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toyota_mdt_tech
Member
# Posted: 26 Jan 2021 08:26 - Edited by: toyota_mdt_tech
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WILL1E, how about that metal mesh with the 1/4" holes, use the strips nailed in place like you mention, but staple the mesh in place to the strips. This will let it breather, not hold moisture and no varmints getting in.

WILL1E
Member
# Posted: 26 Jan 2021 08:33
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To me it seems like having the insulation that visible/exposed on the underside is asking for problems....isn't it? It seems like it would be a haven for the insects to get in there.

Tim_Ohio
Member
# Posted: 26 Jan 2021 08:50 - Edited by: Tim_Ohio
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Oh, man, that's a brilliant idea. I wouldn't worry
about the perforations. They are small. I wish I'd
had that idea, since it was a pain to crawl under and
screw it from below. I would say he has the best
idea for enclosing the underside. That soffit material
can be pre-cut at the metal roofing company, or you
can buy it in long pieces from any hardware, then cut it on site. It's also made in vinyl or plastic. I suppose I'd go with what is least expensive.

Tim

Brettny
Member
# Posted: 26 Jan 2021 09:46
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Sofit can be prety expensive. I think metal pannels or plywood is the way to go. I have had alot of bees make it past my sofit. Even had birds move it and start making a nest behind it. I do use a floating sofit on my house. I had to cawlk that bird area down so they couldn't move it.

WILL1E
Member
# Posted: 1 Feb 2021 09:15
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To recap options thus far.

If i do pink foam under the subfloor and open to the outside, the cost is $0.87 per sq. ft. for R10. If i want to go up to R30 in the floor, i would have to do 3 layers. Only other cost for this method would be cans of spray foam to seal the pink foam sides to the joist.

If i choose to do wool batts in the floor, the cost would be $0.62 per sq. ft. of R-15 or $1.86 per sq. ft. for R30. This option would also require some sort of battier between the batts and the outside such as plywood/metal roofing/soffit.

gcrank1
Member
# Posted: 1 Feb 2021 12:58
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Only way I can get my head around a cost issue is to use all the bits and pieces together as a total package to figure the square footage cost. Then it sorts itself out pretty quick.
Well, there is always the preference and time issues too but I gotta have a handle on the cost first since my pref and time is flexible (I work way cheap for myself).

rpe
Member
# Posted: 1 Feb 2021 13:22 - Edited by: rpe
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I stapled Tyvek underneath to hold the insulation up neatly, and then used roofing nails to hold the metal mesh up overtop to keep the rodents out. I'm happy with how it turned out, and have had zero critter issues, on our 3rd year now. TSC has that metal mesh (1/4" square is what I used), on sale for half price every one in a while.
WP_20181028_002.jpg
WP_20181028_002.jpg


WILL1E
Member
# Posted: 1 Feb 2021 14:13
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gcrank1 For a 16x20 cabin the cheapest option (1 layer R10 pink foam) would be $300 for insulation and most expensive option would be around $1,000 and that's for R30 wool with a metal or wood layer below it.

Quoting: rpe
I stapled Tyvek underneath

Any issues with moisture since your barrier layer is on the bottom?

gcrank1
Member
# Posted: 1 Feb 2021 16:43
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From my past experience I could easily go with the $300 option. Even if I filled the whole depth of the cavity with insul I think Id still have cold feet.....
Imo, priority of insul area is:
Roof/ceiling
Walls
Floor
So Id spend my money saved on the floor on more/better in top and walls.

rpe
Member
# Posted: 1 Feb 2021 21:20
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Quoting: WILL1E
Any issues with moisture since your barrier layer is on the bottom?

No issues. Tyvek is breathable, not like vapour barrier. In fact, I read somewhere it was first designed for under-floor application, and later used on exterior sheathing.

Brettny
Member
# Posted: 2 Feb 2021 07:28
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Tyvex isnt a vapor barrier. It more of an air barrier/house wrap.

Cant remember if you said how high the cabin will be. Have you priced out a diy spray foam kit? No extra steps would be needed prior to putting sub floor down.

WILL1E
Member
# Posted: 2 Feb 2021 08:21
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Quoting: Brettny
how high the cabin will be

Presently i'm leaning towards a single pitch/shed style roof. Low eave side will be 12' and high side i'm thinking will be 17'ish which would be a 3/12 pitch.

WILL1E
Member
# Posted: 2 Feb 2021 08:25
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rpe So you just have unfaced batts between your joist resting on that tyvek and metal mesh? If so, i assume you installed those both from the underside first and then put the insulation in?

rpe
Member
# Posted: 2 Feb 2021 10:59
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Quoting: WILL1E
rpe So you just have unfaced batts between your joist resting on that tyvek and metal mesh? If so, i assume you installed those both from the underside first and then put the insulation in?

Unfaced Rockwool R24 batts were used. This was all installed after-the-fact, from underneath (~18" clearance at a minimum, often 3' or so). In most bays they held themselves in place on installation. In a couple bays where the joist spacing was a bit over, or maybe twisted, they had a tendency to sag and drop out as I was installing them. I just tacked a nail or two in to hold up teh saggers as I was going, then quickly followed up with Tyvek stapled onto the underside of the joists.

Another guy in my area did a major reno on his cottage, including pulling up the floor right down to the joists. He previously had no floor insulation, so he spray-foamed it from above! To provide a backer at the bottom of the joist bays, he nailed those little metal angles (used for supporting hanging ceiling panels) along the bottom of each joist, and then dropped in strips of 1/4" plywood to form the 'floor' at the bottom of the joist bay. Then the spray foam guys shot the bays full of foam from above.

cjm
Member
# Posted: 3 Feb 2021 11:13
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Remember that the r value of insulation is not the same as the r value of the wall. If I remember right, a 2x4 wall, with r13 insulation batts, has an overall r value of r10. I'm guessing that 1 layer of 2inch xps will give you a similar r value as r15 batts. If the batts get wet or have gaps over time, then the xps will probably be better long term. And, if you sandwich the xps between two layers of plywood, you don't have to worry about bugs or rodents at all.

Call it confirmation bias, but I'm happy with our floor with 2" of xps.

rpe
Member
# Posted: 3 Feb 2021 11:26
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Your sandwich method sounds like a good one. I think installation here is critical. A friend put xps in years ago in his cabin floor, and much of it has come loose, hanging down with mice now running like a race-track back and forth between it and flooring. He's thinking of just foaming the whole mess in place now. If ambient air can easily circulate between the foam and the underside of the floor, then the xps is likely of little value.

gcrank1
Member
# Posted: 3 Feb 2021 11:40
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Xps hanging down? How was it installed?
I did mine between 16oc joists, tacked 1x2" ledger strips each side 1" down and set cut to width xps on 'em, floor deck over that. 37yrs and nothing ever came loose, drooped, etc. There is just no weight in the middle of a 15ish" width of a strip of that stuff and set flush with the joist tops before the decking there is no room for anything.
2" would have been better but even then when it was cheaper I was still strapped for cash....guess that part never ends.

cjm
Member
# Posted: 3 Feb 2021 11:41 - Edited by: cjm
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rpe - totally agree and good clarification.

a good-install of option x will beat a poor-install of option y every time.

I should have been more clear when referring to the r values... a continuous, tight layer of xps (e.g., between two layers of plywood) likely has a similar r value overall as r15 batts between the joists. xps between the joists will also have a lower r value overall than the rating of the insulation itself.

WILL1E
Member
# Posted: 4 Feb 2021 15:15
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How much bracket'ing did ya'll do for your joist to rim board and joist to beams? Did you do H1's or similiar at all the beams/joist intersections? Hanger brackets at all the joist/rim board connections?

gcrank1
Member
# Posted: 4 Feb 2021 15:37
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12x24, I used shorter than 24' 2x4s with any butt ends over the stacked block 'piers'.
They were nailed flat to the bottom of the long sides of the box rim frame to create a ledger strip for the end of the joists to sit on.
Old school before hangers (OSBH)

spencerin
Member
# Posted: 4 Feb 2021 23:49
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I don't think 3. (and by extension 2.) was answered sufficiently.....

There are build configurations where certain lengths of the rim board can be load-bearing, especially if they span between 2 piers and support walls, for example. The rim board gets doubled up to improve its load-bearing capacity. It's not an ideal design, but it works fine if done right. As previously noted, you probably won't need it, but that's why you see it sometimes.

Brettny
Member
# Posted: 5 Feb 2021 06:36
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If you use a beam under the floor joists you dont need hangers at the rim joist. It's more there to keep the floor joists inline and not twist. I think a beam under the joists is also stronger and dosnt rely on hangers or end nailing. You can also pull in the structual beam in from the drip line. It also may mean that you dont need PT floor joists.

WILL1E
Member
# Posted: 8 Feb 2021 08:19
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So if I build at least +12" off the ground i don't need PT lumber...but does that mean i need to sheathe the rim board completely so that it's not expossed? I always thought the sheathing stopped jut below the subfloor.

gcrank1
Member
# Posted: 8 Feb 2021 09:55
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I sheathed my studded fill-in walls between the barn beam posts with cdx plywood down over the floor deck edges and about 1.5" onto the rim joist (sealing things up nicely). Over that went 1" exp insul with the actual siding weathered barn boards All The Way Down covering the rim joists.
The way the sheathing and insul stopped up higher left an air gap lower behind the barn boards and the bb bottom became the drip edge. In 37yrs I had no rot/deterioration of the lower bb's or rim joists behind.
Btw, the corner nearest the ground was about 1' above the ground, no skirting around the bottom so good air flow.
I also had a 2' roof overhang front and rear and roof eaves of about 5' either side as firewood cover to one side and a workbench the other, thus the roof runoff was well away from the walls.

Brettny
Member
# Posted: 8 Feb 2021 11:14
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Quoting: WILL1E
So if I build at least +12" off the ground i don't need PT lumber...but does that mean i need to sheathe the rim board completely so that it's not expossed? I always thought the sheathing stopped jut below the subfloor

No. The sheathing should stop below the rim joist. This allows to to properly side the building and cover the rim board with siding. If your going to use a beam under the floor joists I would even extend the sheathing down to cover that too.

WILL1E
Member
# Posted: 25 May 2021 14:35
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What exactly is the purpose of the VAPOR BARRIER between the floor sheathing and the top of the joist? Is it to primarily prevent moisture (spilled liquids inside the cabin) from getting into the insulation cavity (between the joist)?

fish_kc79
Member
# Posted: 7 Jun 2021 20:45
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WILLIE, sounds like we are in similar situations. How is your build going? We are building a10x20 cabin on piers/beams and just going through the same decision making process - not sure if you are done now? What did you decide to do, top layer to bottom?

We just want a bit of insulation, no need for more than 3.5". We are thinking of using rockwool, so out to in: 3/4 ply, vapour barrier(?), rockwool 3.5" thick, resting on 1/2PT ply suspended in the joist bays by strips of PT ply or ripped 2x4. I priced that out compared to buying a 600sq.ft spray-foam kit. About the same. Now I just need to decide what to do.

I don't have an idea of which method is superior. We are in Frontenac Twp Ontario, so there will be snow, bugs, mice, you name it. They are almost the same in price.

Also, stepping back a notch, I also need to figure out how to attach the beam to the concrete piers. Seems like embedding carport saddles during the concrete pour offers no room for error in X,Y & Z, while embedding j-hook bolts during the pour will allow some adjustability in XY. Any thoughts?

WILL1E
Member
# Posted: 8 Jun 2021 11:37 - Edited by: WILL1E
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fish_kc79 I haven't started yet, still hashing out the details + waiting for the lumber situation to tame a bit. Depending when i finally kick off the build, i may wait to insulate the floor until after the shell is dried in.

While your price may be the same for both options, will the spray foam route = the same R value? Also, if you spray foam i don't know that you need the vapor barrier as i think spray foam does the same thing...not 100% about that though.

As for getting things squared up and anchoring, i think doing string lines all over is your best bet for getting things where they need to be pre-pour.

fish_kc79
Member
# Posted: 8 Jun 2021 12:41
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Thanks WILLIE.

I have read that spray foam is about R 6 or 7 per inch, so at 3inch (max recommended), it would be ~R18-21. Rockwool seems to do about R14 for 3.5" (for studs anyway). SO it sounds a bit more effective for a number of reasons, including insulation factor, so far as I can tell.

I've got strings lined up, I still question my ability to get better that +-1" XY with this method. The carport saddles for 6" posts are only about $7 but a jbolt + adjustable saddle is more like $30 total/ea.

WILL1E
Member
# Posted: 8 Jun 2021 14:00
Reply 


What about drilling into the piers after they are cast and the beams are set and dropping bolts in and concreting them in or something along those lines?

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