Is Foam Board Insulation Strong Enough To Be Placed Over Floor Joists And Then Install A Floor Over It?
February 28, 2010 by admin
Filed under Questions & Answers
I am going to be building a 10×14 shed when it warms up a bit. I plan to use it as a shed to store my lawn tractor and garden tools but i can see a time in the future where i may want to use it as an escape (i am going to run power, tv and internet to it). I can insulate walls and ceiling later but the floor is something i think is a lot easier when you are building the shed. My first thought was to use fiberglass but i then thought about mice and potential for getting wet being so close to the ground. So i am now thinking about something waterproof which would be 4×8 sheets of foam board.
My current plan is to build the floor out of 2×6’s 16inches on center. I would then build the walls and put them up. Next comes the floor made from a 1inch foam board (laid directly on the floor joists) and then 1/2 inch plywood. This would bring the floor up to the same level as the 2×4 plate at the bottom of the wall so the floor would be level. The 2×4 would take the brunt of traffic thru the door and the floor would take load from the top.
Other option would be to first put down a sheet of plywood, then foam board, then another piece of plywood on top and then build up the door ways with 1x so the floor level matches.
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When you build your floor be sure to lay some heavy plastic on top of the ground and use pressure treated lumber for the frame and joists. My suggestion for the insulation would be to use 2″ rigid foam and to hold this up you install a 1″ square cleat 2″ down on each side of the floor joist. That way you can measure between the joists when you get them laid, as wood varies from one board to another, and cut the foam on a table saw to the exact measurement and slip it down inbetween each set of joists. Run a bead of construction adhesive on each cleat before lowering the foam pieces on to them. That way they will stay in one place. At this point the foam insulation board should be laying just flush with the top of the joists so you can lay the floor on top of them. I’d use construction adhesive under the plywood on the joists and the foam. I really don’t think you’d like the sandwich, plywood, foam, plywood again. The foam will compress with age and pressure and soon you’d be having problems you don’t need. This way you get great insulation on the floor and good rigidity as well.
I highly doubt the foam board would hold up well directly on the floor joists. Over time it may wear or deteriorate. The foam between the plywood sounds like a good idea. You may also want to lay a moisture barrier down on the ground too before you build.
NO.
Add to the joists with wood then place the foam between the joist. If you are good you could cut the filler just thin enough (1/4″ each side) so you could lay the foam on the joists. Then place the floor.
The site below says you can sandwich rigid foam between a plywood sub-floor and a cement board upper layer. Similar to your plan except you need to support the foam board.
That may work but probably not, one thing for sure, the plywood floor will settle in spots and over time because the foam board is not dense enough to support it well. They do make a roof system that sandwiches foam between 2 layers of plywood, but again they use a denser foam that is stronger than what you can buy in sheets in the store (a roof needs to support a lot of weight due to heavy snows). Foam does work much better than fiberglass to keep the cold out too. You’re on the right track but there is 2 changes to make.
1. Once the floor joists are in place cut the foam to fit between the joists, gluing them along the edges of the foam to the sides of the joists. Be sure to get the right glue for the foam, not all construction glues will work with foam, read the back label. A sharp kitchen knife works best for cutting foam and fiberglass insulation too.
2. Put down the plywood sub-floor before you build the walls, it’s called platform framing and that’s how it’s down now. You want that bottom 2×4 plate to be above the plywood, you may need it for nailing baseboard trim &/or shoe molding later on too.
No way dude! The foam will give too much and begin to deteriorate from the movement of the floor and weight of the floor and everthing on top of it overtime. There’s a few ways you may want to otherwise try. 1 is to put the plywood down, then get under the floor if you have room and make strips of foam insulation, and simply staple these strips from under the floor between each joist.
Now there is another very thin stype of foam insulation thats like 1/4″ or so, and comes in rolls. That I would say you could staple to the bottom of the plywood, then put the plywood down. Its so thin that it will give a little insulation, but won’t bind up at each joist. When you screw down the floor or nail it down, it will simply crush at the joists and lay flat.
PS – its a good idea to buy a decent quality tarp, and stake it into the ground under the entire floor of the shed. This keeps a moisture barrier from the ground.
First put a layer of gravel down.Lay some 30lb roofing felt on top of the gravel and build your platform on top of it. Use paper faced insulation with the paper on top of the floor joists and then put your plywood floor. Now you can construct your walls.
Just make sure the water drains away from the shed. Adding the gravel will elevate the shed from the original ground. You can even dig some earth away from the perimeter for drainage.
Foam Brd?? I assume you mean Extruded Polystirene also called Ridged Insolation?
I would be concerned with the tractor breaking through the floor. Sandwhitching insolation for a floor is usually a bad idea because with the weight it will compress. Suddenly you will notice that your shed feels unstable or spungie. If you were to go with idea two i would recommend you consider using 3/4 or better for the top of the sandwitch.
Personally, I’d Build skids. ( Heavy 2×6 pcs on edge to act as a foundation and to facilitate moving the shead later.) Than i would build the floor frame seperatly. After 3-4-5 ing it( A method of squaring a structure using pathagiras) i’d screw 1/2 ply over the whole thing . Than flip the floor frame plywood down onto the skids. ( doing this you can insolate with cheeper Batt insolation giving a better R value and solving your rodent problem. ) After insolation you can screw your floor down. ( I still suggest 3/4 ply. 1/2 is a little weak. ) From here you can go on with framing walls than fastining them directly to the plywood floor. ( Trying to hit joists of course.) And just cut out the bottom plate from the doorway after the roof is on. ( I diddnt understand how you were goina fasten the bottom plate to the floor frame. )
The problem with this though is that your shead will be a 1′ 1-1/4” off the ground. ( 2×6 skid plus 1/2 ply plus 2×6 floor joist plus 3/4 ply.) You would need a ramp for your tractor.
May i also suggest a window. It’ll get freakishly hot in the summer. I’d talk to window manufacturers. They may have ones that they ordered the wrong size for and therefore may sell cheap. Hope some of this helps… Good luck.