V-Retarder BELOW Joists and Oak Floor ??
I’m thinking that I need to do a little crawl space renovation beginning with an experimental area below the kitchen as I’m preparing to install 3/4″ prefinished oak strip flooring there.
We’re in mixed-humid southwest Arkansas, so theroetically any vapor retarder should be on the outside. Currently, the VR of poorly installed FG batts in floor joists is facing the crawl space. Also, the kitchen has had a double layer of vinyl over particle board underlayment, over 1/2″ ply, over 8″ joists 16″OC since 1982, no problems. Our crawl has no unusual moisture problems. We’re on the top of a hill, good drainage away from the house, about 1/2 of the dirt floor is covered with poly. I’ll finish the other 1/2 as part of this project. Biggest source of humidity is air coming in the foundation wall vents, but one I’m not immediately concerned with. Oh yeah, the metal duct work is down there, insulated with 1″ FG.
From what I’ve read regarding vapor movement, it sounds to me like vinyl flooring should be a no-no over a vented crawl space in our climate (especially with FG batt insulation), as it acts as a VR on the inside of the assembly. However, this apparently has not caused problems with the outside humid summer air circulating through the FG batts and condensing on the bottom of the subfloor. Still, I’m hesitant to install my oak flooring while there is any chance of moisture/vapor migration issues. I’m wary of the ability of tar paper to hold off moisture migration long term as I’ve read horror stories of floors cupping even over tar paper, due to other issues. So it sounds to me like the tar paper is plan B, and I need a plan A but I’d personally rather not go the sealed crawlspace route with this home; $ reasons.
What would be your choice:
Option #1) Since the vinyl inside hasn’t caused a problem in 27 years, why couldn’t I put down poly (instead of tar paper) below the flooring? Everything else in the assembly stays status quo. This just plain sounds wrong.
Option #2) Since it is theoretically proper to have the VR on the outside, how about I detail the floor like a wall? I’ll overfill the joist bays by 1/2″ or 3/4″, glue 2x strips of foam on the bottom of the joists as a thermal break, and put a sealed VR below that (or use foil face EPS like the walls exterior). No VR directly below the oak flooring.
Option #3) Same as #2 but with 15lb. paper below the oak flooring.
Option #4) Same as #3, but use tyvek instead of a VR below the joists.
I’m leaning towards #3, even though I’m still a little wary of the double VR … but that’s pretty much what I have in the walls now (T-111, faced EPS, FG batts VR facing inside, DW, paint), also no problems in 27 years.
Unconfuse me PLEASE before I screw up.
Replies
OW ...
bumped myself.
Any ideas are much appreciated.
couple of comments:
1) You don't mention what is on the crawl space floor. I'd recommend a good vapor barrier. At a minimum 6 mil black plastic lapped 6 to 12" an any joints, and lapped up the walls 6 to 12" and glued to the walls. Even though you feel your CS is pretty dry, there likely is moisture comming up through the ground.
2) re the paper vapor barrier on the FG batts, I wouldn't worry about that too much, as it really isn't a very effective vapor barrier.
1) definitely scared not to finish the VB on the crawl floor as part of this project ... but I do wonder if it'd be necessary if I used poly below the joists. In that case, the dirt floor may even be an escape route for any condensation on the ducts (yes, I'm trying really really hard not to listen to myself convincing myself that a $$$ sealed crawl is what I need to do, but other people get away without it so ... ).
2) Yeah, the paper on the batts, not so much help. Any ideas on what would be a better VR that would still give way to any leaks/spills that find their way that far? Guess I could put the asphalt paper on the bottom side. Hmm.
>> if it'd be necessary if I used poly below the joists. <<
No - don't do that.
>> 2) Yeah, the paper on the batts, not so much help. Any ideas on what would be a better VR that would still give way to any leaks/spills that find their way that far? Guess I could put the asphalt paper on the bottom side. Hmm. <<
No - don't do that either. I don't know why you think you need a vapor retarder at that location. I think you said AR, mixed heating/cooling climate. VB would normally go toward the conditioned side. Here, in NC, a mixed heating and cooling climate, VBs are out of style. Preventing moisture infiltration from the outside is is style, as is properly handling moisture point loads within the home - ie proper exhaust fans in kitchen and baths.
Since you have said you are strpping down to the plywood, do a good job detailing the vapor barrier on the crawl floor. Then lay tar paper down over your subfloor, and then your hardwood. Be aware though that any wide board hardwood is subject to moisture issues. If you want to be safe, get a moisture meter and check the subfloor. You will be looking for around 10% (or less) MC.
If you are just itching to spend money, go with the sealed CS. Seems like you are in the "if it ain't broke then don't fix it" situation though.
How often is your air conditioner running?
Do you plan on pulling up the vinyl flooring or going over it with the wood?
Yes, I'm in the process of stripping the floor down to the 1/2" 3ply which I plan on replacing with 1/4" ply and 3/4 x 2.5" prefinished hardwood.
I'm curious what insight makes you ask about the AC running ... We have a 3.5 ton packaged heat pump serving 1300 sq feet and it runs, IIRC, about 8 hours/day during the hottest part of summer (we're gone 7 hours/day). Think we can improve on this, but that is another post for the most part, and it does a good job keeping the humidity down inside (i.e. huge differential in the summer). The crawl space stays fairly cool in summer and just slightly warmer than outside in the winter) since the ducts are down there. I partially close the foundation vents for the months (3 almost) of winter.
I do currently have what I'd call a light "dusting" of dry mold on some of the joists and subfloor that was never insulated (don't know why). Finishing the VB on the dirt floor may help with that some, but I think a thermal break below the joists will help the most.
I have several worries in making this decision and this being a floor instead of a wall is what confuses me (I know, I know, the conditioned crawl concept is the best answer).
1st, I don't like having two VRs sandwiching the joists (asphalt paper below oak and the foam or poly below the joists), so I think maybe the best scenerio is to skip the VR below the strip oak and just depend on the one below the joists but I'm looking for opinions/options before I go that route.
2nd, I wish I could depend on the kraft face below the FG batts as it would give way to any leaks/spills that might find their way into the floor assembly, but I don't think I'd be comfortable with that even if I detailed it really well and so I'd want to put a better VR on the topside below the oak ... but that's on the wrong side ... but the vinly hasn't caused a problem in 27 years.
I'm chasing my tail here. Unless I'm missing something, I just have to choose which risk I'm willing to take in exchange for not going the sealed crawl space route.
I found this article from buildingscience about vented crawl spaces that has just caused me more questions: http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/?topic=/doctypes/building-science-insights.
Any other alternatives you've seen that work?
Are there still any places in humid climates where codes don't allow sealed crawl spaces? What do they do? They shut-up and get back to work for one thing. Good grief I've written a book here.
Thanks for any insight.
The reason I asked about your A/C use is to get a better idea of how much difference there is between the inside and outside temperature and humidity. I've never been to AR; I would think it's pretty sticky there but I'm frequently surprised at the different microclimates around the country.
With that kind of A/C use, it tells me that there is a fairly strong vapor drive from outdoors to indoors. The fact that you have had no problems with mold or rot probably means that the fiberglass batts are allowing the joist bays to dry to the crawlspace side when the weather settles down. Without the vinyl flooring, I think you might see problems with cupped flooring as the outdoor humidity tries to find its way in to your nice, conditioned living space.
Like Matt (and the Building Science guys) said, the best route is to do a sealed, conditioned crawl space. Make it part of your living space. If air leaks are sealed and you have the proper amount of insulation, you will pay very little extra in energy costs, and could even save some money, after the initial work is complete.
If you don't want to go the sealed crawlspace route, you might consider installing EPS foam (borate treated if possible) on the bottom of the joists. EPS will allow some vapor to pass through, as long as it doesn't have a mylar coating. Or just continue with the system you have, though I would recommend improving on it. By the way, tar paper isn't really a vapor retarder; it's designed to allow water vapor to pass through while excluding liquid water, so while you are smart to be concerned about a double vapor barrier, in this case it's unfounded.