Gaps Between i-Joists and Insulation in Floor Assembly?
Tiziano
| Posted in General Questions on
My questions are: In the floor assembly shown in the diagram, are the 1/4″ gaps between the i-Joists and the Comfortbatt ok? Or do they need to be filled?
Details:
– 12′ x 16′ structure, resting on piers 6″ to 8″ off the ground
– From the bottom, up
– ZIP wall sheathing, upside down, ZIP taped
– 2″ thick EPS
– 24″ OC i-Joists (hangered to LVL rims – not shown)
– Comfortbatt
– Air gap (intentional)
– Avantech sub-flooring
– The joist bays are edged with 2″ EPS, their edges are then sealed with foam, and then the Comfortbatt would be added
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Replies
In theory, 1/4" gaps shouldn't move a lot of heat if the assembly is airtight, but Comfortbatt is available in full 24" widths so I would just order that. You'd have to use two layers because they don't make it in R-30.
Having lived in a building with fiberglass over foam, with a gap above, I would never accept that assembly. Everything will conspire to make a hole in the lower part which renders the air space above the insulation outside temperature. Perhaps not day one, but eventually. I tore it all out and spray foamed. Instant change in floor temp compared to 12" of fiberglass and an inch of foam.
Because of the thin OSB web, I-joist don't have the thermal bridging you get with dimensional lumber. I would skip the rigid and insulate only with fluffy.
What you can do is leave the top subfloor loose until the building is up and dried in. You can then pull the subfloor and fill the joist bays with cellulose. Since it is the floor, you can pack it down from the top and fill it up to full height. Packed cellulose will fully fill the I-joist space plus it is does reduce air leaks a fair bit, much more than typical batts.
No matter the R value, floors over piers will always be colder than a floor over a basement/crawl. Radiation off a wood burning stove can help with cold floors, but adding in some resistance floor heat in high traffic areas might be a good idea.
Deleted
Why wait until the building is dried in to insulate the floor?
Very hard to keep water out of insulated assemblies during a build. Possible but not worth the headache.
You'll need a plan how to insulate the parallel joists under walls as you can't leave the subfloor out there. Maybe drill holes through the Ijoist or the subfloor and insulate through that. You can also ladder frame the edges so you can have a subfloor strip under the walls but the cavities are still accessible.
Makes sense. Attached is a 4-image jpg, outlining the order of the process.
1. Frame as in image 1/4 (except I'd either use i-joists that are full depth or shorten the whole assembly).
2. Temporarily cover with sheeting (not shown), frame and dry in the structure.
3. Remove temp sheeting and install 2" of rigid in the bays (image 2/4).
4. Air seal all edges (image 3/4).
5. Compress dense pack insulation right up to the top (image 4/4).
As for insulating under the plates (I think that's what you mean), how about the second pair of images attached? The rigid, from step 3 above, should solve it. No?
Tizano,
I agree with Akos about the difficulty of keeping the floor system dry, but as we discussed in one of your earlier threads it’s much easier to do with small outbuildings like this. A pond liner would cost about $100. You can frame the exterior walls on top of it and cut it away once the roof is on.
You want y0ur sub-floor to extend right to the rim-joists both to tie the structure together, and create a continuous air-barrier. If you decide to install the sub-floor later you should run strips (about 1 ft wide) around the perimeter under the bottom plates.
There is no advantage to using foam at the rim-joists, or air-sealing it.
Akos, et. al.,
So I'm skipping the rigid and going with full cellulose in the floor joist bays as you and Malcom suggest. As such, I've been reading up on the process (most notably Martin Halladay's article here: https://www.greenbuildingadvisor.com/article/borrowing-a-cellulose-blower-from-a-big-box-store ).
For the 12'x16' assembly at 11 7/8" depth, I calculate 13 to 15 bags of this stuff (https://www.homedepot.com/p/Greenfiber-25-lbs-Cellulose-Blown-In-Insulation-or-Spray-Applied-Insulation-INSSANC/311574454). That's a little more than 1 bag per bay (or half-bay, as it were).
You suggest packing the cellulose down from the top. This, I think, would negate the need of a blower. So to pack it, what if I ripped a 4' length of 3/4" plywood to the width of a bay (between the webbing). Then spread a bag. Then press it down with the ripped ply?
Thanks guys. Good information all around.
This image might give a better view of my initial plan.
Switching sure would make my life easier!
Plus, I could drop down to 9 1/2" LVLs to match the i-Joists. And use 2" EPS in the bays, topped by the Rockwool to give me R-40.
Tizano,
If you detail your sub-floor as the primary air-barrier you also don’t need the foam board or spray foam on the rim-joists the way you do above a crawlspace or basement. You can use either batts or cellulose for everything.
Thanks. I forgot about this!
I would absolutely use loose fill insulation (like cellulose) here. Akos just beat me to recommending it first :-) Loose fill insulation has the huge advantage of filling in whatever random size space you are dealing with, so you never have to worry about a factory cut piece not fitting quite right. Loose fill will also conform to the lower chord of the truss chord.
I don't see a problem using 2" EPS above Zip. I don't think you'd have much problem with critters because the Zip will provide some structural protection, but if that's at all a concern, som 1/2" mesh hardware cloth stapled to the bottom of the I joists prior to installaing the Zip will provide a very good "critter barrier" to stop any critters that manage to chew their way through the Zip. Note that if you're worried about mice at all, 1/4" mesh hardware cloth is a better choice -- Mice can sometimes manage to squeeze their way through the 1/2" square holes in 1/2" mesh hardware cloth. I frequently use 1/4" mesh hardware cloth embedded in canned foam to "mouse proof" random holes.
Bill
I've got fluffy in the attic space of my house. It's been there for what I think is roughly 20 years and has settled a bit as far as I can tell.
I've not really considered it for this project because it just seems like a low-quality way to go about it. However, given the numerous recommendations I should probably rethink my assessment. Ignorance is bliss until you start an online discussion. :)
For most of us here, "fluffy stuff" usually means batts, but can also mean loose fill (probably fiberglass though). I know a lot of DIY people like to think that loose fill insulation is lower quality, because it's cheapest to install, but it's actually superior in many cases. The primary advantage loose fill insulation has is that it will compeletely fill all the oddball size spaces you need to insulate. Batts are never quite a perfect fit, and anything less than perfect reduces overall performance of the batt. In the case of blown cellulose, you get improved air sealing compared to fiberglass batts too, another plus.
The primary advantage of loose fill insulation to production builders is that it has low installation costs, because you don't have all the detail work of cutting and fitting batts into all the bays. That's not a downside though in terms of quality, because the loose fill insulation will actually provide a BETTER installation in ALL cases than you can get with batts, simply because batts can't ever be quite perfectly installed, but loose fill by it's very nature, will ALWAYS conform pretty much perfectly to the oddball spaces.
The things to watch out for when installing loose fill insulation of any type is that you get an even layer, and that you don't have "shadow spots", where something blocks the material during blowing. I call those blocked areas "shadow spots", because if you shine a flashlight in the direction you'll be blowing from, any object that casts a shadow will also potentially block the stream of insulating material during installation, so you need to be prepared to maneuver around to fill those areas in properly at install time. To help ensure an even layer is installed, get a bunch of cheap wooden yard sticks, put some electrical tape around them at the desired insulation depth, then tack those yardsticks around the attic floor so that it's easy to see when you've reached the desired depth everywhere.
If you take some care with the loose fill installation, you'll get a better performing final result than you would if you use batts. Personally, I only use batts in attics with a floor that is sloped too much for loose fill to settle in place. Everywhere else I prefer blown cellulose.
Bill
Thanks for the education, and saving me both money and time.
I just found and read through a couple of FHB articles. Good stuff.
I recall that thread, Malcom. But, I think that idea (pond liner under the plates and then cut away when dried-in) was determined to be flawed due to condensation on the underside. I then decided to get a 50' x 50' 10 mil sheet and drape that over the entire structure each night. Going to be tricky with a 13' tall side...
Anyway, I'll find the thread to disabuse myself of a faulty memory. :)
Good idea re the subfloor strips.