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Building an Insulated Floor for an Air Conditioned Dog House
Floor resting on 4x4 skids
Design of floor
This floor is designed to enable the complete structure built upon it to be movable, although it would probably require a crane similar to the one used to place 5 ton air conditioners into hard-to-reach places. It is also designed to resist water damage and provide some insulation. The 4x4 skids and 2x4 floor joists and bands are treated lumber. Untreated plywood sheathing is screwed to the joists and bands with rustproofed exterior screws. Expanded polystyrene insulation is nailed to the sheathing from underneath.
- 1 4X4X8 TREATED HEM FIR Lowes Part # 86279
- 5 2x4x8 DOUG FIR TREATED Lowes Part # 84652
- 1 23/32 Fir Floor Sheathing Lowes Part # 12182
- 1 1.5 Inx4x8 Expanded Polystyrene Lowes Part # 41505
- 1 2-1/2 x 9PGP EXT SCREWS STAR D Lowes Part # 323899
- 1 Common Nail 8D 2.5” 1# Ace HW Part # 52234
- 1 Roof Nail 1.75” 1# Ace HW Part # 5188230
Cutting the treated lumber
Cut the treated 4x4 and one of the treated 2x4's in half. Cut three inches off one end only of each of the four remaining treated 2x4's. The actual width of each of the two 2x4 bands is 1 1/2 inches, so three inches must be removed from the joists to give a perfect fit underneath the eight foot long plywood sheathing.
However, there is a design variation where you might want to just leave the three inches intact. I had originally planned to have the shed dimensions 4 x 6 feet (its initial use was as a dog house) and have a 2 foot porch on one end of the joists. If you do this, you may as well have a 2 foot 3 inch porch. You might want to use treated decking material, or possibly cedar or redwood, to cover the porch.
For the long term, we are using the shed mostly for storage, so getting the extra covered space took priority over the dog porch. Also, I am growing more claustrophobic and didn't like the idea of being able to stretch my arms in both directions and touch the walls. But if you are going to use this design for a dog house long term then it will probably make sense to shorten it a couple of feet and keep the height of the walls a couple of feet lower than what I ended up building. For all but the very largest dog breeds, a smaller volume will be cozier and much more efficiently air conditioned.
After cutting the treated lumber
Assembling the floor
If you have a power framing gun, then you will want to use that instead of a hammer and the 2 1/2 inch nails. Otherwise, use the 2 1/2 inch nails to attach the two four-foot bands to the four joists, flush on the outside, and with the two inner joists spaced evenly. After subtracting three inches for the two outside joists, that leaves 45 inches divided by three spaces, or 15 inches from the inside of each outer joist to the center of the inner joist next to it. Be very careful to make it square.
Place the 4x4 skids underneath the frame two feet from each end and attach with the 2 1/2 inch exterior screws angled into the 4x4 through each side of the joist where they meet. If you have a framing gun, you might want to use that instead of the screws.
Building the floor and installing insulation
Cutting and installing the insulation
Cutting the expanded polystyrene insulation is an inherently messy job. First I tried a utility knife, but its blade didn't reach all the way through. So I measured and marked the other side and cut again with the utility knife. It still didn't cut through. I finished the cut with an old, serrated steak knife. This process was unacceptable. I won't bore you with the other things I tried.
What I ended up using was the linoleum cutting blade included with the Kobalt 28 Piece All Purpose Knife kit, Lowes part # 12809. This long, curved blade was able to reach all the way through, and by using a seesaw motion through the piece being cut I was able to minimize the amount of crumbling from the edges of the cut. By seesaw motion I mean push the blade completely through the piece being cut and then advance first the front and then the back of the blade in the direction of the cut. When possible, I clamped a straight edge to the piece and used it as a guide, not for marking, but for cutting. Even with this improvement, I was still glad I have a good shop vac.
For insulating the floor, first I measured and cut strips just wide enough to fit between the floor joists, and then slid them in from outside one of the skids and forced them the whole length. If the fit is a little tight, the polystyrene will compress and the snugness will help hold it in place. But there were a few places I had to trim a little. Once in place, I tapped in some galvanized roofing nails to hold the polystyrene in place. If I had it to do over, I would have used a few more nails.