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cyprusrom 09-20-2013 05:50 PM

Insulating a 3 seasons room floor
Hello everyone,
I am building a 12x12 3 season room, and I could use some advice.
I am trying to figure out what would be the best way to insulate a 3 season porch.
The floor used to be a deck, 2x10 pressure treated joists, 16.O.C. The bottom of the joists are only a foot or so off the ground, so no(minimum) access to underside of joists.
My plan was to put 1" x 1" strips on each side of joist flush with bottom of joists. Then drop in inch treated plywood, screwed(does it need to be glued too?) to the 1x1 strips.
Next, was going to put probably 2 layers of 2-1/2" x 4' x 8' R12.5 FOAMULAR 250 Rigid Foam Insulation, and use some can spray insulation to seal any seams/corners.
-Would you consider it to be enough insulation? I live in North Central Wisconsin, winters can get rather cold and long.
-Would that be the proper way of doing it, with the insulation towards the bottom of the joists, and an air space between the last layer on insulation and sub-floor, or bring the insulation right next to sub-floor and leave the non-insulated space at the bottom of the joists?
-Would I need a vapor barrier between insulation and sub-floor? (Planning on putting 6 mil plastic layer right on top of the ground)
Also, would anyone think I need to put any bridging/blocking between joists? That would add more pain in the arse trying to insulate the floor. I plan on finishing the floor with porcelain tile(I thought might be the best choice, given the extreme cold/hot temperature variations). I am thinking maybe the plywood screwed to the 1x1 strips between the joists might add enough stiffness.
Thank you in advance for any input.

Gary in WA 09-21-2013 03:35 PM

Check with your local building department please. Require a perimeter footing or grade beam, 18" code required minimum clearance to wood joists, bearing to soil from roof loads, windows, walls, heating, etc....


cyprusrom 09-21-2013 04:08 PM

The "Deck" was built with a sun-room in mind.The deck is attached to the house by a ledger board.Away from the house,it sits on two beams, each made of 3 2x10" that sandwich (2) 1/2 inch treated plywood between them. Each beam sits on 2 6x6 posts that have concrete footings 4' deep.It has a double rim joist and 16 O.C joists. It cantilevers about 12 inches. The entire structure will be only 12'x12'.
I did apply for a permit, and I was given a go.

Gary in WA 09-21-2013 09:33 PM

Excellent! You'd be surprised at how many "remudels" I've seen by others over the years....

R-38 is code minimum; footnote "g", lol.

You would still get thermal bridging and moisture through the exposed joist bottoms (radiation coupled) with wood at R-1.25 per inch;

The suggested 1-1/4" of plywood on the tops is safe. Do a "search" for topic at box, top of every page...a few good tiles guys around here. I'd need the closest large city near you for a dew-point calculation...

Warm on the toes; airspace at floor;

Vapor barrier; not with that thick a foamboard (any foil-faced?), waiting on the locale...what thickness/type of sub-floor? Air-seal (caulk) the plywood to prevent infiltration.


cyprusrom 09-21-2013 10:19 PM

Thanks Gary!
I live in Wausau, which is about 150 miles north of Madison, or about 100 miles N-NW of Green Bay.
After some research and other suggestions, I was thinking of using Roxul
A bit more expensive, but I thought I would get away without using a vapor barrier, maybe, since they claim is water resistant?
I am thinking, if I use a vapor barrier, would be stapled on top of the joists. How would I then glue the subfloor to the joists?
About the thermal bridging...Could I put a 1" of rigid board on top of the joists, then glue/screw the plywood/T&G OSB? I think they also make some plywood/OSB with a rgid foam board glued together.

Gary in WA 09-23-2013 11:09 PM

It doesn't take much to confuse me these days, describe the floor make-up as planned.... lol. The decking plywood/OSB is around 0.75 perms (hence another vb not needed), the foam varies per thickness, need to know the materials for perms.

Marathon County, Zone 6;

Foam board thickness to prevent cavity condensation for your Zone;

Foamboard strips on the joist tops is another option against thermal bridging if full inside coverage stops drying.


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