Spacing Subfloor OSB Sheets
Is it necessary leave a gap between OSB panels when using them as for a subfloor assembly in a basement?
My subfloor will be composed of Delta-FL then 1″ XPS foam, followed by OSB sheets. The whole thing will be tapconned to the slab. Finished floor will be floating vinyl plank.
In the Delta-FL instructions they don’t say anything about leaving spacing in the panels but the APA (Engineered Wood Association) says that OSB expands and contracts with changes in humidity levels and can buckle if a 1/8″ gap is not left between panels.
We sourced high compressive strength XPS because the room will have heavy exercise equipment in it and I’m concerned about about the ability of the vinyl plank to bridge the gap.
Thanks in advance for any advice.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Patrick,
With the stickup you have on your floor I doubt tightly butted sheets will cause a problem. That said, I can't imagine a situation where 1/8" gap would make any difference to the floor's ability to withstand heavy loading. 1/8" is in the range of gap you might find in spots when you try to tightly butt a subfloor.
I would be more worried about the p0int-loads compressing the foam near the Tap-cons causing the heads to sit proud. Maybe floating two layers of subfloor would work better?
Thanks for the quick reply Malcolm. I'm not too concerned about point loads. I order special high compressive strength foam that is used under roads, etc. It's rated to 60 psi compressive strength. While installing it, I did find that it would indent a bit if for example, you put your whole weight on an elbow or something like that but I would think that the OSB and the 8 mm thick vinyl floor we will have over top of it should be able to spread the weight enough to prevent compression.
One mistake I may have made though was to order regular flat edged 7/16" OSB (the minimum thickness allowed by Delta-FL) instead of tongue and groove. From what I've read, most people use tongue and groove. Do you think that will be a problem? We've installed a small section (no tapcons yet) and there doesn't seem to be much of a ridge (lippage?...not sure if that term applies to OSB) between the OSB boards.
Regarding the spacers, I think I'll go ahead and use them...better than to have a wavy floor at some point in the future.
Thanks again for any thoughts on this.
-Patrick
Patrick,
60 psi foam? That will sure do the trick - and you are fine with the standard OSB. The T&G is only on two faces of the sheets anyway. You may find the edges compress a bit under the Tap-cons as they will be exerting very high p0int loads, but vinyl plank flooring is very forgiving of small imperfections in a subfloor.
Deleted
Would have been better to skip the very high PSI foam. And skip using XPS (it's bad for the environment) - use EPS (perhaps 25 psi). With some easy testing (not elbows direct on foam), you can assure yourself that you won't have a foam compression problem.
Delta-FL is only "< .3 perms" and foam doesn't reduce permeability much more. Vinyl is typically much lower perms. Are you sure that your OSB won't be caught in a damp moisture trap?
So I ended up upgrading to 3/4" T&G plywood. The Big Box store was kind enough to let me exchange my uncut sheets. It made a big difference to the stiffness of the floor. The floor has some unevenness to it and the 7/16" just seemed to conform to the bumps a bit too much. The tongue and grooves also seemed to help. I left the recommended 1/8" spacing on the short sides of the boards or anywhere else that there was no T&G joint.
Malcolm...Yup 60psi. I got it from a dealer that specializes in concrete supply. The foam is made by Owens Corning (Foamular 600) and I understand it's usually specified for under roads, runways, etc. Unfortunately I couldn't find EPS foam in a similar compressive strength and the strength is a bit of a concern because I'll have a home gym down there (think treadmill legs and the occasional dropped dumbell).
Jon, I take your point about a moisture sandwich but my hunch is that I'll be okay. With the foam layer, the OSB will always be above the dew point so there's little chance of condensation due to warm moist interior air contacting a cold surface. I was careful to seal all the joints, foam the edge joints, etc. to prevent any air leaks. Also, the Delta-FL is designed to block vapour transmission from below so I would think that the OSB will not build up more moisture than can be dissipated from between the joints in the vinyl plank. The only way there would be a serious problem is if I have a flood but it that case the OSB is probably toast anyway. I've taken every precaution in the basement to prevent floods (backwater valves, new weeping tile and exterior waterproofing around the whole perimeter, leak alarms, etc.) so I'm willing to take a chance on it.
I'm curious to know whether I've got the physics wrong though...I'm still trying to learn about building science concepts.
The only question I have left now is whether to follow the Delta-FL instructions and put silicone in the tapcon holes. The GBA article "The Stay-Dry No-Mold Finished Basement" doesn't suggest a need for it and I'm finding that it's a serious time suck. Any thoughts on this? I guess the key question is whether the foam will seal adequately around the tapcon threads to prevent any significant air movement.
Thanks again for all the comments.
-Patrick
Thanks for all the comments everyone. So far my subfloor seems to be turning out great.
So I ended up upgrading to 3/4" T&G plywood. The Big Box store was kind enough to let me exchange my uncut sheets. It made a big difference to the stiffness of the floor. The floor has some unevenness to it and the 7/16" just seemed to conform to the bumps a bit too much. The tongue and grooves also seemed to help. I left the recommended 1/8" spacing on the short sides of the boards or anywhere else that there was no T&G joint.
Malcolm...Yup 60psi. I got it from a dealer that specializes in concrete supply. The foam is made by Owens Corning (Foamular 600) and I understand it's usually specified for under roads, runways, etc. Unfortunately I couldn't find EPS foam in a similar compressive strength and the strength is a bit of a concern because I'll have a home gym down there (think treadmill legs and the occasional dropped dumbell).
Jon, I take your point about a moisture sandwich but my hunch is that I'll be okay. With a perm rating of <0.3, the Delta-FL is close to the threshold of being a vapor barrier and Dorken specifically markets it as a way of preventing vapor diffusion up from the slab from damaging moisture sensitive flooring materials. I'm no building scientist though so I would be interested to know whether there are some mathematical formulas or something that could be used to be more precise about this. From the top side, the 1" of rigid foam is enough to prevent the OSB from falling below the dew point so condensation should not ever be an issue. Also, although vinyl has low permeability, I think the tongue and groove joints between the planks would allow air and vapor to pass through pretty easily. We see this all the time where people put T&G planks on vaulted ceilings without an air barrier behind them - this frequently causes massive amounts of condensation in roofs due to moist air just streaming through the joints. Short of a flood (in which case the OSB is toast anyway), I just don't see how the OSB could build up more moisture than can be diffused out through the cracks in the vinyl plank floors.
The only question I have left now is whether to follow the Delta-FL instructions and put silicone in the tapcon holes. The GBA article "The Stay-Dry No-Mold Finished Basement" doesn't suggest a need for it and I'm finding that it's a serious time suck. Any thoughts on this? I guess the key question is whether the foam will seal adequately around the tapcon threads to prevent any significant air movement.
Thanks for all the comments everyone. So far my subfloor seems to be turning out great.
-Patrick
If we take a probably over simplistic approach and say that cracks are 1% of the floor and that OSB is 5 perms and vinyl is 0 perms, then I get .05 perms of drying - which doesn't compare well to ~.3 perms of wetting. Perhaps someone has actual data on the effective vapor permeability of vinyl plank over OSB as an assembly.
See here for concerns even with high perm flooring.
There are some tapeable underlayment products out there for use under hardwood floors. Maybe one of those would also be suitable for use under vinyl plank flooring? Many of the underlayment products are also intended as vapor barriers to help protect the hardwood floors they’d normally be used under.
Bill
This article suggests that vinyl siding has a perm rating of 40 due to air leakage at the joints:
https://www.buildingscience.com/documents/information-sheets/info-312-vapor-permeance-some-materials
It's not really a perfect comparison though because I suspect the joints in vinyl siding are much more leaky than the joints in vinyl plank. Nonetheless, it seems that air leakiness plays a big role in determining permeability.
The other factor is whether or not there's vapor drive. I've done the polyethylene sheet taped to the concrete test and got absolutely no condensation on the plastic. I suspect this is because we had the entire perimeter of the basement excavated and new weeping tile was laid below the level of the footings, providing very good drainage below the slab. We had to open up the slab to reroute some plumbing drain pipes and the drainage stone beneath it was dry to the touch. So we might not even have enough vapour drive to push humidity up through the Delta-FL.
The other issue I forgot to mention is that the airspace below the Delta-FL is connected to the the unfinished utility room so that humidity below the delta-FL might be able to dissipate into the room air. In some cases, it's a long distance (e.g., 30 ft) but it's there. When we framed the walls, we used a product called Floodsill below the bottom plates. It's basically 2 3/4 inch thick EPS foam blocks with air flow / drainage channels carved into the bottom of the blocks. The Delta-FL is taped to the edges of these (tape attaches to the top of the Delta-FL and side of the floodsill blocks) so that the air space under the Delta-FL is connected with the airspace under the floodsill and since the utility room has bare concrete, the air space is connected with the room air via the utility room.
Update on this. I've now had this installed for several months and through a bit of the spring thaw here in Ottawa. No signs whatsoever of moisture issues, though as I noted above, we had a basement fully waterproofed from the outside with new weeping tile installed before we did the renovation so our slab might be drier than most. Also, Dorken has now updated the specs for Delta-FL. The permeance is now <0.02 perms, which is lower than thick polyethylene sheeting. I've sent a message to Dorken to see if the new spec reflects physical change to the product or just a more precise testing procedure. Will update further once they get back to me.
Agreed on the bot post. But it reminded me to provide an update on what I heard back from Dorken. Apparently there was no change to the Delta-FL product they simply pursued a more accurate testing method and were able to be more specific about the permeance. So I'm fairly confident that I won't have problems with vapour passing through the Delta-FL and wetting the OSB subfloor.