Crawlspace insulation and vapor barrier
I need some guidance on the best way to seal the bottom of my floor. I am about to start a remodel on my house and intend to use radiant heat stapled to the bottom of the floor. (Before anyone criticizes that decision, know that it is an old cabin in the mountains of Colorado and the floor couldn’t handle a thin slab without major modification.) For better or worse, my county requires that I have a vented crawlspace. I am at 8,000 ft. so I am in a heating-only climate (that also has very low humidity). The old part of the house is ~400 sq.ft., and I am also adding ~600 sq.ft. of new area.
My understanding is that the vapor barrier should go on the warm side of the insulation. I will be using expanding spray foam insulation (closed cell) for the rest of the house, so the vapor barrier location is irrelevant in the rest of the house (the insulation IS the vapor barrier), but I think that might be a bit permanent for use under the floor. If I want to add any wires, fix a section of pipe, etc., I have to cut out the insulation and will not be able to reinstall it. Does any one have a good solution?
Should I staple 6 mil. across the bottom of the floor joists and try to hang insulation below that? If so, rigid or batt, and how to hang it? It seems that the radiant heat would perform better if it had insulation, etc. closer to the tubes than all the way at the bottom of the joist bays. The county says it wants R-21 in “floors over unheated spaces” and R-20 on crawlspaces. I am assuming that means I need R-21 under my floor and R-20 on the exterior walls of the crawlspace. The spray foam seems like an easy way to handle the walls of the crawlspace, yes/no?
Replies
last question first...yes, foam would be good on the walls of the crawl space.
insight into other questions in no particular order:
Staple-up radiant is fine...alot of folks here and on other forums prefer low mass to high mass for radiant. Have you looked into a sleeper/plate sandwich system on TOP of the subfloor?
You're correct, the insulation should be in the joist spaces. Various ways to do this are discussed in great detail here (try searching) as well as at:
http://www.radiantpanelassociation.org/i4a/pages/index.cfm?pageid=1
Note that you need to be pretty careful about how much airspace remains between the top of the insulation and the bottom of the subfloor. With staple-up, you are using both radiation and convection...heating the airspace under the sub-floor...so you need to be particular about that airspace.
As far as the vented crawl space issue with your county...go ahead and put them in per county specs, then cover them after inspection.
Good luck...more opinions and advice will be along shortly. It would help, I think, for you to provide more information like joist type, size, and spacing; type of subfloor and intended finish floor, working room in the crawl space, estimated heat loss, etc.
If you can afford it, Warm Board or a similar product would probably work better. When I've seen radiant stapled up underneath the subfloor, they are using a type of insulation with reflective foil faced up towards the Wirsbo, held up with wire pieces sprung between the joists. We're in NM at 7200 feet, so the situation isn't much different. No mositure barrier required here from what I've seen, but crawl spaces also must be vented. i would think that the moisture barrier would go on top of the joists, under the sub floor, but I could be wrongh. Latest things I've been reading are to not insulate the crawl space floor, but to insulate the crawl space walls only. I think it was last months FHB that was talking about heating the crawl space with an extra vent and creating a positive pressure crawl space. Maybe it was written by a burnt out insulation contractor.
Jim
Unfortunately, I can't really consider any options that involve an unvented crawlspace, no vapor barrier, or the positive pressure system from the recent FHB. My county (Boulder County, CO) wants it vented and the floor insulated and containing a vapor barrier. No getting around it as far as I know.
That being the case, I was interested by the suggestion of putting the barrier over the joists and under the subfloor. Hadn't thought of that one. Lots of penetrations with all the plumbing and wiring, though. Any one else done this...is it a good way to go?
The radiant subfloor sounds like a great idea and one I haven't ruled out. My understanding from other posts here is that it is fairly expensive compared to staple-up. I do intend to get an estimate for it though. It would make insulating the bottom easier. Just put the VB between the joists and the subfloor (as above) and stuff the joist bays with FG batts, or leave out the VB and spray the whole thing.
A few more specs: The old section (400 sq.ft.) is 2x6 @ 24" o.c. (yeah, it's bouncy.) that I intend to sister with some new 2x8's to add strength. It will have standard ply subfloor and 3/4" narrow-strip hardwood floors. The new section (600 sq.ft.) will be 9 1/2" I-joists @ 16" o.c., same floor except some carpeted areas and some hardwood.
Thanks for the thoughts! Keep 'em coming!
-Rich