Insulating a Floor Underneath the Crawlspace

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About: Hi I'm Linn and on my Youtube Channel I have lots of great videos about building, construction and fun projects. You can also check out my site @ http://darbinorvar.com

To insulate in between the joists underneath the floor in a crawlspace is definitely dirty and annoying work, however it can make a big difference in terms of keeping a space nice and warm / cool. I'm turning a small shed into a workshop and wanted to make sure that the floor wouldn't be the source of letting all the energy out once I heat or cool the space, so to insulate it made a lot of sense. In this instructable I'll go over the process that I used the add insulation underneath the joists.

(For a better perspective, make sure to watch the video that goes over the entire process!)

Supplies:

Step 1: Insulation

For this space I decided to use 1.5 inch Rigid foam insulation, and I picked up four sheets to cover my under 200 sqft building.

Step 2: Cutting to Size

I began with measuring the distance in between the joists. It measured 14.5 inches, which meant that cutting the boards to 14 inch widths was a good idea. I first used a jigsaw for the cuts, however a razor blade knife also works great.

Step 3: Cutting to Size

The individual sections can't be too long, or else you can't fit them inside. I found it easiest to cut them to about 31 inches long. I began by adding some construction adhesive to the pieces and then secured them in between the joists with 2 in. long 16 gauge nails in a nail gun hooked up to a compressor.

Step 4: Clothes

Getting under a crawlspace isn't not exactly a clean and pleasant experience, so it's nice if you can wear clothes that you don't mind getting dirty! I wore nylon rainpants, jacket, hat, boots and gloves, and that made a huge difference! Make sure the area is clean beforehand (I used a leaf blower and rake), and another option is to place cardboard pieces on the ground before crawling under to make it a bit more manageable.

Step 5: Preparation

If you want this job to go smoothly, preparation is key! Cut a bunch of sections to size, add construction adhesive and then place them around the house so you can easily access them when you're underneath. Make sure your air compressor is pumped up and that you've got plenty of nails in the gun! The less trips you have to make crawling in and out of the crawlspace, the faster you'll be able to complete the job.

Step 6: Final Section

Once I was done placing the 31 inch long sections within all the rows, it was time to measure and place the last piece on each. I did this all at the end, and could do it from the side, so I didn't even have to crawl under completely!

Step 7: Spray Foam

To fill in any small gaps, I used expanding spray foam. This way I didn't have to be too precise when cutting the foam pieces to size, since any gaps could be filled with the foam product.

Step 8: Conclusion - Watch the Video

To see the complete process, and my honest reactions to what it was like to crawl under the shed! make sure to watch the video!

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    21 Discussions

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    charlessenf-gm

    Tip 8 weeks ago

    In my experience, chickens eat insulating foam! They de-insulated the lower panel of our garage doors. I wound up covering the bottom section of the shop door with re-cycled dog food bags. I had the problem with white sheets, white sheets covered with silver foil, and green sheets - have not tried the color she is using!

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    TheBuccy.

    3 months ago on Step 8

    A sheet of thin ply or hardboard makes an excellent slide for the body .
    Make sure the temporary supports are based on a good sized material to prevent sinkage into the softer ground .

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    Lorddrake

    3 months ago

    Since you have insulated the floor, how noticeable is the difference in the shed?

    3 replies
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    francatiLorddrake

    Reply 3 months ago

    Why Didn't You Just Put The Insulation On Top Of The Floor & Sheet Out With 18mm Ply

    Yes, You'd Lose Only 2 Inches Height, But Far Far Better & Easier Job !!

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    charlessenf-gmfrancati

    Reply 3 months ago

    "Put The Insulation On Top Of The Floor"
    Well, best to put the insulation outside the package.
    Adding two inches to the flooring impacts the entry way* and 'hides' the plate one would fasten the bottom edge of the paneling to.
    Frankly, I think she would have been well advised to leave space between her insulation boards and the underside of the flooring and used wires to hold the sheets in place until she was able to seal the edges with the spray foam and then cover the entire thing with 8-10 mil plastic sheeting stapled to the underside of the floor joists - as in insulating mobile homes.

    * Trip hazard

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    francaticharlessenf-gm

    Reply 3 months ago

    No Sir/Madam, !!!
    Obviously, one Errects The Cabin/Shed First, PRIOR TO LAYING THE INSULATION and Second Floor Covering, as for
    Impacts The Entry Way ?? This would be no thicker than a Traditional 2 Inch Hardwood Weather Saddle !!

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    aking14

    3 months ago

    There's another easy option, funny I didn't think of it earlier since I've done this before, but it's been a while. First as many have noted skirt, since ground temperature that isn't exposed to cold air tends to be around 60 or 65 degrees, maybe not quite that high if you're in the north but still better than the ambient air temps. Then just harbor freight foam mat on the floor. While it's R value is probably small, that R value with protected ground temperature on the other side will probably put it in range of the outside walls' value with outside air temps on the other side for total heat loss. From that point it is probably be better off to do more on the outside walls before doing more to the floor..

    Very low effort and the mats on sale for $6 or $7 per 4'x4' isn't too bad, plus the added benefit of a padded floor to walk and work on. Carpet squares are similar and also great and are relatively cheap to do a small area. But they are highly dependent on what you'll use the shed/shop for as to whether carpet would be a good idea.

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    farna

    3 months ago

    Just stapling a layer of plastic or house wrap under the shed will make a noticeable improvement. Staple to the underside of the floor joists, leaving a dead air space between plastic and floor. I did that in an old barn under the rafters. It reduced the inside temp by 10 degrees in the summer. Roof was exposed 5V galvanized steel ("tin"). The rafters were open on the ends, so there was some ventilation between the house wrap and steel roof. I was pleased with the temp reduction though, especially for the low cost and effort.

    The floor is the LAST place you loose energy. A vapor barrier and underpinning (with some ventilation) will reduce heat loss through the floor tremendously and not require so much crawling underneath. That and maybe a cheap roll vinyl or outdoor carpet on the floor would be great for a shed. You did a great job, just a lot of work. Don't know that you gained much more than a vapor barrier/underpinning (which can be anything to reduce air movement under the shed -- siding extended to ground, etc) and maybe a floor covering would provide.

    Not criticizing your work, just offering alternatives. Great job, you covered everything well!

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    OldyO

    3 months ago

    lots of useful advice in write up and replies, hat off for darbinorvar for doing theses things, my wife built her own summer house and shed plus service her car. i like women like this.
    big smile and round of applause from this old man from the uk.
    respect.

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    charlessenf-gm

    Tip 3 months ago

    Simpson Strong-Tie; 16 in. Insulation Support (100-Qty);Flexible yet strong with carbon steel, spring wire construction. Mitered tips penetrate wood to keep insulation sheet in place. Easy to use for DIYers of any skill level; $14.31/case
    https://www.homedepot.com/p/Simpson-Strong-Tie-16-...

    By the way, reading all the advice offered here I thought to advise anyone embarking on such a project to get a few grains of salt or, better yet, speak, NO! - better yet ask and LISTEN to professionals and take the time to research a project before you begin!

    If she had jacked the shed up higher on the one end another eight inches (1 block) before leveling it, the insulation task would have been simpler. I suspect that, had she asked a few folks at the professionals desk she might have heard about the insulation supports and a variety of insulating alternatives (they re-insulate mobile homes in a similar fashion everyday).

    Even a simple SHED is designed. The flooring leaves the bottom plate exposed, for instance, so there s a surface to nail the interior wall boards to along the floor and, if youse, the baseboard molding as well. There is a reason for the way the shed was designed - best not to fight 'the program.' Even better to learn it.

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    Colintb

    Question 3 months ago

    Our camp has no insulation in the floor and this was one of the ways I was thinking of insulating it. I'm not sure if the guy who built the place put any vapour barrier down, using this method would I need to apply vapour barrier first? Thanks in advance.

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    charlessenf-gmColintb

    Answer 3 months ago

    A layer of relatively heavy weight plastic under the structure is always a good idea. Also, the area needs ventilation and somesore of screening to exclude rodents and such for taking up residence therein.

    You might look to using insulation wire to hold the insulation in place and then add the spray foam to seal them in place as the idea is to capture and hold 'dead air' that is itself an insulator.

    If the building is up on blocks (as was the case here) raising it up a block and a half first, then lowering and leveling it might be the best approach. And, if you can staple another sheet of plastic to the bottom of the floor joists - it might add to the insulation..

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    UncleEd

    3 months ago

    This is often a nasty job, but is rewarding if it's done well. We moved into a house and had is inspected before we bought it. Unfortunately, we didn't realize that "insulation under the floor is installed with the vapor barrier away from the interior of the house, not against it' meant the condensation was directly against the floor joists. During high humidity, the moisture was quite obvious and replacing a few rotting joists and some bracing ended up rather expensive.

    Read what is printed on the insulation and install it properly, The house will be more comfortable and you will enjoy the results.

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    aking14UncleEd

    Reply 3 months ago

    Yes I'd be tempted to make it a 'drop ceiling', down about 1" from the bottom of the joists. That way it would be insulated but all be one air space, and direct just a bit of heat/cooling into it, and a return, so there is at least a little air circulation. Just to be sure there isn't a trapped air space with humidity cycling. Of course for a shed it's probably not that big of a deal..

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    kris256aking14

    Reply 3 months ago

    Depending on the size of the joists she could put foam between them and seal it to the joists themselves with the spray foam. Venting would have to be added to the ridge and to the soffit, but it's doable. I did this on a single-wide that I replaced the roof on and it worked quite well. I thought the poor R value of the joists would come back to haunt me but it's been over 6 years with some pretty brutal winters and hot summers and haven't had any problems.

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    THolz

    3 months ago on Introduction

    I laud the effort you put in and the concept. However, putting nails thru the foam is not a good idea. It puts a thermal transfer point thru the foam and nails aren’t likely to hold foam board in too well as the nail heads are too small unless you use caps. A better option is to use small blocks of wood nailed or screwed to the joists und r the foam edges; they hold better and won’t come loose or break thru the foam board.

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    Brian SchreiberTHolz

    Reply 3 months ago

    Toooo much work. Both Darbin's and yours. Her effort here is a must experience for anyone who has every tried this. You have to learn the hard way! Darbin is a TROOPER!

    I have tackled similar things with foam insulation and learned that using Great Stuff foam(as Darbin did, in the end) is the simplest for both adhering insulation and best for insulation value. Darbin's hard-Earned and hard-Learned lesson of making the insulation "shorter" than necessary is an important clue here.
    First: this is a two woman job! If the space is 14 1/2" make the insulation 13 3/4" or a bit less. Don't bother trying to make the angles "square!" Ahead of time make some small wood wedges about 2 or 3" long with a 20° or so taper, making about 20, 1" wide. Before any cutting go under the shed with a pencil & paper or call out the rectangle sizes to helper to write down dimensions for the first 4 or 5 areas, in order. (Using a utility knife cut half through the foam board and break off the pieces-- no jig saw work, no "magnetic" foam particles flying around!) Have helper offer them, in same order. Apply foam to the face of each piece, wait a minute or two, and not excessive amount of foam; using the foam to adhere to bottom of floor apply it; use wedges to brace edges of foam against floor trusses to temporarily assist the face-foam to hold them in place. While you are doing this the helper applies foam to remaining foam faces and hands them off in order. This stuff is REALLY sticky which is WHAT YOU WANT for instant adhesion. By the time the 5th area is covered the first foam board can have its wedges removed and RE-USED. Handy. When complete with this work over entire floor go back with the foam and do exactly what Darbin did-- foam the edges around foam to the trusses. The trick here is don't remove the wedges too soon. (Actually, using just two nails per area is sufficient and possibly a better solution vs wedges!) Nails are an insignificant drain in heat transfer; only a tiny fraction of an inch area-- especially in this non-critical situation. The real critical insulation areas are in the ceiling of a structure, not the floor.

    One more point. It may have been a consideration to insulate and cover the floor from above rather than below. Yes, the door would need modification, but perhaps less painful!

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    kris256Brian Schreiber

    Reply 3 months ago

    I suspect that insulating on top of the floor and leaving the under portion could create an issue with moisture and mold as your heat transfer would be taking place within the wood floor itself, vs insulating at the bottoms of the beams and the skirting would work well.

    You are correct about the ceiling and walls being more important though. Could skip the floor until further along and get by for a while with just well insulated skirting.

    Personally my approach for insulating that floor would be slightly different. I would take the entire sheets and attach them to the bottoms of the beams using screws with washer heads to affix the foam. That or remove the interior floor and do the same, but use the spray in insulation to affix the sheet to the bottoms of the beams. I suppose you could also do it in strips and use stringers to allow the foam to settle between the beams at the lowest point and seal the sides and ends with sprayfoam. Problem with that is while it creates a dead air space it also uses an excess amount of spray foam, involves far more cutting and is a loss in efficiency as the wood with it's poor R value will still pass the cold through as the foam is only between the beams and not covering them.

    On a different note Foam with the radiant sheething is really quite nice for this kind of thing. I think it's R-Max? They sell it at my local HDepot

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    DonnH1

    3 months ago

    Someone mentioned about skirting the base. That is something absolutely necessary in cold climates where you are setting up a house trailer. Yes, that really is indispensable and makes a big difference. Skirt it and even insulate that id you can afford it. Insulating the floors of a work shed make a difference too. Another place is in the basement of a house. I see contractors placing high density foam on a ground before pouring a concrete floor. When my wife and I moved into a new house up north we noticed it was chilly in the basement. By just putting down a foam base and a rug that changed the environment in a big way. The kids could play down there and not complain. Insulation is good for keeping heat out too when summer comes.

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    moniqueron.groom

    3 months ago

    I too tackled this project, but with my whole house. I used Roxul for the insulation (less itchy)
    but the biggest gain was skirting the whole structure. This keeps the wind out and the natural heat from the earth keeps the space from freezing. Use plywood or marine grade ply if you're in a wet zone and add a couple of vents to get air flow in the summer months to inhibit the growth of mold.