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Question DetailsAsked on 9/9/2013

whats the best most economical way to insulate a vented crawlspace?

I have a typical vented crawlspace w 6 ml vapor barrier on the dirt floor of it. Its damp in there but no current water problems. I had all of the fiberglass insulation removed because of rodent infestation, and now I need to replace the insulation. Some are telling me to use closed cell foam insulation on the crawlspace ceiling (under the floor above) and to seal the vents and tape the vapor barrier sheet to the walls of the crawlspace, and put a dehumidifier in there. I"m concerned about fire risk of a electrical machine (dehumidifier) down there in the crawlspace. Also concerned about the cost. several thousand. I have Viega PEX plumbing, and heard from the Viega company that the closed cell spray foam can be damaging to the PEX pipes.

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1 Answer


Why do I have to be the bad guy and tell you that unfaced fiberglass insulation is the best thing to use, combined with positive measures to seal all openings rodents could get in. Someone led you astray on that - they should have solved the rodent problem rather than treat the symptom of rodents in the insulation.

Spray foam CAN degrade plastic pipe, particularly polyethylene (PEX) - I have seen a demonstration of it fail under pressure during the spraying process because the foaming chemicals are very active while it is being applied, and can chemically melt right through the tubing. Rigid foam would be very hard to put in effectively around the existing pipes and wiring, plus carpenter ants and possibly other insects like nesting in it.

Do NOT seal the vents unless you are prepared to make that a ventilated space, serviced by your HVAC system, which with a moist ground condition would make for a mold and mushroom farm down there unless you gave it high airflow (and hence energy $).

Hard to evaluate without seeing it - I think you need an HVAC contractor to evaluate it for you, but there are a couple of things you could do:

1) definitely seal the vapor barrier to the foundation walls with ample slack in case a worker has to crawl in there in the future - should be installed on a smooth raked surface without rocks and with a thin sand layer if rocky fill so it might survive a worker on top of it. Tape will not last well to hold it to cold, damp foundation wall - you really need to use firring strips over the plastic over caulk applied at the foundation, but I have seen people use 2x4's along the edge to temporarily hold it down tight at the foundation and then taped up the needed height on the wall, then spray foam over the joint, which actually seemed to seal at the concrete/block foundation boundary pretty well, though not pretty. The vapor barrier should run up the foundation wall at least 1 foot above outside ground level or to the wood sill plate, whichever comes first, because a lot of moisture can come through the foundation also from the moist ground outside.

2) dig a sump and see if you hit water within 3 feet or so - if so, put in a sump pump to draw the water down to reduce the moisture in the crawlspace.

3) put in a dehumidifier - if worried about fire, put a dedicated in-line AFCI/GFCI outlet or breaker in line with it to protect against short circuits, and a hardwired with battery backup fire alarm over it. To go even further, there are automatic fire extinguishers (google this phrase to see what they look like - automatic fire extinguisher Amazon) from $40 on up depending on size, that you would mount over the top and it automatically sprays a fire extinguisher chemical just like a water type overhead sprinkler sprays automatically when it gets hot.

4) put in positive ventilation - a large, low-speed fan running off a humidity switch or timer which xxxx air out one end of the house (the wettest) and in all the vents, though that will also pull conditioned air down out of the house too. Vents close to the fan have to be blocked off to prevent short circuiting of the airflow.

Answered 6 years ago by LCD

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