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hpyjack2013 09-15-2013 08:39 PM

air space yes or no?
I'm having a 3 season porch added to my house. It sits on concrete piers and is approx. 18" above ground. I'm having the floor insulated with the idea that we'll put heat out there some day. The contractor wants to leave an airspace of approx. 4" between the floor and the batt insulation. He says it would be more efficient having a dead air space versus having the vapor barrier up tight against the floor.He also plans on putting pressure treated plywood up to seal off the bottom and keep critters out. I live in New Hampshire. Thoughts? suggestions? air space or no air space

joecaption 09-15-2013 08:50 PM

Never any air space, I would not even use pressure treated plywood.
If it was mine I would use at least two layers of 1-1/2 blue foam and seal the gaps with expanding foam. Then just CDX plywood.

Windows on Wash 09-16-2013 05:04 AM

Ding, ding.

Stick with Joe's advice. Rigid foam is your friend here any any foam to the bottom of the assembly will help thermally uncouple the joists from the floor.

Pay attention to the vapor control layers and make sure you don't make a vapor barrier sandwich with two vapor barriers on opposite sides of the assembly.

Gary in WA 09-18-2013 05:01 PM

Yes on the air space, your builder is smart, Fig.7;

Ply on joist is a vapor retarder as is the floor decking, not vapor barriers. Add some PIC strips before the ply to decouple radiation as link showed.


hpyjack2013 09-18-2013 06:02 PM

thanks Gary. I read that article before posting. I also contacted owens corning, spoke with 2 different people:"Thanks for choosing Owens Corning. We do not recommend for you to have air space. We recommend that the insulation be next to the floor no dead air space should be left next to the room, and the idea for a way to block out the critters is a good idea. If you have any further questions please don't hesitate to ask." Not sure what you mean by PIC strips?

Gary in WA 09-19-2013 03:42 PM

I would opt for a warm floor with socks on, your choice;

Another way to look at it- air space in windows;

PIC strips are foil-faced poliso board insulation cut in strips on the joist bottoms before the plywood if full-sheet ffXPS is not affordable. You would lose much of the benefits though. Hind-sight, I shouldn't have mentioned it....


hpyjack2013 09-19-2013 04:20 PM

air space yes or no ?
Gary, thanks for the follow up. I continued to read and did the "math" Can't seem to find an answer to this question: In figure one is it just batt insulation or batt insulation with a vapor barrier and if it has a vapor barrier which way should it face? Thanks

Gary in WA 09-20-2013 05:56 PM

Which link? Using the correct (foil-faced) foam board for your location to prevent cavity condensation, no additional vapor barrier/retarder is needed;

Unfaced f.b. thickness required per location;

You are still required to meet minimum code R-value for the floor, "click" your county:


hpyjack2013 09-20-2013 06:14 PM

air space yes or no?
Hey Gary, it was the google link. I managed to get some of it to work, but some of the pages can't be viewed. I managed to read about the difference between fg vs spray foam(foam was planned already). The fiberglass batts are already in place approx. 4" from floor. But they have vapor barrier facing towards floor.If I understand you correctly the batts would have to come down and have the vb removed? I sent an e-mail to building science asking for clarification on vapor barrier but haven't heard anything.


Gary in WA 09-22-2013 10:01 PM

IMO, leave it be, more trouble than it's worth to remove. The facing has a variable perm rating; the wetter it gets- the more open it becomes.

Fig,"A"; Your sub-floor will stop the vapor diffusing from the room with a perm rating of 0.75 or so, denser than the facing paper. Let me know their reply... thanks.


hpyjack2013 09-23-2013 05:49 AM

air space yes or no?
Thanks again. I'm going to try calling them and see what they say. I'm running out of time.

hpyjack2013 09-27-2013 04:04 PM

air space yes or no?
Never heard back from Building science. Went with full sheets of foil backed rigid foam and then pressure treated ply. foam was glued to floor joists and all seams taped Thanks for all your help Gary.

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