Addition to house is built on wooden posts with exposed joists. How can I insulate or improve this?

Question

At some point before I bought my house an addition was added to the back of it. The addition is built on wooden 4x4 posts which raise the floor about 6 inches off the dirt ground. The floor joists are exposed to the outside which seems crazy. There is a plastic vapour barrier (thin plastic sheet) hanging under the joists and there appears to be expanded polystyrene sitting on top of the vapour barrier, vaguely between the floor joists.

Should this addition have been built this way? Is there anything that I can do to improve the insulation and weatherproof properties of the floor?

Answer 1

Don't know where you live...........but I'm in Washington st. so here what we do is insulate between the joists and put 3/8 plywood soffit under to stop air infiltration and bugs etc.....if you do this be sure you have phones gas power electric all the things you want out there before closing........but that would be the right way..........get your self a cheap air stapler and shoot it on laying on your back.......would not take long to do........use an insulation with no paper face(plain) we call it stuff insulation.......you want no dampness......but this will help keep your house warm and prevent animals from sleeping in it. All the best.......if you have ? email

Answer 2

Let's do the smartest thing. You need to block in around the perimeter of the addition. This process will be somwhat like putting skirting around a mobile home.

First you need to attach a base around the perimeter which you can fasten blue dow styrofoam into. I suggest using treated 2" x 4" if your perimeter 4" x 4" posts are at least 8' apart or closer. Farther than that use 2" x6" treated boards.

s***w them onto the backside of the 4" x 4" posts.One at the top and another at the bottom. Now take a flat shovel and go around and level out the ground around the addition.

Now purchase some 3" Deck screws and some 1/4" fender washers. Slip the s***w through the fender washers.

Purchase the dow styrofoam in 4' x 8' sheets x 2" thick.

Measure and cut styrofoam so it fits between ground level and bottom of house framing, Take screws with fender washers and s***w it into the boards you put up. Attach it about every 6". Now you have the first layer up.

Now go back and install another layer over the first. only this time you are going to glue it into the first sheet.

Purchase some adhesive that will not eat into the styrofoam.

Now you need to cover over the last layer of styrofoam to protect it from exposure to elements.

I suggest some regular vinyl skirting that they use for mobile homes. You can purchase the ground channel and ground pins. The skirting slips into this channel at the ground, you also need the top channel that the skirting slips in at the top, and the cover trim.

Since you cannot easily get under the addition, I think this will be the best and easiest solution for you.

Go for it.

Answer 3

Keep it simple. Add skirting with foam board glued to the inside it to the 6" gap. Otherwise, rip up the sub floor from the inside of the add-on, add more fiberglass insulation, replace vapor bearer, and put the sub floor back together.