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How do I insulate a crawl space ceiling and walls?

I'm desperately looking for some advice on how to properly insulate the crawl space. I live in eastern Massachusetts where it has recently been in the teens and 20s for low temps in winter.
I have a large crawl space under my kitchen and 3 season porch ranging in height from 1' to 4' with no venting to the outside. I recently air-sealed the entire rim joist with an expanding foam and added a heavy mil plastic sheeting over the dirt floor, making sure to overlap and seal all edges. We also insulated the copper pipes in the space with closed cell insulation tubing and taped all joints to prevent the pipes from freezing. The kitchen is still very cold.
I would like to ideally insulate the space in between the floor joists and possibly the exterior brick foundation walls but are getting conflicting info on what to do.

- Could use a poly spray foam kit, but the max R-values I have seen are only in the 7-10 range and its not anywhere close to warm enough outside to prevent it from failing. Everything I read says not to use apply when below 60 degrees or it will not cure properly. Its barely in the 30s right now! Also, the exterior walls are not all that clean and the foam might not stick.

- Floor joists are not evenly spaced and range from 17"OC to 22"OC so batt insulation is going to be a pain.

- Could use blown-in insulation but how do I get it to stay up there? Someone told me to use a plastic sheeting across all the joists with furring strips for extra support and then simply blow the insulation into each joist cavity, but I am worried that the plastic sheeting is too much of a vapor barrier and will cause mold inside the joist cavities. If the crawl space is already moisture sealed, am I OK? Do I need some sort of barrier up inside the cavities on the bottom of the floor?

Also, do I really need to insulate the brick foundation walls and will this show a big difference? Because the dirt is not evenly distributed and some areas are tight to get into, getting rigid foam board would be nearly impossible.

Please help!!!

Re: How do I insulate a crawl space ceiling and walls?

Here in the midwest we always use a rolled fiberglass batt insulation. It's a 4' tall roll of fiberglass insulation that comes in varying thicknesses and R-values. It can either be spray glued to the foundation wall or you can attach it mechanically. If the wall is dirty like you say they you may have to mechanically attach it.

Power actuated fasteners or drilling fasteners with large washers will work.

I would recommend using the poly face insulation so that it can also act as a vapor barrier. The poly face goes to the heated side (interior).

You're right that the spray foam may not work in these cold temperatures.

Stay warm!

Re: How do I insulate a crawl space ceiling and walls?

How thick is the foam around the rim joists? If you have 4 to 6" of foam against the rim joist all around, and it is a spray foam that has sealed all the seams between the joists and the flooring material, then you don't need to do much more. Any warm air in the crawl space will rise up against the floor and the air below the floor should be warmed up by radiation from the floor.

If the floor is still cold, then you must had air circulating around under the floor. You could use a heavy plastic across the bottoms of the joists to stop air from circulating and the air trapped between the joists would then warm up.

If the joist cavities are shared between the three season porch and the kitchen, then you will need to put some blocking between the joists along the wall between the kitchen and the joists and they will need to be insulated. Since you don't need any structural support, the blocking could be made from foam board, 2" thick. That would give you the blocking plus the insulation in one step.

As long as the space immediately below the kitchen floor shares any space with the 3 season porch, you will have cold floors in the kitchen. You don't need to divide the whole crawl space, just the joist cavities and then seal them under the kitchen floor. Moisture should not be a problem as you probably have a moisture barrier on the kitchen floor now.

You might also be getting cold air in under the wall at the floor. You have a sill plate on the wall that sits on the flooring and the wall studs attach to it. depending on the siding used, sometimes air can blow under this sill plate and spill onto the floors. You have to take up the trim around the edge of the room and caulk this seam to stop it. In a kitchen, this might mean removing some cabinets.

Floor insulation is different than walls and ceilings (attics). Warm air rises so in a wall or ceiling, the warm air rises away from the surface. In a floor, the warm air rises and is trapped by the floor. dead air in a floor has about the same R-value as fiberglass. That doesn't work in walls and ceilings though, just floors.

Re: How do I insulate a crawl space ceiling and walls?

Joel- Thanks for the feedback. Ill look into that for the walls.

Keith- the rim joist foam sealant is not all that thick- maybe an inch at most. Its basically the equivalent of those cans of Dow Gap Filler sealers you get at Home Depot that you use once and then throw away. Perhaps I need to double check around the edges again for any leaks or areas that are not completely 100% covered or have shrunk and cracked since it was done this summer.

Luckily, the joists run left to right of the house, while the porch is in the rear, so the joist cavities are not shared. Ill try the plastic sheeting across the joists as you suggested. Do you think its worth adding any insulation as well? I was contemplating the blown-in kind, but I am nervous about any moisture buildup since there will be plastic sheeting holding it up inside the cavities on the cold side. If there is a vapor barrier underneath our linoleum floor, will that be enough to keep moisture buildup out of the cavities? Its also an extra $600 or so for the extra insulation, so I don’t want to waste the money if it wont make all the difference after the plastic sheeting is up and air tight. What you said about flooring dead air being good potential R-value is encouraging.

I didn’t think about the air leakage from the sill plates. That might make sense, since there is evidence our exterior walls are cold already (my husband has to put hot water in his coffee cups each morning before coffee because the cabinets are so cold.) Is there anything we can do from the outside to help this? Obviously gutting the kitchen and insulating the walls and ceilings would be ideal, but that’s at least a few years away…

Thanks for all your help!!

Re: How do I insulate a crawl space ceiling and walls?


I also live in eastern Mass. & bought a house here some years ago that had some of the same problems you are experiencing---namely, a crawlspace varying from 1' to 2' without a cellar.

The previous owner had built a small 6' X 6' empty storage area in one part of the crawlspace in a corner below the kitchen--as YOU noted this made the kitchen floors impossibly cold in the winter; in addition, since there was no cellar, the previous owner carved out a small utility room on the 1st floor to house the heating system (forced hot water) & electric panels---this was at the expense of adequate space for an impossibly small bathroom that had nothing but a small shower, commode & tiny bathroom sink---and I also was forced into a seemingly endless battle of installing various types of insulation, plastic sheeting & other like procedures that just didn't seem to work.

After years of suffering with this, I finally had a contractor excavate a large enough area inside the crawl space to relocate the heating system and install adequate drainage to eliminate any moisture problems; the planned project turned out very well----the heating system was moved to the new utility room under the kitchen, the heating system was moved down there, and the space formerly taken up by the 1st floor utility room now allowed the inclusion of a full bathroom with a tub/shower tiled enclosure and mentioned fixtures---wonderful, what a joy to have a full bathroom.

The crawl space and new utility room remains at roughly 55 degrees even in the coldest New England weather because the near-boiler distribution piping keeps the area warm enough so no additional heating is required---the kitchen & floors on the 1st floor are warm, and the apt. itself is much easier to heat in the winter.

If you have a host site where you can post some photos of your crawl space/project area, I'm sure many of us would love to see them, or you can click onto "BOARD JUMP" below, then onto Electrical & Lighting, then onto moderator JL McDaniel to upload the photos to him so he can post them here.

Clearly, what I did to my house may not have any relevance to YOUR situation, but I could see a lot of similarities as I read your post, and I wish you well on your project, I know you will be successful.

Re: How do I insulate a crawl space ceiling and walls?

Did you actually use those small cans of foam to do this? If so, I would suggest that you go back and re-insulate with fiberglass batts. Don't remove what is there, just cut the batts to fit. At the joists ends where they meet with the rim joist (perpendicular rim joist), cut the batts about 2-3 feet long. Fold the pieces into a J shape with the short end of the J against the floor, the bottom of the J against the rim and the longer section of the J along the bottom of the joist cavity.

Along the rim joist that runs parallel with the floor joists, cut a batt to width to fit the height of the rim joist, or fold it in half length wise so that it goes up the rim joist and along the underside of the floor. Then put another batt along the bottom bridging the rim joist to the first floor joist. Now just enclose the whole thing with a thick plastic sheet stapled to the bottoms of the joists.

The linoleum makes a pretty good vapor barrier by itself. Same for vinyl flooring or vinyl tiles and ceramic tile floors. A floating wood usually has a plastic sheet under it as well. only hardwood flooring might be a problem. But it can be covered with a good coat of clear polyurethane that will seal it as well.

Ultimately, you will need to address the walls though. Cant do much from the outside unless you are putting up new vinyl siding with some foam board under it, and not just that 1/8" thick stuff they try to pass off as insulation. You need a minimum of 3/4" just to make a measurable difference. 1.5 to 2" would be better.

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