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WILL1E
Moderator
# Posted: 5 Jan 2021 08:59
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I've got several questions regarding the construction of the flooring system for a cabin build which will be an elevated platform likely on piers/columns with beams on top of that for the joist system to rest on.

If you answer a question, please reference the question number so it's clear what you are responding to.

1. You have the floor joist that rest on the beams...but then around the perimeter you have the box that frames out the footprint of the cabin. What is the correct term for those perimeter boards? I've seen band joist, header joist, edge joist, etc. Which is it?!

2. Sometimes i've seen the above mentioned perimeter boards doubled up around the full perimeter. Are those secondary boards called the same thing as the answer to #1 above?

3. What's the purposes of those secondary board referenced in #2 above and what dictates whether or not you use them?

4. For insulating this floor (i'm in northern WI) i was thinking the following going from inside to outside: 3/4" plywood/OSB, vapor barrier, unfaced wool battens between the 2x8" joist, 1/2" treated plywood and lastly some treated strips for the treated plywood to rest/anchor to. Will this method give me cold lines on the floor surface because the joist aren't insulated? If so, how would i remedy that keeping in mind there won't be enough space to crawl underneath so everything has to be done from the topside.

5. Instead of #4 above, would this be better assuming i'm building a single story with self supported lofts?

6. What is the determining factor if you use OSB or plywood for your floor sheathing?

Brettny
Member
# Posted: 5 Jan 2021 09:17
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1 rim board

2 and 3. A second rim board isnt needed as you will have horizontal beams under all of the floor joists. You can add a second rim board if you want but it's not needed. It's mostly there to keep the floor joists from twisting and in the correct space.

Im not sure what your asking in 4 and 5.

6 osb if your going to have s dry structure quick. Plywood if your not. They make 3/4in tongue and grove plywood that has a special brown colord wax like coating on it. It's called plytanium dryply, it works very well and holds screws/nails better than OSB.

For the price of your floor insulation method have you priced out spray foam? If you want to get rid of any cold lines then you need to break the thermal barrier. The easiest way to do that is to use foam board on your subfloor.

WILL1E
Moderator
# Posted: 5 Jan 2021 09:57
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Brettny 4 and 5 are just showing 2 options for insulating the floor...i was just asking if one would be better than the other. Based on your last paragraph, it sounds like the method in link #4 would solve the problem of cold lines.

I'll have to look and see if Menards or HD sell that plywood you referenced, not sure i've ever noticed or seen that particular type.

WILL1E
Moderator
# Posted: 5 Jan 2021 10:00
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This kinda looks like that plywood you referenced.

Nate R
Member
# Posted: 5 Jan 2021 11:23
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1: Rim Joist is what I've heard.....

4/5: The option in 5 certainly avoids any thermal bridging, which is nice. But I'd look at cost increase for that vs change in heat loss between the 2 options, and/or how much of a premium is floor comfort worth?

I don't recall, how do you plan on heating this place?

Builditsolar.com has a great heat loss calculator to figure out BTU heat loads, etc.

6: For subfloor, it is worth considering something that has a coating if you won't be roofed immediately. Menards SKU 1242888 is another option, an OSB equivalent of that plywood subfloor. A bit cheaper. For me on subfloors, that's how I decide between the 2, I'll take whatever's cheaper.

Nate R
Member
# Posted: 5 Jan 2021 11:25
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Oh, and for #3, I bet it's for wall support? Like in your image you linked to, having the 2x4 wall rest only on the 1 2x board is a bit much cantilevering....

gcrank1
Member
# Posted: 5 Jan 2021 11:31 - Edited by: gcrank1
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For #4, on my '83-84 build in south central WI I used 1" blue DOW styro between the joists which were open to the ground beneath. I could have just as easily used thicker. I didnt like the idea of the styro sandwiched between the joists and deck, styro will compress some and I wanted the decking to tie all the joist framing well together. I certainly wouldnt do more than 1" that way.
It was fairly easy. I used a couple of scrapwood 'tee's', one edge rides the top of the joist the leg down along the side, in my case 1". With each of us using one we could set the 't', pull a ledger strip up to it and tack it down. Then it was just a case of cutting strips of styro (btw, Do NOT use cheap styro 'beadboard'!), laying them in and putting decking of choice over.
We used no vap-barrior nor sealing of the styro edges; I might reconsider that nowadays. A plastic vap-barrior stapled over the styro before the decking would have been way cheap and easy.
I made my cabin sit high enough that I could worm my way under if needed, and it was on uneven ground so that one corner would have maybe not been accessible.
I was under a ways a time or two, saw no evidence of critters getting at the unprotected styro.
We never noticed the floor being colder where the joist were. We had area rugs on much of it and indoor slippers a lot, uninsulated joist tops were not something we ever became aware of, in fact, the diff between our meager 1" insulated floor and a friends uninsulated floor made ours the clear winner. Our current, new to us this year cabin has an uninsulated non-t&g board floor with one thin layer of cheap old carpet. It is a cold floor! If I really have to pull this floor out I will do as above and look at 2" rigid styro (hopefully salvage from some construction rather than new).

gcrank1
Member
# Posted: 5 Jan 2021 11:35
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On that #3 Nate, it isnt just supported by the rim joist, it has the (likely) 16"oc joists meeting it as well as the decking; ie, lots of support for a cabin.

WILL1E
Moderator
# Posted: 5 Jan 2021 11:53
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In the architect software i got and have been playing around with, it seems to refer to the first wrapping of joist simply as joist. It then automatically adds a second wrapping of joist, defaults to 1 1/8" thick, which they consider the "rim joist". This is what got my started on the path of confusion because i've personally always seen just 1 level of joist which i've always called rim joist.

I'm leaning towards pellet with propane as backup, or just propane....depends which is easier inspection wise. I would love a wood stove, but i'm not sure how much timber i'll really have year to year on my property. Plus, less time splitting and cutting for inside heat is one less thing i need to worry about when we go up.

My wife has a deep hatred for the cold and even though she always wears slippers plus 2 layers of smartwools, she'll detect cold spots in a heartbeat. We stayed in our 5th wheel last weekend which has the "arctic" package and even with a sealed underbelly, electric fireplace and propane furnace she was cold.

GCrank1 I like the simplicity of just doing rigid foam on the underside of the floor sheathing and vapor barrier in between the joist. That way no insulation can get soaked with moisture.

gcrank1
Member
# Posted: 5 Jan 2021 12:05 - Edited by: gcrank1
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The styro cant hold moisture anyway and iirc comes with a very thin plastic on one side, probably just to make them separate from stacking so maybe dont count on that as a vap-bar. I do have to wonder just how much moisture does go 'down through a floor' since it is carried with the heat that goes up? If you put down linoleum or a faux wood flooring would that be a suitable vap-bar for a cabin?
I suspect you are caught between the rock and a hard place with your wife's foot issues. Heat rises, the floor is always going to be the coldest side and you cant make her stand on her head, lol. I can sympathize, I have neuropathy in my feet, chronically cold, too. Ive been considering making a heat diverter, a tube that will go up toward the ceiling where the heat is stacking and pull it down through with a low cfm volume fan (12vdc computer fan?) with a directional vent floor to 'aim towards feet'. It would also help recirculate the room air to make the whole place more uniform in temp.

Brettny
Member
# Posted: 5 Jan 2021 13:01
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I see what's going on in the linked picture now. If you have electric have you thought about in floor electric heat?

Yes that menards link is the dryply I'm talking about.

WILL1E
Moderator
# Posted: 5 Jan 2021 13:51
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@Brettny, i was actually going to suggest electric floors to gcrank!

I had a buddy who rigged up in-floor heating in his main home which was a 2 story house with a full basement and natural gas furnace. He did it on the first floor in a couple rooms and basically had the line run through the fireplace that he always had burning because he didn't like paying for gas and he has endless access to firewood. Worked pretty darn good.

WILL1E
Moderator
# Posted: 5 Jan 2021 15:53
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Going inside out:
-3/4 floor sheathing
-vapor barrier
-R13 Kraft faced fiberglass between joist
-R10 pink rigid foam between joist (edges sealed with spray foam to joists)
-treated strips to hold foam

This option puts me right around $1 sq.ft. and can easily be done from the topside.

When layering R values, is it straight total of all R values or do i loose some? For the scenario above, would i have R23 or less than that?

gcrank1
Member
# Posted: 5 Jan 2021 17:48
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I have no grid and the place is an occasional use cabin. Yesterday we went there, at 10am it was 24*f outside, 34* inside. Got the infrared lp heater (way better than a 'blue flame', imo) and the woodstove going and it took 3hrs to cook the deep chill out though the steady, gradually warming to saturation heat of 76* still felt great. I was burning two scrapwood and slash piles with the snow cover and no real wind so I was plenty warm.
If I had grid I probably do similar but maybe have a portable elec heater to aim at my feet where I sit. (though my wife says I 'dont sit', I land on a chair, catch my breath and fly off quickly to another project)

Brettny
Member
# Posted: 6 Jan 2021 08:23
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Quoting: WILL1E

Going inside out:
-3/4 floor sheathing
-vapor barrier
-R13 Kraft faced fiberglass between joist
-R10 pink rigid foam between joist (edges sealed with spray foam to joists)
-treated strips to hold foam

This option puts me right around $1 sq.ft. and can easily be done from the topside.

When layering R values, is it straight total of all R values or do i loose some? For the scenario above, would i have R23 or less than that

So the foam would be on the bottom side exposed? Is the bottom of your floor joists high enough to get under there later if needed?

I did 2in foam board between my rafters on my temp shabin. I held the foam board up with 2 nails then foamed around the edges. Those foamboards are very tight once the edges are foamed in. Leave a gap big enough for the foam straw around all the edges, makes it easier to foam.

WILL1E
Moderator
# Posted: 6 Jan 2021 08:43
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Quoting: Brettny
So the foam would be on the bottom side exposed?
Yes.

Quoting: Brettny
Is the bottom of your floor joists high enough to get under there later if needed?

No, hence doing everything from the topside.

Quoting: Brettny
Leave a gap big enough for the foam straw around all the edges

Since step one from the topside would be installing the treated strips for the foam to sit on, i was thinking i could run beads of spray foam on top of the strip perimeter before i would set the rigid foam boards down. This way it acts as a glue and filler to the strips and joist. Might look ugly on the underside as it expands but no one will ever see that.

snobdds
Member
# Posted: 6 Jan 2021 10:32
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I just got two kits of closed cell spray foam. My cabin is on piers about 3 feet high so I can get under it. I put the spray foam kits on a pallet I can pull from front to back to help moved them around. Started at one end and put about 2-3 inches of foam directly on the underside of the subfloor. Two hours later I was done.

Four years later, no mice, the floor has no echo, it's never cold and it really put some rigidity into the floor.

Not cheap, but my time is money. I was able to apply my time to other places and move farther ahead.

Brettny
Member
# Posted: 6 Jan 2021 10:49
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I would suggest spray foam right on the under side of the sub floor and joists. But being that you cant acess this later I wouldnt leave any space for mice to get in. This may mean putting plywood on the under side and if your doing that you can just put your joists on it and spray that.

Not leaving acess later could really come back and bite you. How are you going to fix a pipe or level the floor if needed? Why are you building so low?

Nate R
Member
# Posted: 6 Jan 2021 11:12
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Quoting: WILL1E
When layering R values, is it straight total of all R values or do i loose some? For the scenario above, would i have R23 or less than that?


You'd have R23 BETWEEN the joists. R values are handy in that they are additive. Not sure if you want faced batts in there AND vapor barrier....Gotta have a way for the batts/interior to dry out....

This R-Value calculator is SUPER handy, and can give you R values for the whole wall/floor/ceiling, taking into account the reduced R-value of the framing, etc. You can add or delete layers as needed, etc.

https://www.ekotrope.com/r-value-calculator

WILL1E
Moderator
# Posted: 7 Jan 2021 09:51
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Quoting: Nate R
Not sure if you want faced batts in there AND vapor barrier....Gotta have a way for the batts/interior to dry out


Wouldn't unfaced batts plus a vapor barrier between the joist and floor sheathing be the same in terms of breathability as faced batts and no additional vapor barrier?

snobdds
Member
# Posted: 7 Jan 2021 10:46
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I would never put batt insulation sandwiched between two air impermeable materials. I would never put batt insulation in a floor system.

We have access to good foam products, I would use them.

WILL1E
Moderator
# Posted: 11 Jan 2021 08:48
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Thanks everyone for answering all my questions thus far!

New question:
I've seen some folks here set their joist assembly directly on concrete post they've poured or blocks they've stacked or built up. Other times i've seen beams set on those post/blocks and then the joist assembly sitting on top of that. I know for WI if you go the beam route, the beams can't be set in from the rim joist more than the thickness/depth of the floor joist. So with that said, what are the other pros/cons between the 2 methods?

Biggest pro i can see for the beam method is that ALL of the joist can rest on the beam vs. just relying on hanging from the rim joist.

I ask this because when i go to build a storage shed it will not have a poured foundation, so there will be a platform. The ground is slightly uneven so i'll have to do blocking or posts, however i don't think i want the shed to be elevated more than it needs to be and a beam setup would increase it even more.

Tim_Ohio
Member
# Posted: 11 Jan 2021 11:20
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I did it this way. I did have enough room to crawl under, only about 10 inches. I managed to screw the tin in place, although it was not fun.

Here is the pic of the floor without insulation. At first, I had a plan to locate the floor joists on top of the beams. Then, I realized how high it would be, so I decided to put them in between. The second picture is when I started insulation and the sub-floor. The insulation is mineral wool, R32. Weep holes were drilled in the metal for any moisture to escape. Also, at the ends of the tin, where the ridges left an opening, I stuffed stainless steel scouring pad in the openings to keep the rodents out.

I probably should have doubled up the floor joists at
the gable end. The span was less than 6 feet and a 2x6x12' sill plate or bottom plate of the gable wall spanned from one side to the other. So far, no problem since the gable end is not the heavy, I hope.


Tim_Ohio
floor with tin
floor with tin
sublfoor start
sublfoor start


WILL1E
Moderator
# Posted: 11 Jan 2021 13:20
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Tim_Ohio This is interesting, i've never seen this method before. Are the massive beams really adding anything on the eave sides? I could see the center one being that big because of all the joist hanging on it and potentially having walls or post on the topside that might be taking some of the ridge roof load.

Brettny
Member
# Posted: 11 Jan 2021 14:18
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Quoting: WILL1E

New question:
I've seen some folks here set their joist assembly directly on concrete post they've poured or blocks they've stacked or built up. Other times i've seen beams set on those post/blocks and then the joist assembly sitting on top of that. I know for WI if you go the beam route, the beams can't be set in from the rim joist more than the thickness/depth of the floor joist. So with that said, what are the other pros/cons between the 2 methods?

Biggest pro i can see for the beam method is that ALL of the joist can rest on the beam vs. just relying on hanging from the rim joist.

For a storage shed use runners on blocks. That will be totally fine for a shed.

For a cabin you plan on having plumbing and heat in I would use the vertical posts ontop of sonto tube method. At some point a pipe is going to leak or mice could get to the insulation. At that point your going to be real happy your cabin is 2ft off the ground and not 2in.

I have also seen the sono tubes sticking out of the ground 2ft and the rim board on that. The issue I see with this method is you need more PT lumber, skirting the cabin is harder later and if you dont cover the top of the sono tube with your siding your allowing water to get to your joists.

WILL1E
Moderator
# Posted: 11 Jan 2021 15:48
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Quoting: Brettny
use runners on blocks
Again, is this just to get the joist themselves off the blocks and to distribute the load onto each joist vs. just the rim boards?

Brettny
Member
# Posted: 11 Jan 2021 17:00
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