Floor insulation in a building on skids

[Zachary Hoyt]

I am getting close to starting work (probably in 4-6 weeks) on my smaller-than-desired shop building on skids and I have the design pretty clear in my mind except I am baffled about the floor insulation. I was thinking of putting foam sheets between the joists, but that is expensive, environmentally unfriendly, and awkward. It may still be the best option, but I'm wondering. Rockwool batts are another possibility, but I am afraid of them holding moisture against the pressure treated wood and attracting varmints to make nests. Another idea which occurred to me yesterday was to run foam sheets vertically down from the bottoms of the walls and bury them in the ground a foot or so. This would be a lot less foam, and would perhaps be easier to seal well than all those strips of foam between the joists. The other thing is just to not insulate the floor at all, and live with it. I think I read somewhere that only 5-10% of heat is lost through the floor, so floor insulation is less effective than roofs or walls. If I did that I would still seal up the walls to the ground in the winter so that the wind didn't go howling through. I'll be grateful for any advice about this.

[Maurice Mcmurry]

Mold and mice can be a problem. Uninsulated floor with plenty of ventilation is how our building on skids is. The floor and floor structure are constructed with treated lumber. I have thought about putting foam on top of the treated floor and another plywood floor on top. I am uncertain about that idea.

[Frank Pratt]

As long as the rock wool is not in contact with the ground, there won't be a moisture problem. Put some steel mesh under the joists to hold the insulation up & keep out the critters.

[Jack Frederick]

Perhaps 2” of foam on the floor between 2x stringers and more plywood above. You loose 2 3/4” of so of height, but it gets the job done. Electrical can be run in it as well.

[Jim Becker]

Rockwool is actually a good material for this application as it really doesn't hold moisture nor is it friendly to critters. It's also formatted to fit in pretty snug. Frank is spot on about using a mesh barrier to prevent physical damage from below as well as account for any potential of the Rockwool to sag downward over time, but that's minimal compared to material like fiberglass because of the density and resistance to moisture.

[Zachary Hoyt]

Thank you all for the input. I'll have to look into options for mesh to go under the joists. I'd rather use Rockwool than foam for environmental reasons and because it is not as messy as cutting 4x8 sheets of foam with a table saw.

[Scott Brodersen]

Whatever you do, be sure not to trap moisture below the floor and ensure there's plenty of air circulation under the floor -- I speak from experience!

[Zachary Hoyt]

Thanks, that's a helpful thing to know too. I'm thinking now that what I may do is build it without insulation under the floor, with the idea that I can put rockwool in from below with mesh under it at some point. It would be easier to insulate the floor before putting down the plywood, but I would need several days with no rain in order to get the walls and roof built to keep the insulation dry. Rigid foam would be almost impossible to retrofit from below, but rockwool should be manageable, if not fun. I might even lift up the plywood sheets that are not under the walls and do the center of the building from above, to save some crawling.

[Jim Becker]

How much clearance off the ground are you going to have here? They type of building you are constructing are generally "on the ground" so future access from below wouldn't normally be possible.

[Zachary Hoyt]

I'm planning to have 1 4x8x16 block above ground and one in/on the ground, and maybe more in spots where the ground is low. Then there'll be 6" or 5-1/2" of skid below the joists, so I should have 10" of space. I'm fairly skinny so I should be able to squeeze in, though it would not be a lot of fun.

[Jim Becker]

If you are going to raise it up like that instead of sitting the skids on gravel as would be customary for a "portable" structure, you may want to consider pouring stable columns to below frost level to make sure your building doesn't "rock and roll". Ground heave in your area may not be pretty at all, especially with a structure of the size you are planning.

[Zachary Hoyt]

I would prefer to have columns (I wanted to pour a slab, really) but I can't put things in the ground near to the town water line. I'm planning to have concrete blocks in a 4 foot grid, so there will be 45 support points for the building. I could do gravel instead, but I think it should be better drained and ventilated on blocks, and I won't have to wait for the spring road weight restrictions to be lifted like I would if I had a dump truck come in to bring gravel. I can bring the blocks in myself. The garden tool shed here that I built in 2009 is 12x16 feet and sits on 6 large rocks, and it has not had any trouble with frost heaving. I am hoping that this new building will be okay as it will have more support points even though it will be bigger.

[Brian Gouldman]

I have a 12x20ft on typical 4x4 skids on gravel foundation I’m building out to be my shop. I’ve insulated the walls and ceiling with rockwool. I will be putting down 2” foam sheet continuous and topping with 1 or 2 layers of advantech followed by some nice 4” prefinished red oak I got for a song. The ceiling is finished out as a cathedral, so the lost height isn’t that impactful since the ridge is about 10ft. I think how you finish the ceiling will play a significant role if you build up from the first subfloor.

[Zachary Hoyt]

That seems like a good way to insulate the floor. Will you need to move the door up so you don't trip over the new higher threshold, or was it built with that in mind? I am planning on a cathedral ceiling so I can store bulky light things up there like bags of shipping peanuts and banjo cases. I was surprised to see yet again a price spike in sheet goods when I spent some time this afternoon thinking about my plan. I found a sawmill about an hour from there that sells rough cut pine and hemlock for 70-75 cents a board foot depending on length, and I am going to see if either local Amish or Amish up there are at a similar price point. I came up with a plan to put up the whole building except the skids from rough lumber, including the floor, and it looks like I would need about 4k board feet so it would cost about 3k. A board floor will dry out and have gaps, but I can put something else down over it when I "finish" the building into proper shop space. My plan is to build the shell in the spring and use it for storage and a place to run the tablesaw while I'm working on the house. Then in the fall when the wood has had some time to dry out I can insulate, cover the insides of the walls and roof with something, run electrical and so on. I just can't see spending $75 a sheet on 3/4" plywood that's not that great anyway. That's about $2.30 a square foot, so pine or hemlock at 70 cents looks a lot better.

[Brian Gouldman]

My shell was a prebuilt, delivered to site, shed I ordered just months before prices started spiking a couple years ago. So it has a vinyl double door that overlays the rim joist on the front. So I will be making my entrance a little shorter, again nbd because of vaulted ceilings, and I plan to just use while aluminum flashing and some sort of transition to make the rim joist there look nice. Currently it has the typical diamond plate angle stock at the corner transition and the bottom of the rim is just exposed.

Main downside to my building is the 7ft walls vs you can build yours as tall as you want and fit a normal single or French door with a threshold if you want. If I were to redo it I’d probably have site built with 9ft walls and a nice full lite French door.

In either case, you could frame the walls on the initial floor, pad under the door with solid wood the same thickness as the foam, put continuous foam down, new subfloor over and screw through to joists, and then just do proper weather flashing like you would any sill. Then you can make it pretty with aluminum flashing like I will, you could have your siding of choice, or just like a trim board. Whatever works for the aesthetic you are going for. In my case I need something thin because of the type of door I have already on the building. But once it’s done you’ll never know there is a double layer subfloor on the structure.

If you go with XPS you can get it in high psi so you shouldn’t need any sleepers within the floor between subfloor layers. It also has some vapor permeability which should help prevent any moisture related damage.

I’m sure someone will come in and say for the price of wood right now you could probably go concrete floor for the same price, maybe even cheaper. I wouldn’t disagree with them at all. I’m stuck with this option because I ordered a prebuilt shell. If I were doing site built I’d prob have a monolithic slab floor.