Replacing rotten floor joists; should I also tackle the crawl space?
I’m buying a house in the San Francisco Bay Area with a vented earth-floor crawl space and a lot of rotten, termite-eaten floor joists (see pic). The crawl space access is down through a closet floor in the master bedroom addition and then in through the original exterior wall hatch: doable for a human, but not for a joist. The contractor says he can have people go in through the hatches and joists through the vents, though.
Presumably, these joists got this way for a reason. I’m from The Netherlands, where people like to gripe about those newfangled concrete roof tiles that last only 30 years. I understand that the Dutch probably overspend on quality, but it seems downright irresponsible to me to put brand new joists in a damp crawl space and wait for them to rot again.
So here start my questions:
- Am I correct to think that crawl space venting never really works and the only real way to keep the crawl space dry and the new joists from rotting is to seal and condition it (e.g. cleanspace)?
- Am I correct to think that with a sealed, conditioned crawl space the bat insulation between the joists (not in the pic, but the addition has it) is useless and might as well be torn out?
- In my climate, if I get the crawl space sealed, does it make sense to have insulation underneath the plastic seal material?
- Is there any advantage to using a thin concrete slab in the crawl space instead of the plastic stuff?
- With an eye to future work like this, is there any good reason to create a proper crawl space access, or is the current two-hatch path just fine?
I hope my questions aren’t too stupid and that I picked the right subforum. Feel free to shoo me away if I’m in the wrong place.
Replies
Venting has nothing to do with termites, they only need a way to get to the ground - that is what you need to interdict.
Concrete is good
#1 - see #5, make a basement
#2 is incorrect, unless more than the batts insulating outside walls of crawl space.
With $$ costs in SF, might as well install PT joists, wont get termites in them then
#3 gravel under the plastic, no insulation
#4 plastic under concrete also
#5 - Very proper, why not dig it out and make a basement? Have done that in multiple geographic locations, always worked out well unless you get into water table.
Thanks for the response! As for a basement: the house is a mile from the bay and a mile from the creek and FEMA zone AE. I could check with the city, but I doubt a basement would be a good idea.
Wood rot, condensation, crawl space.
San Francisco Bay often has high relative
humidity to temperature.
In such a location a vented crawl space is wrong.
Your best bet, is to remove the existing floor and
joists, fill the crawl space with
sand/stones/rocks/pebbles/earth anything that is
solid and is not effected by water vapor/condensation.
Topped off with a concrete slab, 1mm thick damp
proof plastic sheet, floating polystyrene insulation
and floating OSB glued t&g floorboards.
Good info here...
Our sister site, Green Building Advisor, has some good info on this topic.
http://www.greenbuildingadvisor.com/search/node/crawlspace