View Full Version : Basement XPS insulation gaps

06-07-2012, 09:57 AM
Hi I have these panels of XPS covered with OSB. The OSB is shiplaped and is a very tight fit. However the xps behind it has 1/8th gaps at the seams. Is it necessary to fill these seams if the OSB is tightly sealed over the seams. Here are some pictures.1654116542

06-07-2012, 08:15 PM
Because the XPS is acting as both insulation and a vapor retarder, it should be sealed with spray foam and tape at all joints.

Open seams such as that in your picture will greatly reduce any improvement which the XPS would otherwise provide.

Ideally, all your shiplap joints should be offset as to not align with the joints in the XPS.

06-08-2012, 09:19 AM
Laying a bead of low-expansion foam on the edge before butting up the next panel when installing panels on foundations & masonry walls works a heckuva lot easier than trying to fill it in later. The can-foam would air-seal it and fill in the gaps in R-value. But the 1/8" gaps don't mean much in the way of total moisture transfer or heat loss (the area of an 8' long 1/8" gap is only 12 square inches, after all), and half-inch OSB is semi-vapor retardent by itself, so even those 12 square inchs are only ~3-5 perms. Moisture transfer via vapor diffusion will be slowed signficantly by the OSB, but it has to be air tight to limit air-transported convection- making the seams of the OSB air-tight is more important than filling the foam gap.

Don't use tape on the OSB seams- it would fail (and quickly) unless you first painted 2" either side of the seam with a high-quality latex-acrylic primer to act as a bonding surface for the tape. It's quicker, easier and more reliable to use fiber-reinforced duct mastic slathered on ~1/8" thick extending 3/4" or more either side of the seam. At the top of the foam where it's accessible it's worth sealing with can-foam to prevent convection from any potential leakage.

BTW: Is this an above-grade wall or a foundation? If above grade, is the OSB on the exterior or the interior, and what is the total wall stackup from interior to exterior?

06-09-2012, 12:48 PM
Thanks for you help! It is below grade on an interior walkout blockwall foundation that is eight feet tall. Foundation is concrete block (8 inch) with full bricks mortared on the exterior. The panels are 2 inch of XPS and about 1/2 inch of osb with shiplaped edge on the osb. It is actually all already on the walls and I was posting to see if it would be beneficial to take down the panels and seal the seams with the spray foam or just make the osb airtight. I'm worried about the 1/8 " gap letting moisture vapor through to the OSB and creating a mold issue. I don't know enough about this stuff and convection currents to know if that is a realistic concern or not.

06-11-2012, 08:49 AM
The only material susceptible to moisture in the assembly is the OSB itself, which is essentially on the interior of the thermal & moisture boundary. So as long as you don't put a highly vapor retardent material up as a finish (such as foil/vinyl wallpaper or heavy alkyd paints), the moisture content of the OSB tracks the average humidity in the room.

The above-grade bricks produce a very high moisture drive when the sun hits it, but if there's an air gap between the brick and the concrete masonry unit (CMU) blocks it wouldn't likely create an issue. If the CMU cores are hollow, same thing- the air gap creates a partial moisture barrier. At 2" XPS is fairly vapor retardent- more vapor retardent than the OSB, so even if the CMU were saturated, the OSB would still be able to pass the moisture into the room without loading up and getting moldy. But if the gaps become an air infilration point it's theoretically possible that enough humid summertime air moisture could come through and condense on the exterior side of the OSB in a cool air-conditioned space, but air sealing the seams with mastic would be enough.

If you're going to finish the interior with wallboard, stagger the seams of the wallboard with that of the OSB, and use only latex/acrylic-latex paints as an interior finish. Wallboard is fairly permeable to water vapor, and standard latex paints are semi-permeable, 5-10x more permeable than the XPS, which is what you want in order to keep both the OSB and wallboard facers dry and mold-free.

06-11-2012, 09:23 PM
Thanks. I was planning on using 4x8 sheets of 1/4 inch paneling. Would that be ok if I staggered the seams over the mastic sealed seams of the OSB.

06-12-2012, 09:31 AM
Staggering the seams is BETTER than OK, as it further improves air-tightness.

06-12-2012, 01:52 PM
Is there a recommended mastic for OSB? Should I use fiber reinforcement tape with the mastic?

06-12-2012, 02:43 PM
Any tub of fiber-reinforced duct mastic from a box store would do. For this application tape buys you nothing over what the fiber mixed into the goop already provides. It holds up for decades under the thermal cycling and vibration of metal ducts, and will handle normal seasonal moisture cycling of the OSB, but not necessarily a full-on flood. It bonds better than most paints or caulking ever could.

The key is to find the word "fiber" on the label, which will protect better against cracking over time.




06-12-2012, 08:58 PM
Can I use the mastic to seal the tops and bottoms of the XPS as well instead of using sprayfoam?

06-13-2012, 08:37 AM
The can-foam would adhere better to the XPS, has similar vapor-retardency as XPS, and expands for a sure-seal, but sure, globbing mastic in there will work.

06-13-2012, 08:39 AM
Thanks for all your help.

06-13-2012, 10:08 PM
One more question. If I decided to take down some of the panels to fill the gaps could something like this work? http://www.emseal.com/products/Architectural/Backerseal/Backerseal.htm Or maybe cutting strips of 2 inch thick XPS to stuff in the gaps and then covering it with the fiber mastic before putting the OSB back up.. I'm trying to avoid using alot of sprayfoam as I read it has some toxic things in it like isocyanates.

06-13-2012, 10:30 PM
Isocyanates are only used in 2 component urethane foams...

06-13-2012, 11:56 PM
PRODUCT NAME : GREAT STUFF (TM) PRO Window and Door Gun Foam Sealant 20 OZ HC
MATERIAL TYPE : One component system
ISSUE DATE : 07/19/2007
REVISION DATE : 05/09/2007
Ingredient CAS Number %
Prepolymer of MDI and mixture 40-70, 60-100%
Polyether polyol
Polymethylene polyphenyl Isocyanate 9016-87-9 5-10,10-30%
containing approx. 40-50% MDI
(4,4'methylene bisphenyl isocyanate)
CAS# 101-68-8
Liquified Petroleum Mixture mixture 10-30%
containing Isobutane (CAS#75-28-5),
propane (CAS# 74-98-6) and
dimethyl ether (CAS# 115-10-6)

06-14-2012, 08:08 AM
Sure- most any acoustic sealant type caulk would work, and an low-expansion sealing caulk like Backerseal fills the bill.

Toxicity of the isocyanates used in 1-part gun-foam are pretty low compared to others in that class, but those few people who have an allergic sensitivity to isocynates probably shouldn't be the persons doing the installation. The total quantity/exposure is pretty low post-installation, given the extremely low surface area involved- nowhere near the same lingering exposure as spraying 2" of 2lb foam as the insulating layer (which is also pretty low.) When used solely for seam sealing, even very modest ventilation rates I'd be a bit surprised if it would be detectable even with a gas spectrometer in an air sample 10 days post-installation.

06-14-2012, 11:00 AM
Thanks for the info on the Great Stuff isocyanates. That makes me rethink using it alittle. If I decide to go with the Backerseal, does it matter that it is open cell foam between closed cell XPS foam. I read that open cell foam is more susceptible to moisture.

06-14-2012, 11:29 AM
Open cell is far more vapor permeable (passes more water as water vapor) than closed cell, but it's still air-impermeable, and is more flexible than closed cell. But in this application it doesn't really matter. Open cell foam is not damaged by liquid water, but it it will take on water when submerged, but it releases that water after flooding events. The OSB nailer face is far more susceptible to water damage than open cell foam. Your primary goal here is air-tightness, not water vapor tightness, since far more moisture can be moved via air-convection than through via vapor diffusion, even the most vapor-permeable of foam.

And the total amount of cross sectional area of the sealant facing the room is small- no matter what the vapor-drive is, the amount of moisture diffusing through literally ANY sealant would be of no consequence compared to even single square inch of air leakage.

06-14-2012, 11:51 AM
I really appreciate you taking the time to answer all my questions? Do you think the benefit/cost of taking down the panels to seal the foams gaps(I have 33 gaps total) is worth any benefit gained or do you think I should just seal the OSB with the fiber mastic you suggested earlier? If it was your basement, what would you do?

06-14-2012, 12:23 PM
So, with 1/8" wide x 8' tall you have 12 square inches per seam, times 33 seams is 396 square inches, or about 2.75 square feet, maybe half of which is above grade. The entrained air in a sealed gap won't convect much due to the narrowness, so it probably has an effective R value of at least 3 at that point, which is about the same amount of thermal bridging you'd get with 2x4 timbers. Adding all the gaps together is the same thermal briding of about three 2x4s- not a big deal.

Were it my basement I'd air seal the facers with mastic and plug the top (& bottom, if accessible) with can foam, forcing the tip of nozzle in a half inch or so and squirting until I see it clear the top. I'd also lay a bead of either can foam or acoustic sealant along the seam between the XPS and CMU (where accessible.) Stopping air motion behind the panels and through the panel seams is 100x as important to the thermal and moisture performance as any conducted heat loss or vapor diffusion issues through that ~400 square inches of seam area.

06-14-2012, 12:32 PM
Ok thanks. My wife will be glad to hear that. She wants to leave it as it is and seal with Mastic. So the 2 inch deep space doesn't factor in the cubic inches calculation? Just the 1/8 inch gap width x 8 foot height? It's amazing the detail that needs to be addressed rather than just throwing up some walls. Glad I found this site. Thanks again!

06-14-2012, 01:26 PM
The volume/cubic inches don't matter. The surface area (square inches) presented to each side of the assembly is what matters most for both heat & moisture transfer. (R-value is 1 divided the U-value, and U-value is denominated BTUs per hour square-foot per degree-F, permeance is denominated in grains per hour per square foot per inch of mercury vapor pressure difference. The volume of the material doesn't matter.)

If the gaps were wider, lets say an inch or more the model of convection within that mini-cavity gets more complicated and the effective-R would be lower, but in a narrow crack like that it can't convect as much. But if the gaps were that wide they'd be worth filling too, eh?

06-14-2012, 09:11 PM
Thanks again. Sure glad I posted here! This has been very educational and extremely helpful!

08-02-2012, 02:45 PM
Hi I recently made this post, but was unable to log in under my old user name, but I had another question. I haven't been able to do anything with the gaps yet, but am getting around to it now. I saw a Expanded Polystyrene Sill Sealer at my local hardware store. Could I cut 2 inch strips and place it in the gaps to seal them or would I still need something like a BG44 this at the very front just behind the shiplap? Would it be ok that it is Expanded Polystyrene sandwiched between Extruded Polystyrene? Here are some links to the products.

http://www.conservationtechnology.com/building_gaskets.html 16959

08-03-2012, 03:08 PM
It's fine to sandwich those materials in, if you like- they're quite chemically compatible.

08-03-2012, 03:13 PM
It's fine to sandwich those materials in, if you like- they're quite chemically compatible.

Thanks. Do you think I should use both or just the sill seal and mastic over the front of the panels?