Subfloor Question

[pcarpe01]

I have been searching high and low for the answer, even called the support line for the insulation I plan on using. I live in NY and remodeling a small 20' x 8' room that is above a vented crawl space. I plan on using Roxul insulation between the joists that does not have a vapor barrier. Roxul recommends plastic as a vpaor barrier for my area. The sub floor is all new plywood. I would like to glue and use ring shank nails to fasten the plywood directly to the joists, however, if I use a plastic vapor barrier over the top of the joists (under the plywood), the glue will not be effective as I will be essentially gluing the plywood to the top of the plastic.

Any recommendations on what I should to are appreciated. Thank you in advance!

-Paul

 

[pcarpe01]

To add....I am not looking to convert the space to conditioned.

 

[Sponsor]

 

[Dana]

I have been searching high and low for the answer, even called the support line for the insulation I plan on using. I live in NY and remodeling a small 20' x 8' room that is above a vented crawl space. I plan on using Roxul insulation between the joists that does not have a vapor barrier. Roxul recommends plastic as a vpaor barrier for my area. The sub floor is all new plywood. I would like to glue and use ring shank nails to fasten the plywood directly to the joists, however, if I use a plastic vapor barrier over the top of the joists (under the plywood), the glue will not be effective as I will be essentially gluing the plywood to the top of the plastic.

Any recommendations on what I should to are appreciated. Thank you in advance!

-Paul

IRC code min for floors over vented crawls or pier foundations in your US climate zone 4A location is R19, but R19 batts compressed to 5.5" in a 2x6 bay only performs at R19, and is barely more than an air-filter for resistance to air infiltration currents. A rock wool batt designed for 2x6 framing is R23 and FAR more air retardent, so you're thinking along the right lines.

First be sure to caulk all the seams of the band joist & foundation sill with polyurethane caulk. Use can-foam for any gaps bigger than 3/8" or so. Air leaking into or by the cavity insulation undercuts it's performance. And a square inch of air leaks moves far more moisture than vapor-diffusion through 1000 square feet of 3/4" plywood subfloor.

The only time moisture accumulation in the subfloor would be a problem in your area during the cooling season, when humid summertime air with an average dew point north of 65F enters the crawlspace (which is much of the summer.) In the winter the air in the crawl space has a dew point much lower than the temperature of the (warmer, insulated) subfloor, which will tend to dry it out. A couple of alternatives:

A> If you paint the bottom side of the subfloor with "vapor barrier latex" it will reduce both the drying & moisture accumulation rates of the subfloor. If the crawl space has ground vapor retarder to limit the ground moisture from entering the crawl space and the vents are open in winter it offers quite a bit of protection- the moisture content of the joists will rise and fall seasonally (which is potentially an issue for the joists), but the subfloor's moisture content will stay pretty stable. To prevent thermal bypass air currents, the rock wool needs to be snugged up to the subfloor, with no gaps. Having it snugged up like that keeps air from moving between the insulation & floor, but there will be some temperature striping on the floor over the joists detectable with bare feet during the coldest weather.

B> If you leave the subfloor unpainted and fill the joist bays completely (are they 2x6? 2x8? 2x10s? 2x12?) A layer of 1-2" foil faced polyiso cap-nailed to the bottom of the joists, seams taped with foil tape, and edges caulked to the foundation sill with polyurethane caulk (or can-foam) is more vapor retardent than 6 mil polyethylene, and will reduce the temperature striping issue completely. It takes at least 1" of polyiso to double the R-value of a 2x6, and 2" to double the R-value of a 2x12. More is always better, of course. Thinner polyiso can still deliver the moisture protection, but for the amount of effort it takes to hang sheet foam anything less than 1" hardly seems "worth it", and may not have sufficient R value to prevent condensation/frost at the foam/fiber boundary during winter.

If it were my house I'd be inclined toward B, unless it's simply impossible to get the sheets of foam in there. A 4' x 8' sheet of foil-faced goods can be scored at the mid-point with a utility knive and folded on the other facer into a 2' x 8' to fit through a 26" wide vent space, assuming there's sufficient room to unfold it there. Before the subfloor goes down it might be possible to slip them in from the top side in full 4' x 8' sheets, in which case it would be worth caulking them to the framing from the top side for enhanced air tightness, installing the batts from the top side prior to installing the subfloor.

What you DON'T want to do is to skip the foam or VB latex and staple a sheet of 6 mil poly on the underside of the joists. While that is protective of the subfloor in winter, it's all but guaranteed to be wet in winter when it's on the cold side of the assembly.

With a more detailed description of the floor assembly there may be other options that can work too.

 

[pcarpe01]

Awesome - thanks for the detailed reply. The joists are 8" and I have a decent amount of plumbing under the joists - copper for heat, water to washing machine and drain. So is looks like option b is out of the question. Are there other options? Can't I just lay down the barrier on top of the joists, then plywood and screw using SPAX? Or is gluing definitely preferred?

 

[Dana]

If you have plumbing under the joists it will be at high risk for freezing up during cold snaps once you insulate the floor. Putting the heating plumbing on the exterior of the insulation boundary of the house guarantees that the heat loss from that plumbing is truly lost, and does not heat the house. (Distribution losses inside the house are really just doing some of the heating with a long skinny radiator called a "pipe".)

Are you SURE you don't want to put down a ground vapor barrier, insulate the foundation wall, which makes the crawlspace nominally conditioned space? That would keep the potable pipes above freezing, and the drain won't ice up. Drains are a real PITA to thaw once they have become ice clogged.

Gluing the subfloor is the tried & true long term prevention of floor-squeaks, but screws can work pretty well too (far better than nails.) A sheet of poly under the subfloor would also be a slip-surface for any motion of the wood, making it less squeaky too.

Without any bottom side cladding there is potential for periodic summertime wetting of the poly sheeting if the space is being air conditioned, since the subfloor will spend at least some time below the outdoor dew points (which are in the mid 70s F right now on L.I..) With the polyethylene vapor barrier the subfloor is protected, but there will be some mold/rot potential to the joists. If there were sheet OSB or plywood layer the bottom edges of the joists it would behave as both an air barrier and a "smart" vapor barrier, limiting the amount of summertime moisture that can accumulate in the joists. This is a bigger problem for vented crawlspaces in the (even more humid) southeastern US coast or gulf coast states, but coastal NY/NJ is not immune. (If I were the Code Diety I'd outlaw vented crawlspaces in the eastern US. On average they bring in more moisture than they purge in these climates.)

Air sealing and insulating the foundation sill & band joists, a ground vapor barrier sealed to the foundation at least 6" off the floor and a continuous layer of 1.5-2" of foil faced polyiso (R10 is code-min, 1.5" polyiso is close enough) strapped to the foundation with 1x4 furring & TapCons, with a couple of floor grilles in some inconspicuous places, to potentially actively ventilate the crawl if it ever flooded or got damp from a water or drain leaks, or a leak in the floor vapor barrier) would fix the freeze-up potential and fix the seasonal wood moisture issues.

 

[Jadnashua]

Using construction adhesive between the joist top and the subflooring makes a HUGE difference in the flooring system. Ring shank nails (cheaper and faster than screws) will work fine, as once the adhesive cures, the fasteners are mostly extraneous. Without the adhesive, you don't really create a fully integrated flooring system. The resonance of the whole floor will be lowered when you use adhesive, and squeaks should be non-existent, at least in the floor (maybe not in how the walls are attached to it). Making the floor a monolithic structure helps to distribute the loads better.

 

[pcarpe01]

Thank you. All excellent responses. So I will plan to semi-encapsulate the crawl space, meaning, floor vapor barrier and polyiso on the walls. I would still love to glue to sub-floor, but unsure about vapor barrier that gets installed on top of the joists. Can I just install the barrier from underneath the floor, against the sub floor and seal at the joists? Or should I just use the a vapor retardant paint?

 

[Dana]

Thank you. All excellent responses. So I will plan to semi-encapsulate the crawl space, meaning, floor vapor barrier and polyiso on the walls. I would still love to glue to sub-floor, but unsure about vapor barrier that gets installed on top of the joists. Can I just install the barrier from underneath the floor, against the sub floor and seal at the joists? Or should I just use the a vapor retardant paint?

With the vents blocked, a ground vapor barrier, and air-tight R9+ polyiso on the walls neither need nor want a vapor barrier on the joists. Go ahead and glue the subfloor to the joists without any vapor barrier or vapor barrier paint.

With insulation on the crawlspace walls you also don't need or want insulation in the joist bays, which wouldn't save much energy, but could lower the temperature in the crawl space to potentially mold-hazard levels in the summer. Once the crawlspace walls are insulated, insulating any hot water distribution plumbing or heating plumbing with R3 would likely save more energy than adding R20-R23 between the floor joists.

With polyiso foundation insulation it's doubly important to be sure the ground vapor retarder is affixed to the foundation, between the foam @ foundation, a half-foot or more above the bottom edge of the foam board. Polyiso is hygroscopic, and if the cut bottom edge of the foam is on the dirt-side of ground vapor barrier there is potential for it to wick moisture over time, losing performance. Depending on how clean the foundation is the vapor barrier can be either sealed to the foundation with housewrap tape prior to installing the foam, or with duct mastic (which is messier and slower.) When installing the foam, a bead of polyurethane caulk or foam board construction adhesive on the flap of vapor barrier will seal the exterior side facer of the foam to the vapor barrier, for an air & water tight seal.

Some code officials would insist on a thermal barrier against ignition for any foam board in the crawlspace, despite the lack of an ignition source. Sheet rock nailed to the horizontal 1x4 strapping that's keeping the polyiso in place would qualify, but there are a couple of vendors of fire rated polyiso with beefier facers that would also pass muster in most building codes which would save time, and maybe money too. The fire-rated polyiso is more expensive than the standard stuff. Dow Thermax is one such product, but I believe Rmax also has fire-rated versions. If it's going to be inspected it's good to know which way you're going to go on this before you buy the stuff. Even standard grades of foil faced polyiso without the fire ratings are pretty safe in this application- safer than polystyrene (EPS & XPS) due to having a higher kindling temperature, and the fact that it doesn't melt even when fully engulfed and burning.

 

[Reach4]

Can I just install the barrier from underneath the floor, against the sub floor

Do you mean between the prefinished oak flooring and the subfloor?

 

[Dana]

Do you mean between the prefinished oak flooring and the subfloor?

There shouldn't be a vapor barrier there either.

A vapor barrier blocks the transfer of water vapor via diffusion through the layer. Sometimes that's useful for protecting moisture-susceptible materials when there is a large humidity or temperature difference on different sides of the assembly, but it also blocks drying of those moisture susceptible materials. With the foundation walls insulated and vents blocked the temperature on the bottom of the subfloor will be very near that of the top of the finish flooring, and without the outdoor air infiltration sealed off and a ground vapor barrier on the floor of the crawlspace the absolute humidity levels should be about the same too, especially if there are a couple of floor grilles to allow at least some air transfer between the crawlspace and room above.

The subfloor is a fairly stiff vapor retarder- less than 1 perm but it is not a true vapor barrier, moisture can move via diffusion through the floor. But any significant moisture that manages to find it's way into the crawlspace can't dry quickly via vapor diffusion alone. The total volume of this crawlspace is pretty low- probably less than 500 cubic feet, and not big enough to need continuous active ventilation with fully conditioned air from the room above. (A conditioned crawlspace under a whole house usually does.) Even at 1 cfm it would get about 7 complete air exchanges per day, far more than would be needed to keep the humidity levels in the crawl space about the same as in the room above. But monitor the humidity down there with a cheap AcuRite or sniff the air at the floor grilles occasionally (does it smell like "musty basement"? ) until you're sure it doesn't need at least SOME active ventilation.

If it's chronically at 75% RH (any temperature) at the bottom of the joists or just under the subfloor it might be worth installing a tiny ventilation fan running at some duty cycle on a timer to force the air exchange. The air at the uninsulated crawlspace floor's vapor barrier will be much cooler, with a higher RH even at the same absolute humidity, but the ground vapor barrier isn't moisture susceptible- the wood is. The temp at the ground vapor barrier will probably run about 55F, and in a 70F room the subfloor will probably run about 65F. A reading of 75%RH @ 55F on the floor would not be a problem, since the same air would run about 53% @ 65F, well below the level that would constitute a mold & rot risk.

 

[Bearhunt]

Take an airless and spray a vapor barrier paint underneath and be done

 

[Dana]

Take an airless and spray a vapor barrier paint underneath and be done

No, DON'T impede the ability of the subfloor to dry into a crawlspace by painting it with anything- least of all vapor barrier paint!

Vapor barrier latex has about the same vapor retardency of a 3/4" plywood subfloor, but unlike plywood it doesn't become significantly more vapor open when the moisture content of the subfloor is high. With an insulated walls and a ground vapor barrier on the floor of the crawlspace, vapor barrier paint buys you nothing, but makes the assembly less resilient to moisture, cutting the drying rate by about half.