Insulation help

[Steve Silberberg]

Hi,
I live in the Boston area My house has no basement, just a crawl space between the main floor and the ground. The batts of insulation beneath the floor are drooping or worse so that there's nothing between the breezy crawl space and the floor.

I had a guy come over and he suggested that instead of roll insulation, I have foam sprayed between the joists to form a hermetic seal.

Question: Is this a good idea? I like that fact that winter winds will no longer whip up through my floor, but will the hermetic seal cause a moisture problem under the house in warmer, wetter times?

Many thanks in advance.
Steve

 

[Slumlord]

The best way to insulate a crawl space is to turn it into an UNVENTED space. To do that, you will first need a vapor barrier on the floor (especially if it is dirt) using 6 mil plastic, taped at the seems. Masonry walls, plates, and other leak points can be sealed with 2 component expanding foam (material costs are NOT cheap). You can do the joists above. To save money, I would recommend you have the crawl space joists netted and filled with cellulose. Urethane foam give you better insulating value, but the costs/benefit ratio is wacked for the joists due to the high cost of foam.

 

[Stretch]

Steve,

I agree with slumlord about sealing the crawlspace. See the following link to a crawlspace article:

Crawlspace problems
http://www.contractingbusiness.com/...aspx?CID=7253&HBC=GlobalSearch&OAS=&NIL=false

The foam may be a good idea to reduce infiltration though. It depends on how leaky your house is and how high your utility rates are. We have one contractor in my area that sprays Icynene foam (water based foam, no nasty outgassing) for $1.10/sq ft. He does not remove the old fiberglass for that price though.

Stretch

 

[Edwin Pawlowski]

Urethane foam give you better insulating value, but the costs/benefit ratio is wacked for the joists due to the high cost of foam.

I'd use sheets of foam. Not far from the Boston area is Insulation Technology in W. Bridgewater. They cut sheets from large billets and can give you any size and thickness you want. Figure R-4 per inch

 

[tbasc]

Look at the Building Science Corporation web site for more information on crawl spaces. If the crawl space is often damp from surface drainage or from ground with high water table, a sealed crawl space can be difficult and expensive. If it the environment is right for it, a sealed crawl space is a good idea. Many building codes make that arrangement illegal and some require an engineer's report. As I remember it, Steven Winter Associates is in your area and specializes in this sort of problem.

TB

 

[Slumlord]

My only problem with using sheets of foam is the labor involved. Lots of caulking to make sure all those seams are sealed. The two component foam is just so easy to use (be sure to ventilate well during installation).

 

[Tony Hwang]

My only problem with using sheets of foam is the labor involved. Lots of caulking to make sure all those seams are sealed. The two component foam is just so easy to use (be sure to ventilate well during installation).

Hi,
Tape is no good?
Tony

 

[Guest]

My only problem with using sheets of foam is the labor involved. Lots of caulking to make sure all those seams are sealed...

If you seal up the floor above, where will the water vapor from the imperfect vapor barrier on the ground go?

Automatic foundation vents could help, but that's more complex and less energy-efficient.

Nick

 

[Stretch]

Nick,

If foam insulation is used in the floor, the water vapor will not go into the house, that is for sure The water vapor won't hurt the block/rrick foundation, but it can hurt the wood floor. If the joists amd floor is covered by foam insulation, the wood will be OK. Put a dehumidifier in the crawlspace and drain it to the outside. If the crawlspace is low or coners a large area. put in a fan to circulate the dry air so the dehumidifier dries the entire crawlspace. According to ASHRAE fundamentals, with a high water table, the ground can evaporate 10 gallons per day per 1,000 Sq. Ft. into the crawlspace with bare dirt. If you put down 6-mil plastic, overlapped 12-inches, and fastened to the foundation walls, that drops to about 1 quart per day per 1000 Sq. Ft. (per Craig DeWitt, PhD, PE, RLC Engineering, Clemson SC. www.rlcengineering.com )

Nick, automatic vents open when it is warm, which is when the outdoor humidity levels are highest. That makes the crawlspace worse not better. Dehumidistats installed in the crawlspace will bring on fans hwen it is humid inside, but if it is humid outside, they never shuts off and the crawlspace gets wetter, not drier.

Stretch

 

[Steve Silberberg]

I agree with slumlord about sealing the crawlspace. See the following link to a crawlspace article:

My crawlspace actually is sealed, but not very well. I live in across the street from Boston Harbor and winds come whipping in over 50 MPH gusts several times during the winter. When this happens, I can feel the wind come up into the living room through the old hardwood floor. So while it's sealed in theory, wind gets in pretty easily.

In the summer, winds die down and it gets humid. I haven't seen any rot under there, but it's hard to access, so I haven't looked in a year.

The foam may be a good idea to reduce infiltration though. It depends on how leaky your house is and how high your utility rates are.

I'm actually in the mood to fix it "for good", so while price is a consideration, staying warm in the winter (and not getting rot in the summer) are my primary motivations here.

So I guess I should re-phrase the question as "Is there a reason NOT to use the foam?" or How can I insure it won't get too humid under there if I use foam?

Thanks for all your replies.
Steve

 

[Goedjn]

I'm actually in the mood to fix it "for good", so while price is a consideration, staying warm in the winter (and not getting rot in the summer) are my primary motivations here.

So I guess I should re-phrase the question as "Is there a reason NOT to use the foam?" or How can I insure it won't get too humid under there if I use foam?

My understanding is that the way insulation combines with water-vapor to create a problem is if there's water-vapor that enters from the warm side, migrates through the insulation to the cold side, hits an impermeable and/or cold barrier, and condenses there. If you put fiberglass under the floor, and covered it with plastic sheet, *THAT* would cause a problem when the crawlspace was colder than the house. If you put permeable insulation in WITHOUT covering it with plastic, then that would cause a problem when the floor is colder than the crawlspace. (during the AC season.)

Since spray-applied foam *IS* the vapor barrier, you shouldn't get any significant water migration through it, and any water that condenses on one side or the other should be able to get out the way it got in.

http://www.ncfi.com/Insulation/JLC_SPF_Artcle.pdf

--Goedjn