Insulate Crawl Space Floor

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  #1  
Old 02-24-15, 11:37 AM
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Insulate Crawl Space Floor

I'm in the process of insulating my crawl space and putting down a drainage mat and vapor barrier. The other thing I believe I should do is to put some insulation between the drainage mat and the vapor barrier. My question is: What should I use? I can buy styrofoam that will give me about 3R value, and I can line the floor of the crawl space for about $300.

The second question is whether or not I should use more insulation on the walls or not. My understanding is that R3 - R6 isn't very much. Should I double up my insulation?

Finally, I understand that air sealing is a big deal, but is air sealing a big deal when I'm putting it on the ground? I'm going to be sealing it with the vapor barrier, so do I need to tape the boards together or something? If I don't tape them together I might get little gaps where there is no insulation, but does that drastically impact their effectiveness?

One last thing, I'm not really sure if I need to "air seal" the top of the floor joists. Light does not get into the crawl space from the outside in any space except for the one vent, which I intend to seal. So Do I need to do anything else? How do I test for this?

Thanks guys!
 
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  #2  
Old 02-24-15, 11:45 AM
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Hi Kyle and welcome to the forum,
First question, where are you? Your climate zone will determine how much insulation we can recommend.
What is on the floor now, dirt or a concrete slab?
Is this just a crawl space or is this a space you intend to use?

In most cases adding a layer of insulation over the floor would not gain a lot and would be better used as additional insulation on the walls or the rim cavity above the walls.

Is there or will there be heat added directly to this crawlspace?

Bud
 
  #3  
Old 02-24-15, 01:45 PM
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Bud,

Thanks for responding.

I live in Michigan. The weather drops to zero for about a month each year, and it hangs around 90 for a few weeks in the summer. The humidity is about 50% in winter, and nearly 90% during the summer.

The flooring is dirt. This is just a crawl space. The purpose is to save on energy costs. I may use it for storage, but that's all I would use it for.

There is no heat blowing into the crawlspace, but the duct work is running throughout the crawl space, and it is fairly well heated because of that. The floor can't be more than 10 to 15 degrees cooler.

Thanks again!
 
  #4  
Old 02-24-15, 01:52 PM
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Yep, get a vapor barrier on that dirt. After that, two schools of thought on crawlspaces:
1. Insulate the walls, seal off from the outside and expose to the conditioned air of the house.
2. Insulate the ceiling, seal off from the house and expose to the outside air via vents.

Sounds like you're closer to the first option, which is generally my choice anyway.
 
  #5  
Old 02-24-15, 01:59 PM
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If it were me, I would spend my money insulating the ductwork and floor of the home. Unless you have water lines in the area, why waste heat in a crawl space you are not using?

In either case, get some plastic on the floor and cover it will a thin layer of crushed rock to protect it.
 
  #6  
Old 02-24-15, 02:31 PM
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Keith,

I do have water pipes in the crawl space. I have read a lot of reviews that say that insulating the floor joists with fiberglass insulation can lead to mold. So I didn't want to go that rout. If there's a way to insulate the floor joists with a mold resistant material, I would love to know!

Stick,

I am definitely closer to the 2nd option. I only have one vent, and I'd like to seal it. I'm guessing I would need a HVAC guy to come in if I want to condition the space. I'm wondering how that compares to insulating the duct work.
 
  #7  
Old 02-24-15, 03:10 PM
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Here's a link on crawlspace issues. Photo #5 is my dream crawlspace:
BSI-009: New Light In Crawlspaces — Building Science Information

Bud
 
  #8  
Old 02-24-15, 03:25 PM
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I am in Michigan as well, and agree with Bud in liking photo # 5. My personal opinion, based on everything that I have read, is caulk and insulate the rim joists first, either foam or rigid and foam, vapor barrier on the ground and up the walls a foot or so, then rigid on the walls. At that point, I doubt that you would want to insulate the ducts because any loss there would have no place to go but up.
 
  #9  
Old 02-24-15, 03:31 PM
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You certainly can insulate the floor joists with fiberglass without worries about mold. The key is moisture Control. That would consist of poly on the floor and good air circulation In the crawl space. And a thermal break under the joists. The problem is most crawl spaces are severely under vented, even if they meet code. The same is true for attic spaces. The ideal alternative material would be closed cell spray foam on the floor, but that can get expensive.

Given the water pipes in the area, you are probably better off insulating the crawl space from the outside air.
 
  #10  
Old 02-26-15, 02:20 PM
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Keith,

I don't have good ventilation. I only have one vent. That being said, I think it's pretty dry down there. I just don't want to get mold.

Bud,

Thanks for the link. I called up the hvac guys to get a quote for conditioning the crawl space. The other option that looked good was to put the reflective rigid foam on the joists. It's nice to understand the science behind it.

Pedro,

I will not insulate the ducts! Thanks.
 
  #11  
Old 04-23-15, 11:48 AM
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I've gone with the option of srayfoaming the joists in our crawlspace. In the two years since insulating I have not encountered any issues. Though upon reading this thread and the linked documents I find myself questioning if what we have done is correct.

I live in south western Ontario. The floor space above the crawlspace is carpeted. All the furniture in that room is elevated off the floor. The adjoining brick wall of the crawlspace and house is sprayfoamed on both sides of the wall (in the crawlspace side and the interior basement). The crawlspace is not used for storage and is sealed off for all but one vent/access point. The dirt floor in the crawlspace is lined with a 6mil plastic barrier with sand to keep it down and is not sealed to the interior crawlspace walls except for where the interior crawlspace wall is sprayfoamed. Their is insulated supply and return ductwork from the basement through the crawlspace to service the room above the crawlspace.

I plan to add another two vents to the crawlspace in order to increase airflow.

Any other suggestions?
 
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