't you want to tape the insulation seams on the walls if you are putting them directly onto the foundation walls, again as a failsafe to have a barrier between drywall and wall?

c) I see what you are suggesting.... moisture condensing on the interior surface of the poly and creating more of a moisture problem. I guess its a balance to consider.

floating walls - I haven't even gotten that far into plans, however after googling and watching some videos on it.... now I plan on building floating walls I really don't know how much heaving will occur under the slab.... the ground here has a pretty shallow frost line... but again, as a failsafe, its not a bad idea and is just a little extra work. From what I have read, floating walls along the interior are more critical than the perimeter walls (if needed at all?). Still trying to figure out if you build a floating wall, leave a gap at the floor with the drywall, how do you install baseboard and make it flush with the floor without it buckling upward if there is any heaving.


milnerptUser is Offline
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29 Nov 2015 11:29 PM
Searching and reading threads about layering of the vapor barrier is well summarized here
http://www.greenbuildingadvisor.com/blogs/dept/musings/polyethylene-under-concrete-slabs

It seems mostly related to the issue of blotter sand, and moisture getting between the foam and poly (if you layer foam on top). Given my placement will be enclosed, Im not worried about rain.... and plan to layer (bottom up) rock, poly, insulation, pex/concrete.

Additionally, I don't plan on adding flooring.... just finished concrete.

It also goes into detail of why you don't need to seal the layers as a vapor barrier, as you suggested. Although, im not sure if the foundation walls would be a different story.


smartwallUser is Offline
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02 Dec 2015 08:16 AM
Option 3 use an icf and make things simple


sailawayrbUser is Offline
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02 Dec 2015 01:31 PM
The concern with placing the vapor barrier below the EPS/XPS is that there will be water in the concrete mix that will seep down and get trapped between the vapor barrier and the EPS/XPS during the concrete pour. Since this water can not be absorbed and removed during the concrete curing process, it may cause near-term residual moisture in the slab that may adversely affect the floor finishing process.

Placing the vapor barrier above the EPS/XPS causes all the water in the concrete mix to be contained, absorbed and removed during the concrete curing process. This may result in a stronger slab and may minimize the likelihood of having near-term residual floor moisture that may adversely affect the floor finishing process.

The number of holes that you will likely introduce by stapling should not significantly effect the overall low moisture permeance capability of the vapor barrier.


Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
milnerptUser is Offline
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02 Dec 2015 11:26 PM
ICF - out. my foundation and footings are already poured.... im just getting the slab in there.

VB over versus under the poly... ive read up on this enough to not be terribly concerned. I think I may just tape the seams and call it good. no blotter sand is to be used, and Im not installing anything over the finished concrete flooring.

Not to get too far into that wormhole of a discussion, but it seems like there are several different aspects of concern that kinda get all lumped together... Out of curiosity's sake, can you think of any DOWNSIDE to having VB above AND below the insulation, if you are that anxious about the situation? VB comes by fairly cheap if it means sleeping well


FBBPUser is Offline
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03 Dec 2015 12:05 AM
The primary reason to use a vb below concrete is, as sailor alluded to, to protect the integrity of the concrete. You don't want the water (and cement) draining out of the fresh floor slab. As the paste and water drains out, you have a weaker finished product. The second reason for the vb is to prevent any capillary moisture creep up through the slab after the fact. Both of these can be attained by simply taping the joints of the foam, but it is much simpler to just put in the poly.
If you place it under the foam, you will still get a small amount of water weeping out of the fresh slab but not a significant amount. If the foam shifts during the pour, you might get small areas of uninsulated slab. If you place the poly over the foam and is shifts, these will just be voids, which is much better then concrete in contact with base.
If you do both, you will get some moisture trapped due to the few holes from the staples, but unlikely to make a significant difference.



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04 Dec 2015 04:20 PM
The primary reason we use VB is to keep moisture from entering our conditioned space.

We have many thousands of feet of PEX over XPS with VB on the bottom. We have many without the VB since the standard 2"XPS has a perm less than 1.

http://commercial.owenscorning.com/assets/0/144/172/174/e45fe07d-5cc9-4e4b-866a-5e35d75090ec.pdf

I used 6 mil under 2" XPS under one side of my slab-on-ground HRF building and XPS taped on the other. Perfect concrete on vapor issues on either side.


MA<br>www.badgerboilerservice.com
milnerptUser is Offline
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04 Dec 2015 10:17 PM
"The primary reason we use VB is to keep moisture from entering our conditioned space. " agreed.... it seems the argument some make is that the VB (on bottom) causes concrete curing issues. Now I don't have practical knowledge of this... and theoretically its feasible... however I don't feel it plays that big of an impact. Thank you for your input on what you did.... as I believe that's exactly the setup I plan on doing.


sailawayrbUser is Offline
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05 Dec 2015 11:16 AM
Vapor barrier has been placed EITHER above OR below insulation since time and memorial. Each contractor typically does it the way they have always done it and each contractor will tell you that their way is the best. We don’t know any contractors that would place vapor barrier BOTH above AND below the insulation.

A perm rating of 1.0 represents 1 grain of water passing through 1 square foot of material in 1 hour. One pound or 16 fluid ounces of water is equal to 7000 grains. If the building assembly layer has a permeance of 0.1 perms or less, it is considered vapor impermeable and classified as a Class I vapor retarder or a vapor barrier. If the building assembly layer has a permeance of 1 perm or less and greater than 0.1 perms, it is considered vapor semi-impermeable and classified as a Class II vapor retarder. If the building assembly layer has a permeance of 10 perms or less and greater than 1 perm, it is considered vapor semi-permeable and classified as a Class III vapor retarder. If the building assembly layer has a permeance of 10 perms or greater, it is considered vapor permeable (also referred to as breathable) and it is not classified as being a vapor barrier/retarder.

XPS being about 1.1 perms/inch would need to be over 11” thick before it would be considered vapor impermeable and classified as a Class I vapor retarder or a vapor barrier. EPS being about 2.7 perms/inch would need to be over 27” thick to be considered vapor impermeable and classified as a Class I vapor retarder or a vapor barrier. Typical polyethylene vapor barriers are about 0.06 perms. We don’t know any contractors that would consider only XPS or EPS to be an acceptable under-slab vapor barrier.

So it is really left to the astute reader to decide what makes the most sense for their unique situation.


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FBBPUser is Offline
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08 Dec 2015 12:15 AM
Read Martin Holladay http://www.greenbuildingadvisor.com/blogs/dept/musings/polyethylene-under-concrete-slabs.

Moral of the story. Use a low water ratio cement. Don't water it down to get it to flow, use a plasticizer (water reducer) if you don't want to work to hard.

All the old spec's will say - use blotter sand and never use a vapour barrier. These days we know better. Use just enough water in the mix to rehydrate the cement and use a vapour barrier to prevent the loss of this water. By the way, the last spec's I saw for Walmart store slabs called for no rebar and prefect concrete, no cracks. Right mix, right water cement ratio, right slump, right finish. No cracks.

If you can find a copy
H. Maynard Blumer, “New Justifications in an Old Debate: Specifying Vapor Barriers Under Slabs-on- Grade,” The Construction Specifier,
Construction Specifications Institute,
601 Madison St., Alexandria, VA
22314, February 1990.



milnerptUser is Offline
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08 Dec 2015 10:41 PM
FBBP:

Ive looked over the article before. Makes inherent sense. Already spoke to the concrete supplier and they suggested a plasticizer as well. Plan is for pea gravel, vapor barrier, 2" insulation (taped as an extra measure), stapled pex. Vapor running to the inside of the wallboard insulation.... which again will be taped.

concrete mix plan is for plasticizer. 6 bag cement, no rebar/mesh, fibermesh additive. Hard trowel finish, control joints. Not certain yet on stain versus colored concrete.

as far as vapor barrier goes, local guy is strongly advising of 10mm poly rather than 6.... not sure if its just a durability thing or what the real reason is... wouldn't think the water permeability is a concern, but walking around on it as you lay it down and cover with rigid insulation and damaging it may be where he is coming from.


again, thanks for the discussion and points from all aspects.


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