Cold floor insulation: Are batts enough?
wineyarders
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I am insulating a room above a screened porch (’20’s craftsman, new england zone 5). I plan to air seal the floor with a primer/sealer, and then fill bays with rockwool batts. I’m using batts because I already have them.
I’m also trying to decide if XPS foamed in above the batts would provide any particular benefit. I initially was going to foam in XPS on the bottom layer but due to framing it does not seem practical.
Any advice is greatly appreciated.
Ryan
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Replies
Ryan,
I noticed spray foam in your picture. Are you attempted to create a conditioned attic?
Yes, the roof has flash/batt installed and will not be vented.
Ryan,
No amount of insulation will make an overhang floor truly warm. I have about 18" of insulation bellow the floor of an overhang that is well sealed. With the floor heat turned off, it eventually goes cold enough to be uncomfortable in the winter. Depending on the R value of the assembly it settles to a couple of degrees bellow room temperature, which feels cold to bare feet.
Adding in a layer of foam above or bellow the joist to eliminate thermal bridging from the studs definable helps, but won't completely solve cold floor issue. Foam between the joists above the rockwool doesn't increase the whole assembly R value much, not worth the effort.
If you want the room to be comfortable, I would recommend some form of floor heat or a nice carpet.
I agree - with a R49 floor, figure that the floor will be cooler by .025F for every degree below 70F outside. So at 0F, the floor will be about 1.75F colder than the room. Somewhat cooler over joists.
Besides floor heat or less conductive flooring (carpet, cork, wood, etc), airflow (to break up the air film above the floor) can make a big % difference. Or radiant heat from somewhere above.
Electric floor heat used only to raise the floor to room temperature and only located in places where people walk uses very little energy (< 1% of seasonal floor losses).
Best to verify your air sealing (don't count on visual inspection). Any infiltration has a large effect on floor temp.
I see no reason for cut-n-cobble XPS.
Your cross joists address thermal bridging.
Funny you mention that- I always regretted not insulating the floor of my unheated master closet which is above an exposed front porch. We started with a space heater hooked up to a timer for the mornings but now we just get in get out. It's an ugly sight on the thermal camera.
Hi Ryan -
I am not sure what you mean by "air seal the floor with a primer/sealer." Since you are using an air-permeable cavity insulation, best practice would be to have a continuous air barrier on both sides of the floor cavity and of course, air control is key to the energy and thermal comfort performance of this floor.
I have a kitchen addition that I built on piers (https://www.greenbuildingadvisor.com/green-homes/a-fast-kitchen-addition-made-with-sips), so that portion of the kitchen is much like your situation. I used SIP for the floor, wall, and roof of the addition, very well air sealed so the energy performance is pretty good. However, when it gets really cold and for a good stretch, the floor is a thermal comfort problem when my family walks on it in bare or stocking feet.
The solution to this issue is to decouple the finished floor from the rest of the assembly--something Joe Lstiburek did a great ASHRAE Insight on: https://buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance.
While an air gap helps a little, my opinion is the Joe L did a poor job of explaining it. He didn't mention that the floor temperature isn't a single number (joists thermally bridge, so it's colder immediately above them) and he didn't quantify the effect. The effect is smaller as thermal bridging decreases (which Ryan has done).
My guess is that you could put 1.5x2x1 blocks of wood on the joists every 12" and then cover the rest of the joist top edges with strips of polyiso. This would further reduce thermal bridging to the flooring.
XPS is the least environmentally friendly foam board there is. Stop using that stuff! There are several vendors selling reclaimed roofing polyiso & EPS at a fraction of the cost, and a comparable or better R value.
Not to mention, expensive high-R/inch foam thermally bridged by framing is a waste (beyond what's needed for dew point control):
https://www.finehomebuilding.com/membership/pdf/184243/021269086NRGnerd.pdf
R30 is IRC code min for joist insulation in exposed floors in zone 5. It looks like you have full-dimension 2x6 perpendicular to full dimension 2x4s, for about 10" total depth? The 2x4 spacing looks a bit random too. Blowing that full of 3.0-3.5 lbs cellulose would deliver about R37, and with 4" of thermal break on the 2x6 you'd have no cold-striping to be concerned about. If you don't quite hit 3lbs density don't sweat it- this isn't a wall, and it won't sag or settle even at 2lbs.
This is DIYable with a rental blower, using landcaping fabric as blowing mesh. Or if you're installing plywood/OSB under the new finish ceiling you can drill & fill that layer.
Even if it's milled 2x4 and 2x6 with only 8" you'd still be at R30, but outperforming R30 installed between full-depth joists.
The flooring above is a class-II vapor retarder- nothing really needed here. Detailing the whatever the new ceiling is as a primary air barrier would still be a good idea, but not critical for moisture control. At 2lbs + density cellulose is quite air retardent-it'll find and fill the bulk of any leakage seams with fiber.
Thank you all for the feedback.
Peter - By sealing I meant I painted the topside of the t&g porch ceiling with primer sealer to fill any gap and reduce air intrusion. Thanks for link to article. Interesting that an airgap is more efficient than additional insulation. I was planning to place 1" insulation board for final layer but sounds like I can skip that part.
Dana - I will skip the foam board. I have roxul batts and will end up with r46.
Thanks again,
Ryan
Ryan,
You can't use primer to seal air leaks. If you want to stop air leaks through a board ceiling from above, you'll need to use spray foam.
For more information on this issue, see "How to Insulate a Cold Floor."
Fitting batts into the oddball framing without lots of voids or thermal bypasses will take some patience, but the result will be worth it!
Good luck with the project!
Consider buying a few bags of cellulose to fill any voids in the batts.
"with a R49 floor, figure that the floor will be cooler by .025F for every degree below 70F outside. So at 0F, the floor will be about 1.75F colder than the room."
I can't imagine worrying about a difference that small. That's comparable to the areas of my house with the least heat loss (e.g., an upstairs room in the newer part of the house, with a heated room below and by an inside wall, the floor is a degree or so cooler than the wall, according to my thermal leak detector). I walk around barefoot there with no problem (wood/lino/area rugs).
I would like to do something about the uninsulated floor in the bay window areas downstairs, but the area underneath them is not easy to access.