# Sealing off a Crawl Space



## drelldrell (May 12, 2014)

I had a similar issue last year. Not sure where you live, but I take it that the winters are fairly cold. You need to do three things. 

First, insulate all pipes and ducts. Effectively you are going to have / already have an unconditioned crawl space. The insulation will prevent pipes from freezing and reduce energy loss from the pipes and ducts as air and water travel through them. If you do one thing, this is THE thing to do. Wile likely see a drop in energy bills from this alone. Maybe see better performance of heating unit. 

Second, seal all holes - like where the black PVC pipes gouges through the wall. It will stop air travel and reduce you heating load assuming this air travels to the downstairs floor. Air sealing should be a never ending mission. 

Third, insulate the underside of the floor joist. Closed cell spray foam is best. You should hire someone to do this. Much better than DIY approach. An alternative that is a DIY approach is to use fiberglass or rock wool insulation. If you use faced insulation, place the faced side against the floor joist. No need to insulate sides of crawl space. 

Check with your utilities as there are usually very good incentives/rebates for insulation. I good about $350 off of a $2,500 insulation job for attic and crawl space. 

Related dehumidifier, can't you have a water drain into the same outlet as your sump pump or have a condensate pump that exits to the exterior? Very good that you have a vapor barrier - keep it in tack. 

Hope this helps.


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## Windows on Wash (Aug 30, 2011)

If you have ductwork in the crawl and insulation across the stem wall, air seal the perimeter, make sure the vapor barrier across the floor is sealed and probably install a new one, seal the stem wall up, install some more foam and cover it with a code approved ignition barrier (fiberglass will work), and condition the crawl.


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## arblargan (Sep 4, 2014)

My apologies, I thought I put my location in the original post. I currently live in Alaska, so as you can imagine, it gets pretty cold. 

My main thing with insulating the ducts is that they're about 18" in diameter and I have probably 300 ft of ductwork just for the furnace. Now I know that insulating them will obviously help with utility costs and heating efficiency, but does the benefit outweigh the cost? My chief complaint right now is how cold my downstairs living area is. I feel like insulating the floor to isolate the crawl space would be a better starting point to alleviate this problem. 

How well is Fiberglas batting at insulating crawl spaces? I would love to have the spray foam insulation, but I definitely can't afford that for all 1200-1300 sq. ft. of my crawl space. Fiberglas is still pricey, but at least it's affordable. 

If I completely close off the outside vents, will a dehidifer be the only thing I need to keep my crawl space up to code? If not, what else do I have to take into consideration when sealing off my crawl space? 

And lastly, do I just cut a hole in one of the furnace ducts to allow the forced air to enter the crawl space? I'm not quite sure how to go about this step. Also, do I have to adjust the output of my furnace after this due to how large my crawl space is?


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## Windows on Wash (Aug 30, 2011)

Use rigid foam and small amount of spray foam to seal up the bands.

The floor will be warmer and the space better performing if you move the insulation to the stem wall.

Search the forums here for a bunch of posts on the conditioning of crawlspaces and you will get all the feedback you need. 

Just cut a hole once its sealed up.


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## arblargan (Sep 4, 2014)

Ok, so after a boatload if research, I think I've come up with a game plan. The first thing I'm going to do is make sure every inch of dirt and cement are completely covered by poly. First question, can I lay more vapor barrier on top of existing vapor barrier, or do I have to rip out the old one? I don't want to create a nice little swamp between layers of vapor barrier. I've noticed that I have a little bit of moisture seeping through the vapor barrier, or at least that's what it seems like. There's no holes, but what does this mean?

Next question, do I have to place more insulation on the side wall if I already have that white styrofoam block stuff over the foundation cinder block? If so, what product should I use? When I put my hand on the white block stuff, it doesn't feel cold to the touch, so I think it's doing a good enough job of insulating the side wall. 

Next, I'm going to be sealing the crawl space vents immediately, except for one vent. In order to cut costs, I'm going to install a humidistat controlled fan over one of the vents instead of using a dehumidifier. How much will this affect the to temperature of the crawl space? I feel that if I create a hole in the ducting to condition the crawl space, this one vent that only blows out will have a negligible impact on the crawl space. 

Next I'll be insulating the ducting to help improve efficiency. My main question here is, do I have to insulate the supply and return ducting, or can I get away with just sealing and insulating the supply ducting? Remember, I'm trying to save money and cut costs where I can. How much of an impact does an insulated return duct make on efficiency?

Of course during this process I'll be going around and air sealing the crawl space except for the one vent that will have the fan. 

A few last questions. Will creating a hole in the ducting to condition my crawl space create too much of a load on my furnace? Do I need to have the settings adjusted to compensate for the increased coverage area?

And lastly, as said earlier, the walls don't feel cold to the touch, but the ground is freezing (dirt that's covered in vapor barrier). Is there a product that I can put down to insulate the crawl space from the ground? I feel like that's where the majority of the cold is coming from. My main concern is that anything I put down will mold and mildew because of the water that seeps throughout vapor barrier. It doesn't pool, but anything directly touching the vapor barrier does get moist. Also, is this product affordable and available at local stores? Due to living in Alaska, I'd like to save as much as possible on shipping costs. 

And one last question. Is it worth it to place batted insulation in the floor joists, or should I do all of the above first and see if it makes a difference? Everything I'm reading says to insulate, seal, and condition the crawl space to help with a cold living area. From what I'm reading, insulating the upper floor joists won't have much of an effect on the issue I'm currently having. Can anyone confirm or deny this for me? If it's worth it to insulate the floor joists, then I'll do it. I'm trying to get everything together to hopefully finish this project during the first week of October.


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## Nailbags (Feb 1, 2012)

Windows on Wash said:


> If you have ductwork in the crawl and insulation across the stem wall, air seal the perimeter, make sure the vapor barrier across the floor is sealed and probably install a new one, seal the stem wall up, install some more foam and cover it with a code approved ignition barrier (fiberglass will work), and condition the crawl.


you do not want to block up the vent blocks in a foundation. Just insulate the floor and have a good vapor barrier down. also insulate the ducts and pipes.


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## Nailbags (Feb 1, 2012)

arblargan said:


> Ok, so after a boatload if research, I think I've come up with a game plan. The first thing I'm going to do is make sure every inch of dirt and cement are completely covered by poly. First question, can I lay more vapor barrier on top of existing vapor barrier, or do I have to rip out the old one? I don't want to create a nice little swamp between layers of vapor barrier. I've noticed that I have a little bit of moisture seeping through the vapor barrier, or at least that's what it seems like. There's no holes, but what does this mean?
> 
> Next question, do I have to place more insulation on the side wall if I already have that white styrofoam block stuff over the foundation cinder block? If so, what product should I use? When I put my hand on the white block stuff, it doesn't feel cold to the touch, so I think it's doing a good enough job of insulating the side wall.
> 
> ...


You don't know the science behind the vents in a crawl space do you? they are not there for fresh air for the vermin to breath better, you have to have 1.5 sf of vent space per every 100sf of crawl space. other wise you will have rot and moisture issues. sealing it up will cause other problems too. like Radon buildup if you live in that area of the world. Insulation is the key to it. you have to have ventilation to your crawl space and attic year around failure to provide adequate ventilation to both will cause problems.


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## arblargan (Sep 4, 2014)

I know the principle behind a crawl space and the ventilation required. I think that your sf estimates of vent per sf of crawl space may be off. I have about 2 sf total of ventilation for ~1200 sf of crawl space from the time the house was built. Everything I've read about encapsulation of crawl spaces says that as long as the vapor barrier is down with no holes, moisture buildup should not accrue in the crawl space. Regardless, you either need 2 ventilation options, or a standalone dehumidifer in the crawl space after encapsulation.

The first ventilation method I read states: "continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area." So for me, this would be about 40 cfm exhaust fan over the vent duct. 

The second option states: "requires that the crawl space have a forced-air register delivering 1 cfm of supply air from the furnace or air handler for each 50 square feet of crawl space area." This just basically means creating a conditioned environment of at least 40 cfm from my HVAC ducting correct?

If so, then I plan on doing both of the above things. With the vapor barrier down and all air leaks sealed, the only thing I'll have to worry about down there will be stale air buildup. These two ventilation options should alleviate that issue. 

I'm familiar with the fundamentals behind what a crawl space does and why my downstairs living area is cold. My main questions is what is the most efficient, cost-effective, way to insulate/condition the crawl space to help keep my house comfortable. Also, I'm not really familiar with the required materials needed to complete this project. I know about the vapor barrier, but ideally, I'd like to place some type of insulation over the dirt ground to help isolate the crawl space from the cold ground even more. I'm not really sure if there's a product that does this, though. Does anybody know of one, that's relatively affordable for 1200 sq. ft. of crawl space?


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## Windows on Wash (Aug 30, 2011)

I will respectfully disagree with nailbags.

Seal up the crawl and condition it. The ductwork is already there.

You can cut and cobble rigid foam and do it cost effectively.


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## Nailbags (Feb 1, 2012)

Windows on Wash said:


> I will respectfully disagree with nailbags.
> 
> Seal up the crawl and condition it. The ductwork is already there.
> 
> You can cut and cobble rigid foam and do it cost effectively.


so waste energy by conditioning it? run the heat in to it in the winter and cool it in the summer? the passive way to do it is insulate it have a vapor barrier down and not worry about it.


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## Windows on Wash (Aug 30, 2011)

The poster mentioned that his primary issue was with the cold floors. Insulating the ductwork and putting insulation up against the underside of the floor will do a good job of warming those up but when the temperatures are low enough and there's that little duct leakage into that space the floors will ultimately be somewhat cool.

If the space is well sealed and well insulated on the perimeter the amount of energy and conditioned air that will require to keep that space warm is miniscule.

It is also worthy of note that in most applications were your insulating underside of the structure and have copious amounts of ductwork in the structure that it is more difficult to do that properly then it is to insulate the stemwall. 

Whatever option can be done most properly and comprehensively should bear some weight on the decision process.


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## HomeSealed (Jan 3, 2008)

+1. It sounds like the space is already halfway set up to be conditioned, so I'd finish that off as WoW described. Insulate and air seal the sill boxes, air seal all of the penetrations in the space, seal the vents, etc. 
There are two acceptable ways of addressing a crawlspace depending on various factors, 1) establish the space as inside the building envelope (insulate the walls, airseal, poly the floor, etc) or 2) vent the space and insulate/seal the floor from the space. Given your climate along with the presence of all of the ductwork, plumbing, etc, I would probably continue along the line of making it conditioned space. Should be more cost effective as well... Right now it sounds as though its kind of half and half which is not efficient. It either needs to be treated as interior space or exterior space to put it simply.


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