I’m faced with a remedial insulation challenge in a sealed crawl space (20″ minimum crawll room with most ~ 36″) where the rim joist areas are developing signifigant moisture condensation.
This is a pressure treated wood foundation that has 6-mil poly placed on the dirt, and steps are being taken to better seal that ground vapor barrier at the edges. There is no evidence of groundwater penetration, as the outside of the PTW was backfilled with pea gravel with socked drain tile at footing plate level and the ground slopes away from foundation adequetely on the outside. Ground under the VB seems mostly dry, some moist, but certainly to standing or running water. We have had heavy rains this summer and fall.
We originally (2 years ago) just stuffed unfaced FG batts in the rim joist spaces, which now obviously is not adequete to control condensation in those spaces as well as on the inside portions of the PT plywood sheathing that are above grade.
I had a couple sheets of 1″ (R 6.5) foil faced poly-iso left over and have cut pieces to fit snuggly insome of the worst rim and stud spaces where moisture appeared, and also used can foam to seal the corners. So far, it seems like that has stopped the condensation, although the true test is today with outside temps in the single digits and inside crawl space at 62*F with 52% RH. There is a radiant heated floor sandwiched on top of the subfloor, with faced batts of FG in the joist spaces.
Real question is should I use more foil faced 1″ poly-iso to insulate the remaining spaces, or should I go with 1 1/2″ extruded poly styrene? Then, after the rigid foam is in and sealed, should I replace the FG batts in there and seal with poly VB?
Concerns are that the foil faced will act too much as as a vapor barrier, and perhaps the extruded will be a better match for this application?
Of course, in retrospect, I should have had those rim joist and stud spaces professionally fomed with closed cell urethane before the subfloor went down and I suppose I could retro-foam these spaces now. I have searched on this topic already but couldn’t come up with a matching situation. I’ll be happy to supply more information if needed. Thanks in advance!
Replies
Hi,
I don't know the direct answer to your question, but
The extruded polystyrene will not absorb moisture or pass moisture through it. Its better than Polyiso this way.
The Polyiso will absorb some moisture -- that's why they put the foil faces on it.
Since you are effectively turning the crawl space into a conditioned space, I don't see why the foam board acting as a vapor barrier would be bad, but maybe I'm missing something. On my crawl space, I used 2 inch extruded foam board and foamed around the edges with the cans of "Great Stuff" PU foam. I've had no problems, but I live in a very dry climate.
Gary
http://www.BuildItSolar.com
Thanks for the response...it's just that I heard or read somewhere that extruded polystyrene will pass a small amount of moisture, not much, but more than polyethelene that's typically used as a vapor barrier.
To get 2" in a rim joist space would be challenging, unless under cut 1/4" or so all the way around...how did you keep it in place until you foamed the edges?
Hi,
I cut the blocks so that they were a bit of a friction fit, and then buttered the back with a bit of the PU foam, pushed them in, and then put more foam around the edges -- that seemed to work fine -- I'd hate to have to take them back out :-)I agree that if you have to trim the 2 inches to make them work that its not worth the effort.Gary
The polyiso is more expensive than extruded.
Polyiso has a higher R-value per inch, but over time outgassing causes the polyiso to lose much of that advantage. I suggest the extruded as a better value.
If you have much of this work to do, you will want a foam gun
Edited 12/5/2005 3:53 pm ET by csnow
Thanks, but FWIW< Just looked at HD 1" foil poly-iso @ $14, versus 1 1/2 extruded @ $16. The Thermax brand days the R values are "stabilized" which I take to mean outgassed values.
Place the fiberglass in first and then cut foam to fit snugly and seal well.
Well, the jury's in. Even the R 6.5 polyiso is covered with a thin film of condensate...at least in the rim joist space. The other interior plywood surface, the PT foundation sheathing, seems dry where the foam was applied and sealed. So I guess I'll just have to leave the spaces open, continue remedial sealing around the vapor barrier edgws, where I suspect alot of the moisture is coming from, and try to get the wall spaces dry enough next spring for spray foam.
I may try 2" extruded, cut with a 1/8" - 1/4" gap, and then sealed with can foam. Seems alot easier to seal with can foam if it has someplace to go.
Buildingscience.com suggests a max thickness for XPS of 1" I think (look up the article). This is to allow for air to leak through the insulation and condense on the outside wall, then permeate back in (ow you'll have a nice mold factory behind that insulation). 2" may be too thick, permeability is determined by thickness as well as perm rating.(Unfaced) polyiso might actually be better than XPS if it permeates better. Bottom line, you don't want a VB in basement insulation, gotta allow drying to the interior.
Correct me if I'm wrong (as if I needed to say that!) but the way I understand your post, you have the faced fiberglass in the floor joists between the two conditioned spaces. The last crawlspace that I did, I removed that insulation (which was soggy as heck), VB'ed the sand floor and insulated the walls and and 2 ft of the perimeter floor with XPS, sealed with foam. It seems to have worked really well.
Yes, that's right, the faced FG is in the floor joist space, with the intention of cutting down on the downward loss. Those spaces all seem to be OK, indicated by spot checks. Not reaching a dewpoint there. I'll keep an eye and dry hand on it.
The real problem seems to be only in the rim joist spaces and some foundation walls that are above grade (many of them are actually below grade, as it's a sloped site and the backfill berms up on one whole long side and parts of the short sides).
I have controlled the moisture penetration that was coming from the seam between the foundation and the floor VB, which seems to have helped through lowering the RH. ICO Acoustiseal is great, but messy and smelly stuff.
Also, thanks to the many helpful comments here, my future strategy is to #1 wait for the rim joist spaces to dry as much as they will...may take till spring or summer...then #2 either do the 2" XPS and can-foam or, if finances allow, call in the spray foam contractor.
Edited 12/16/2005 12:23 pm ET by johnnyd
Just an update on this whole project...I left off last December when I had just finished carefully sealing ALL the edges of the 6 mil vapror barrier on the dirt floor of the crawl space. I also pulled all the unfaced FG out of all the rim joist cavities, and insulated the most exposed with foam.
I haven't had the heart to crawl down there to check for moisture until just this week, after some heavy rains, but I'm happy to report that ALL the rim joist cavities are nice and dry. There is no evidence of abundant moisture or mold anywhere in the crawl space.
So the real ticket apparently was better sealing against ground moisture, and letting those cavities dry.