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Old 05-24-2020, 08:55 PM   #1
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Crawl Space Options - Encapsulate or Insulate Floor


Hello,
I am seeking advice regarding how to best insulate an addition over a crawl space.

A planned flooring renovation has led to the floor deck above a small crawl space to be removed and I need to seal it up again.

The room:
13'x19' Dimensions. Crawl does not have enough space to get under once floor deck is installed. (About 10" up to floor joists). Floor deck currently removed due to foundation repair project.
Location: NW, Indiana
Project: Installing Central HVAC to an addition that had none. Replace Deck. Install Flooring.

This room was previously insulated with fiberglass between floor joists. Floor always cold of course. Much of the fiberglass had fallen off since installed.
Since I have the space open, I see two options:

Option 1: Insulate as was done before, wrap duct in the R3 or R6 bubble stuff, Install Floor deck. Make sure sufficient venting exists for crawl. Floor will likely be cold. I thought about using a mesh or something to keep insulation suspended against the floor.

Option 2: Use 4" (or whatever the thickest I can get is) foam around the crawl. Seal the crawl using plastic sheet. The room has a void left between the original basement wall and the crawl. This is what I'm using to run my duct for central air.

My main questions:
1. Which option is preferred and why? I'm leaning towards Option 2 but have been advised otherwise by someone who's opinion normally holds merit.
*Assuming Option 2*
2. Would I fully seal the space between basement and crawl or would I leave that open for airflow into the crawl that's now sealed?
2a. If open, should I install a register in the crawl or is leakage out of unsealed duct sufficient? It would 'return' through the old window into the basement.

If you'll indulge more questions (Still Assuming Option 2 is preferred):
3. What is used to seal a crawlspace? Is it just the heaviest plastic sheet that I can find in a home improvement store? Construction adhesive to attach it to the 4" foam?
4. Foam I assume gets installed just inside the first joist and sandwich the plastic between foam / joist so the foam is inside the sealed envelope?

I've seen radon come up in other threads about this topic. House currently has an active radon mitigation system to drain tile for basement. Also, I have an electronic monitor.

Water issues in crawl: Currently yes but already being addressed. Will no longer be a problem.

If I have missed something, please feel free to set me straight. Looking forward to the advice and conversation.

Last edited by bmax1985; 05-24-2020 at 08:58 PM. Reason: Added data
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Old 05-24-2020, 09:46 PM   #2
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Re: Crawl Space Options - Encapsulate or Insulate Floor


I live in the Chicago area and have a half crawl space. For years I dealt with freezing floors above the crawl space, which is the kitchen, living room and dining room. I had planned on insulating under the floor and purchased a bunch of rolls of R19 fiberglass insulation. Then further reading lead me to the link below from Building Science. I learned that insulating the rim joist with foam boards and sealing off the two static vents to the outside was the proper way to deal with it. Once I did that, the temperatures in the crawl space stay at about 63 degrees year round (I have a remote temp sensor down there), when before, it would get down to just above freezing during the winter, making me afraid that my water lines would freeze. I have heating ducts down there but no vents feeding the crawlspace itself. I used the newly purchased insulation up in the attics instead.

https://www.buildingscience.com/docu...ace-insulation
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Last edited by Dave Sal; 05-24-2020 at 09:49 PM.
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Old 05-24-2020, 10:45 PM   #3
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Re: Crawl Space Options - Encapsulate or Insulate Floor


Quote:
Originally Posted by Dave Sal View Post
I live in the Chicago area and have a half crawl space. For years I dealt with freezing floors above the crawl space, which is the kitchen, living room and dining room. I had planned on insulating under the floor and purchased a bunch of rolls of R19 fiberglass insulation. Then further reading lead me to the link below from Building Science. I learned that insulating the rim joist with foam boards and sealing off the two static vents to the outside was the proper way to deal with it. Once I did that, the temperatures in the crawl space stay at about 63 degrees year round (I have a remote temp sensor down there), when before, it would get down to just above freezing during the winter, making me afraid that my water lines would freeze. I have heating ducts down there but no vents feeding the crawlspace itself. I used the newly purchased insulation up in the attics instead.

https://www.buildingscience.com/docu...ace-insulation
What temp sensor do you use?
Is the space open to a basement or anything like mine would be?

My core concern with completely sealing it is around potential humidity. (not sure from where but I guess possible)

I'll give that article a read too. Thanks.
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Old 05-24-2020, 11:22 PM   #4
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Re: Crawl Space Options - Encapsulate or Insulate Floor


The thermometer with remote sensor is an Oregon Scientific like this one:
http://www.infiniteelectronix.com/or...lockblack.aspx
My house is a tri-level, with a below grade family room, half bath and utility room. The living room, dining room, kitchen, and side entry are on the main ground level floor, and there are three bedrooms and a bath on the upper floor. The crawl space is mostly closed off from the below grade areas except in the utility room, where there is about a one foot tall opening, about 4 foot wide, which goes over the lower bathroom into the crawlspace. Air can pass between both spaces, but now that the crawlspace is no longer exposed to the outside air (through the two static vents on either side which were sealed off when I insulated the rim joists) the temperature stays about the same as the rest of the house except during winter, where it stays at 63 degrees, like I mentioned before. It is also less humid than before and feels pretty much like the rest of the house.
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Old 05-25-2020, 01:31 AM   #5
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Re: Crawl Space Options - Encapsulate or Insulate Floor


It is a no win situation. Just some more things to think about.

We do a lot of walk out basement houses with garages under bedrooms upstairs.
The garage is like a great big crawlspace the outside walls are never insulated.

The floor of the bedrooms above a 2x10 or more and are filled with insulation.

As heat rises in the room above, no amount of insulation will make it warm, just less cold. If you can keep the floor at the low 60s you are winning.

For the living part of the basement the footing goes down to frost depth and to stop the freezing from creeping under the floor We can put foam board on the inside of the foundation down to the footing. or we can put the same amount of foam board under the slab around the perimeter. For us that would 4" x 18" and the usually get the the full 24" width on the flat.

There is no advantage to insulate the rest of the basement floor as heat rises and the floor will stay in the mid 50s with or with out insulation. They do insulate it if they are going to heat the floor.



Both systems loose if you have water problems

Most of our craw spaces are vented and was changing when I retired.

The crawl space was treated much like a basement, they got a skim coat of 2" concrete.

We glued sill gasket to the foundation at the line of the top of the lab and the 6 mil poly glued to the sill gasket

So I think if you are going to seal the crawl space, you want foam board on the flat around the perimeter the same as frost depth, I would want the poly to to up and be sealed to the sill at the top of the wall and then the foam board up the exposed part.
But then you have to heat the space or your floor will be just like mine. Mid 50s.
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Last edited by Nealtw; 05-25-2020 at 01:34 AM.
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Old 05-25-2020, 08:56 AM   #6
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Re: Crawl Space Options - Encapsulate or Insulate Floor


I fully agree that a crawlspace is no win. Slab or basement IMO. Unfortunately it's what I've got. The water issue won't exist by the end of the week. The addition was built over half a patio that's sunk and directed water under.
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