Floor joist insulation in crawl space

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Nobody

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I am looking for a good way to insulate the floor of one section of my house (35ft x 25ft) that is above a dirt floor vented crawl space. Most of the vents are right below the bottoms of the floor joists. This section of the house was built in 1967 and has a 4 2x12 girder that runs down the center of the house. The code calls for R30 and I just finished installing a new 6 mil polyethylene sheeting over the dirt floor doubled up in many areas.

I am a bit nervous about this decision because I spent many difficult days crunched under the house replacing/sistering about 30 of the 2x10 floor joists....the originals were rotted on the outside ends (blocking between them on the sill plate) and the rot extended about 6-12 inches inward. The wall sheathing was rotted to; just in the lower part near the floor joist outside ends. Bought the house in this condition didn't know about the rot and cant figure out why they all rotted this way.

Anyway, I dont want all my new joists to run into the problem of growing mold/potentially rotting from moisture being trapped in fiberglass batts installed between the joists. Ive read and seen the horror pictures of this happening (moisture in batts and joist rot) in the Southeastern hot and humid climates with vented crawlspaces. From what I have read, vented crawlspaces seem to work fine in the Western Washington climate.

Would installing the fiberglass R30 batts between the joists and then holding them in place by stapling Tyvek on the bottom of the joists be a good idea? I read an article where this was done and also heard that installing rigid foam (1" or ) board over the bottom of the joists has been done....but the rigid foam board option seems to be a very expensive option and seems like it would really seal in the moisture if the joist bays were developed moisture.

Thanks for ideas.
 

Dana

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Rigid foam under the floor joists keeps the joists warmer, which in turn keeps them drier. It's the cold joist edges and the lack of a ground vapor barrier that led to moisture accumulation in the joists. If you add rigid foam, the foam it self is vapor retardent (a true vapor barrier, if foil-faced), and would keep moisture from migrating from the soil to the joist edges. As long as the ratio of foam-R to total-R is high enough to keep average winter temp at the foam/fiber interface above 40F (the presumptive dew point of the conditioned space air), the joist edges will stay dry & rot-free/mold-free. With a ZIP code we would be able to tell you what the R-ratio would need to be.

Now that you sistered on joists you now no longer have standard batt widths in the joist bays, which makes it hard to get good performance out of batts since there will be compressions & potential voids at the edges. What's more you've doubled the thermal bridging, which is an even bigger argument for a foam-under treatment, since that dramatically reduced the thermal performance of anything you stick in the joist bays.

But it may be cheaper/better to go with an unvented crawlspace, if the foundation wall surface area is less than the total floor area, which is pretty common in crawlspace foundations. The subsoil temps in WA are all over the place, but typically in the low to mid-50s F. If your ground temps are 50F, sealing & insulating the crawlspace walls (and not the joists) would be sufficient to keep the crawlspace well above 40F, probably above 55F, with the rooms above it running 60-65F at the floor. If you have heating ducts or plumbing down there, that would be an even greater rationale for insulating the walls, not the joists.


geothermal-subterrainean-temperature-contour-map.jpg





If you decide to go with the "conditioned crawlspace" approach, in most of WA 2-3" of EPS or 1.5-2" of foil-faced polyiso on the wall would be cost effective. (About $0.80-$1.20 per square foot, if using virgin stock new foam, or 20-25 cents/ft^2 if using reclaimed/used foam.) It can be held in place with 1x4 furring through-screwed to the foundation with TapCons or similar. Your ground vapor barrier should extend at least 6-12" up the foundation walls, and should be mastic-sealed to the foundation, with the foam on the interior side of the v.b.. This detail is particularly important if using polyiso, which unlike EPS, is hygroscopic- the cut edge should never be in direct contact with soil or concrete. (The facers are fine in contact with a concrete foundation wall, but not soil.)

With the walls insulated to at least R8 (R12 is better), it's really optional whether you insulate between the joists. But if you are insulating between the joists, given the narrower than standard width the best option would be to use blown cellulose, which is hygroscopic, protecting the joists by sharing the moisture burden (which it does without loss of function or damage.) You can use landscaping fabric stapled every 2" to the underside of the joists as a blowing mesh which you slit 5-6" to be able to squeeze the blower hose into it a few feet, first blowing one direction until it backs up and stalls, then blow the other direction. Prior to blowing it's a good idea to screw some 1x furring to the bottom side 24" o.c. perpendicular to the joists as a means of supporting the cellulose over the long term without the mesh sagging too much. In a 2 x 10 joist bay that would end up at about R30-35 center-cavity.

With both the walls & joist bays insulated the temp in the crawlspace will stay a bit above your deep subsoil temps, but will vary a bit with outdoor temperature. Air-tightness between the crawlspace & outdoors would be key to keeping that crawlspace temperature variation low, which keeps the joist-edge temps from from falling too low.
 
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LLigetfa

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I decided to make my crawlspace part of the conditioned space so my walls are insulated and drywalled and I poured a concrete floor. Basically it is like a basement except for the headroom.

Putting batt insulation between the joists would be impossible to effectively vapour barrier. Spray foam may be an option but the flammability may be a concern. I have heard of spraying stucco over the foam to address the flammability aspect.

Placing foil lined rigid foam as Dana mentioned assumes you have the room for it and a way to get large sheets down there. The foil may or may not address the flammability concerns adequately.

In a former home I had moisture issues with how the builder insulated the crawlspace walls. I had to take down all the poly, and pull out all the glass batts to dry them out. I then cut rigid foam to fit snugly between the studs and put them against the sheathing on the cold side. I then put glass batts on the warm side and then the poly. The R value of the rigid versus the R value of the glass batts, put the dew point somewhere in the rigid so the glass batts stayed dry after that.
 

Nobody

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Thanks for the advice and ideas

Im in zip code 98077

Insulating the foundation walls/ closing vents would be less work and expense. All of the plumbing and heat ducting is down there too.

Do you have to "heat" and run a dehumidifier in the crawlspace if you call it "conditioned". Will the inspector /building dept "ding" me if he crawls down there someday if he sees its not heated and dehumidified?

Good point LLigetfa......getting 4x8 sheets down there probably cant happen......a 2x8 yes. There is only about 2ft or so of height so it makes working down there with large pieces of anything a real pain.
 

LLigetfa

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Do you have to "heat" and run a dehumidifier in the crawlspace if you call it "conditioned"...
Dana would be better positioned to answer code issues. I am just a DIYer. I had electric baseboard heaters in the crawlspace that the building contractor originally installed. The contractor screwed up by placing a traditional thermostat in the living space so it was not sensing the temperature in the crawlspace. Code required the control to be in the living space but the sensor should have been in the crawlspace. Because of that, it raised the temperature in the crawlspace which subsequently raised the humidity. Cooler means less moisture is drawn from the soil beneath the vapour barrier.

You would want it only warm enough that you don't get cold feet, not toasty warm like it was in-floor heating.

My current home has forced air heating and that exchanges a small amount air in the crawlspace via leaks in the ducting despite being sealed with mastic. The heating contractor installed three takeoffs to heat the 1200 sq ft space but I keep the dampers closed most of the time.

You probably have enough air exchanges with your ducting and some air infiltration from outside.
 

Dana

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Thanks for the advice and ideas

Im in zip code 98077

Insulating the foundation walls/ closing vents would be less work and expense. All of the plumbing and heat ducting is down there too.

Do you have to "heat" and run a dehumidifier in the crawlspace if you call it "conditioned". Will the inspector /building dept "ding" me if he crawls down there someday if he sees its not heated and dehumidified?

Good point LLigetfa......getting 4x8 sheets down there probably cant happen......a 2x8 yes. There is only about 2ft or so of height so it makes working down there with large pieces of anything a real pain.

In 98077 (which is in US climate zone 4C) you don't need or want a true vapor barrier on the interior side of the conditioned space, and on a foundation wall it simply doesn't matter very much where the vapor retardency ends up. If you're putting foam on the walls and making it air tight you don't need go higher than a foot up from the dirt with the vapor barrier, as long as the foam laps over it.

Since it's both cheaper and better to insulate at the foundation walls, that's the route you should go. If you used fire-rate Dow Thermax as the wall foam you won't need a thermal barrier against ignition to meet code, but it's not necessarily the cheapest option if you have a source for reclaimed foam. (Search the local Craigslist for "rigid insulation", and don't pay more than 3 cents per R per square foot for used stuff especially if it's dinged up rather than nearly-perfect.)

With 2" foam it's pretty easy to just glue the foam to the wall with foam-board adhesive then strap it in place with 1x4 furring trough screwed to the foundation with 3.5" TapCons. With thicker foam than that it's sometimes hard to find the screws, and trapping it in place with a non-strucural 24"o.c. studwall is sometimes easier, in which case it's worth installing R13s in the studwall for a super-insulated approach. If you want to do it primarily with batts, in your climate you can drop back to 1" foam (any type), and UNFACED R13-R15 batts, with no interior vapor barrier, but with air-tight wallboard, similar to this basement wall detail:

bscinfo_511_figure_03.jpg


With the thermal bridging of the studs the studwall part comes in at about R9-R10, average performance, but with even 1" of EPS (R4) it's performance rises to that of a 2x6/R23 studwall.

When insulating the crawlspace it's also important to air-seal and insulate the foundation sill & band joist, at the same time. In your climate you could get away with just rock-wool batts at the band joist in a fully heated basement, but it's better to cut'n'cobble some rigid foam board in there (sealed in place with can-foam) if it's only a semi-conditioned space that runs cold.

Some useful links with pointers & alternate details:

http://www.energyvanguard.com/crawl-space-encapsulation-method/

http://www.greenbuildingadvisor.com/blogs/dept/musings/building-unvented-crawl-space

http://blog.energysmiths.com/2011/07/basement-insulation-progress.html

http://blog.energysmiths.com/2011/07/basement-insulation-part-2.html

http://blog.energysmiths.com/2011/08/basement-insulation-part-3.html


From a code point of view some amount of air exchange between the conditioned space & conditioned crawlspace is necessary, but you don't have to actively heat it, but since the heating ducts are already down there you might as well use them to provide at least the minimum air exchange. If the ducts aren't meticulously sealed at every seam & joint they may already be providing more than sufficient air exchange, but it would be better to seal the ducts, then give it 2-4 square inches of both supply & return plenum air at opposite ends of the crawlspace, or a 2-4" of supply with a grating through the first floor to provide the return path.
 

Dana

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I neglected to mention...

In your climate, the absolute minimum ratio of foam-R to fiber-R for a foam + studwall solution would be R1 of foam for every R6 of fiber, but in a cool crawlspace it's safer/better to go with at least a R1/R5 foam/fiber ratio.

With R4 EPS and R15 rock wool between 2x4s you'd be closing in on an R1/R4 ratio, which has a very comfortable dew-point margin (in your neighborhood anyway, not so much in mine), which is why I suggested 1" foam.
 

Nobody

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Dana would be better positioned to answer code issues. I am just a DIYer. I had electric baseboard heaters in the crawlspace that the building contractor originally installed. The contractor screwed up by placing a traditional thermostat in the living space so it was not sensing the temperature in the crawlspace. Code required the control to be in the living space but the sensor should have been in the crawlspace. Because of that, it raised the temperature in the crawlspace which subsequently raised the humidity. Cooler means less moisture is drawn from the soil beneath the vapour barrier.

You would want it only warm enough that you don't get cold feet, not toasty warm like it was in-floor heating.

My current home has forced air heating and that exchanges a small amount air in the crawlspace via leaks in the ducting despite being sealed with mastic. The heating contractor installed three takeoffs to heat the 1200 sq ft space but I keep the dampers closed most of the time.

You probably have enough air exchanges with your ducting and some air infiltration from outside.
 

Nobody

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Wow....Thanks...... lots to think and learn about.

There wasn't any insulation in the floor or crawl when we bought the place last April. We have a gas forced air furnace with an insulated large round (14-16 inches diameter ) metal duct running down the center of the crawl. All the offshoot ducts from the central duct in the crawl are flex duct piping. The flex duct goes to each room vent. So I imagine that there are leaks somewhere since I don't know how long ago the ducting was installed......doubtful it is original to the home which was built in 1967. So I am probably getting air exchanges from it. There is a spare sealed duct opening in the main central duct I could potentially use if need be.

I replaced all of the subfloor (5/8 plywood (replaced some of these) with 1/2 OSB on top) above the crawl and caulked around the edges of each sheet of OSB I screwed down to the plywood layer. Also caulked all around the sole plates of the exterior walls.....so I am thinking the floor is well air sealed in relation to the crawl.

Fortunately in my climate it never gets "really" cold like in others so the floors above my crawl don't seem really cold but since the house had zero floor insulation I'll definitely benefit from something.

There was virtually no attic insulation either in a low slope (up 1ft over 4ft roof situation)....but that will be another post someday.

Thanks for all the ideas.
 
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