Stopping air flow between crawlspace and living space

ferroplasm Zone 7bSeptember 17, 2013

Split level house and the lower level shares a wall with the crawlspace. Musty odors make their way through into the living space despite fiberglass insulation in the bays and some old asphalt/wood fiber sheathing on the crawlspace side. The sheathing is beginning to deteriorate. How should I replace the insulation and sheathing for a better seal?

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Stopping all air flow in a crawl space is a bad move. In times of lower humidity conditions the dry air circulated in the space actually drys the moisture laden dimensional lumber in the space that has been introduced there during times of high humidity. It actually sound like you don't have enough ventilation if you are getting a musty smell. There actually should be 1 square foot of net venting for every 200 square feet of floor space at the corners if there are corners. Check that number on the Building Science website.

If the space is dead ended you need a circulation fan below it.

Never block a crawl space from exterior air.

Don't seal it for goodness sakes.

Search "crawl spaces" on buildingscience .com. for an excellent explanation.

    Bookmark   September 18, 2013 at 4:13AM
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ferroplasm Zone 7b

I live in the piedmont of NC. Building scientists are actually advising that crawlspaces in southern homes be converted to conditioned space or at least sealed and dehumidified. How do you consider more air flow in the crawlspace to be a solution? If I had more airflow, hot and humid air in the crawlspace would still be moving into the living space during the summer and cold air would be blowing in during the winter.

The wall needs to be sealed regardless of what I do with the crawlspace. I am asking what materials would be best for insulating and sealing this wall.

    Bookmark   September 18, 2013 at 10:50AM
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Of course if there are 2 scientists there will be 2 opinions. It's the dry cold air that dries the dimensional lumber in the winter, it is a delicate balance. A well insulated floor above the crawl space is very important. Southern climes may be somewhat different but this is what I have come to learn, including humid southern Florida.

If you are intent on sealing there will be numerous options available to you. Closed cell insulation panel,then plywood and vinyl siding well caulked and sealed being 1.

I've also used solid sunroom wall material panels with great success around crawl spaces as it provides insulation and has vinyl or aluminum sandwiching it. Look for a Temo Sunroom dealership near you where they may supply you with the sunroom panels.

This post was edited by SouthernCanuck on Wed, Sep 18, 13 at 16:30

    Bookmark   September 18, 2013 at 4:25PM
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Ferro, you need a vapor barrier between the unconditioned space and the conditioned space. Neither fiberglass insulation nor wood sheathing does that job. Vapor barriers = stuff like tyvek or thick plastic sheeting that's taped closed at the edges. How that's accomplished on a a retrofit, and in a humid climate, I don't know, so I'll stop here. You'll get condensation in the summertime at the border between the cool dry air inside and the hot humid air outside, so it matters where the barrier is put. I just don't know any more about it. I'm happy to live where the summers are not humid and not too hot ;-)

Ask a general contractor who does remodels. You should not have so much air-inflitration that it's noticeable, either because of drafts or because of odors. There could be something else going on that is pulling air in, maybe leaky a/c ducts causing negative pressure in the house?

Building science is a new and developing endeavor but there's not diversity of opinions on many of the basic issues. It's applied science, not theoretical science.

    Bookmark   September 18, 2013 at 10:15PM
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It is hard to know what to do. If the moisture is coming from the earth, sealing the crawlspace is going to make it worse. You'll have to seal off the crawl-earth boundary first. If the moisture is coming from the air, sealing the crawl is the right thing to do. I'd hesitate to do that with my home over an earth-only bottom on the crawlspace even if it is dry.

    Bookmark   September 19, 2013 at 5:25PM
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the air flow between crawlspace & living space happens when there are openings in the floor.
air sealing the floor...then insulating...then putting a vapor barrier to bottom of floor joists would solve this problem.
I've used foam sheathing under joist as both air & vapor barrier...lot of work!!

without a vapor barrier under the floor joists the batts will
sag over time..condensation forms on underside of floor &
floor rots.

putting just batts with no vb under joists will allow wind washing of batts reducing R-value. and moisture migration from ground will wet batts. wet insulation has no R-value.

with a vented makes little sense to put a vapor barrier on the ground. if you close the crawlspace..then vb & all that...but it isn't something I'd do in my hot humid climate.

I'd pull the batts out of the floor joists & spray closed cell foam. vapor barrier & insulation in one fell air sealing.of course seal any large holes first...flashing with caulk, screwed into place.

having done both air sealing, adding batts & foam sheathing....& closed cell foam of floors..they break out pretty close. labor, time & materials of diy...or extra cost to have it done in half a day...half a day of prep work beforw foam install...sealing under tubs etc & you are done.

where are ducts?

best of luck.

    Bookmark   September 19, 2013 at 6:30PM
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I wouldn't expect a remodel contractor to understand
about negative pressure pulling crawlspace odors into
the house.
remodel contractor would just put batts & call it done.
no air vb or air barrier.
unless they have experience with building science & blower doors...

when the central unit runs...the holes in the house...floors ceilings etc will depressurize pulling ambient or attic air into the house. identifying these leakage sites & sealing them
prior to any insulation install is the correct way to address this issue.

by the same token...any leaks in duct system will also suck
in air of surrounding area.
thus mastic seal of ducts is also recommended.
it is truly amazing the amount of duct & house leakage that
exists in our homes.
blower door testing of house, testing ductwork is a good investment...always.

best of luck.

    Bookmark   September 19, 2013 at 6:37PM
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ferroplasm Zone 7b

Ionized - I've installed a vapor barrier completely isolating the earth from the rest of the crawlspace. Six mil fiber reinforced plastic has been taped and mechanically secured up the block walls about 6 inches above grade and the same type of barrier has been installed over the earthen floor. All seams have been taped with waterproof tape. The only moisture now entering is in the humid air from the crawlspace vents.

Energy_rater_la - I'm almost certain I've pin-pointed the major source of air leakage. It's through a wall on the lower level of the house. It's a split level house and the 2x4 framed wall on the eastern side of the crawlspace is shared with the lower level. There's a closet that gets really musty and in that closet I can see the fiberglass insulation. On the crawlspace side of that wall there's asphalt impregnated fiberboard sheathing that is beginning to break apart and allowing air flow into that closet. This is where the moist air is entering.

I just need to know which side of the insulation to have the vapor on and which materials would be most appropriate.

    Bookmark   September 20, 2013 at 1:58PM
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It seems like you are part way to a sealed crawl, but you'd still have to do the sides. If the crawlspace is sealed, you can't have a vapor barrier between it and the house at all. It has to be able to release moisture in one direction or another. If you do want to seal both sides, I suppose another approach would be to monitor the humidity in there and install a dehumidifier. That is a lot of expense or trouble and I don't think it is usually done.

Keep in mind that one advantage of sealing the crawlspace is that it couples the house to the ground which has a relatively moderate temperature. That way you don't have the expense of insulation.

In a climate with cool inside and moist, humid air outside, the vapor barrier needs to be on the outside. Your wall may be rotten because you are periodically sucking in warm, moist air every time the AC cycles. It hits the cool wall in the closet and condensation happens. Is air coming in when the AC blower is running?

It may be that the wall is rotten because the impregnated material started to decay allowing air to get by into the fiberglass and onto the cool interior wall. If the closet does not dry well to the inside, the wood will stay too wet.

    Bookmark   September 20, 2013 at 7:06PM
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vapor barrier goes to warm side.

insulating with an unfaced batt & sealing the area
with foam sheathing would possibly work.
caulking the foam sheathing, taping seams &
securing with button cap nails is the best way
I've found to air seal areas.

difficult to say with out seeing the area.
any pics?

best of luck.

    Bookmark   September 21, 2013 at 10:58AM
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