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Srollins
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# Posted: 26 Jan 2017 10:44
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Hello all,
question: after pouring my concrete sonotube pier footings and I run my rim joist all the way around the perimeter and install my floor joist if im approx. 2' off the ground how can I finish off the underside of the joist? Im thinking of that as it could also help hold insulation in, I wouldn't think you crawl underneath to install the floor insulation would you? and if so what keeps it from falling out overtime? speaking of floors after the insulation and underside is complete would it be better to bite the bullet and use PT plywood or just standard T&G ?
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bldginsp
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# Posted: 26 Jan 2017 12:53
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They make a cheap plastic netting that you hang between joists to hold up the insulation. But that still leaves the insulation exposed to critters who will nest in it. So you either need to cover the bottom of the joists with plywood or perhaps fine mesh screening. Or seal up the perimeter to the earth around the piers to keep the critters out.
I would not use PT on the floor. The floor should be kept dry by the building envelope. If not, and it gets wet regularly, something is wrong. PT floor joists and girders are a good idea if you have moist conditions, but not necessary if all is held up at least 8 inches off the dirt, you have ventilation, and you don't have problems with ground water or extreme humidity or nasty termites.
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MtnDon
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# Posted: 26 Jan 2017 14:33
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Simpson makes Insulation Supports, spring steel wire that is pushed up against the insulation to keep it up against the floor where it should be. HD usually has them. About $10 for 100.
Just curious as you only mention rim joists... are there beams, built up or otherwise on the pier tops?
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Srollins
Member
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# Posted: 26 Jan 2017 18:31
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Sorry I should have been more specific , yes 3 total beams consisting of double 6x6 Pt sitting on top of the piers , piers will be 8' apart then 2x10 PT on top of those 16" on center. I may space the piers closer not sure yet. I think as far as under the joist after insulating I may go with a vinyl soffit system ?
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rockies
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# Posted: 26 Jan 2017 19:31
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Don't leave the underside of your floor assembly open. Batt insulation is extremely porous and does nothing to prevent cold air from moving through it. If the joist bay is left open then any moisture carried by that air will condense on the underside of your t&g subfloor and eventually cause rot. The situation is even worse if you leave the joist bay open underneath and close in the perimeter of the cabin. You're just creating an unvented crawlspace suitable for growing mushrooms.
If you don't want to crawl under the joists to install pressure treated plywood and insulation after you've put the joists in place then may I suggest that you lay the pressure treated plywood on top of your beams first and then install the joists on top of the plywood. You can then go underneath to just screw the plywood to the joists and then pack the bays full of insulation from above. You'll probably need to add a vapor barrier on top of the joists before you put down the subfloor to keep warm moist air from inside the cabin getting down into the joist cavities.
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Srollins
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# Posted: 27 Jan 2017 04:24
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Thanks ROckies that's exactly what I was afraid would happen, I don't mind crawling under not really an issue just looking for an easier way. So PT joist , PT under side plywood say 1/2" ? , insulate the floor, vapor barrier , then standard 3/4" T&G sound correct?
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sparky30_06
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# Posted: 27 Jan 2017 06:45
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how do you allow each joist space to breath if you fully enclose them with PT plywood on the bottom and subfloor on top??
If you are more that 18" above ground you should have enough airflow to prevent condensate and it would depend on your location too.
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Atlincabin
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# Posted: 27 Jan 2017 11:02
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Agree with Rockies, use the plywood to keep stuff from falling out as well as things from crawling in. And it helps with overall insulation value.
Plywood will breathe enough. It is not a vapor barrier.
Depending on location and the local porcupine population, it may be advisable to staple some hardware cloth over the joints in the plywood. Just enough to keep the porkies from chewing on the plywood edges - yes, they like PT stuff just as much as regular plywood.
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littlesalmon4
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# Posted: 27 Jan 2017 17:35
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I nailed a 1"x 4" to the bottom of each joist, filled each bay with scrap plywood then insulated.
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bldginsp
Member
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# Posted: 27 Jan 2017 18:33 - Edited by: bldginsp
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If you are in an area with a lot of humidity seasonally, you could ventilate the plywood on the bottom with circular vents. They come in 2, 3, and I think 4" diameters. Real easy to instal with a hole saw. Two per bay.
Littlesalmon's way of installing the plywood sounds like the easiest way to me. And the most obvious- why didn't I think of that....
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rockies
Member
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# Posted: 27 Jan 2017 19:45
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I found this article on the "Green Building Advisor" website about insulating a raised floor on piers.
http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/best-way-insulate-floor
I would pay attention to the third paragraph under the heading "The first job is to meet minimum code requirements" and the first paragraph under the heading "Our experts opinion".
The one thing I noted in the second recommended paragraph was the statement "you need a continuous air barrier". An "air barrier" is different than a moisture barrier. If you seal both sides of the joist bay with a moisture barrier (like using a plastic vapor barrier above the joist and rigid insulation or plywood below the joist) any moisture that is inside the bay will be trapped.
The moisture must be able to get out so by replacing the upper moisture barrier with an air barrier the air movement in the bay is stopped but moisture can still vent to the cabin's interior.
I wouldn't cut any vents into the joist bays. The more air and moisture you allow to get in means the more you'll need to get out (and it usually doesn't. It condenses on the cold surfaces inside the bay).
So the way I read the article - from the top down, t&g subfloor, air barrier, joist (fill the bay with mineral wool insulation) 1-2 inches of rigid foam insulation (taped at the seams and to the beams) and then pressure treated plywood.
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Srollins
Member
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# Posted: 28 Jan 2017 10:08
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Thanks guys I appreciate the good info, I do like the idea of nailing 1x on the bottom of the joist way easier then crawling under. Cabin will be at least 2' off the ground , if I had a good idea on how to support I would have her 7' off the ground and enclose the underside for storage or quad parking or something useful , possibly CMU piers filled solid with non shrink grout and rebar would no doubt get me up there. Decisions decisions
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CLguy
Member
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# Posted: 28 Jan 2017 20:19
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Go with 2 inches of spray foam insulation. It is a little pricier than the other option but way less labor and once it's there you will never have to worry about it again. Batts of insulation will eventually get moist and cause problems and as was mentioned earlier the critters love the stuff. With expanding spray foam the whole underside is sealed and the critters don't like it.
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Full Choke
Member
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# Posted: 28 Jan 2017 21:10
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I went a simpler route.
We are in dry - sandy area of Michigan's Upper Peninsula.
Approx 24" above grade. Post and beam(like a deck)
Treated joist 16" on center with a row 2x4 blocks placed flat every 4' at the top of the joist -- not near the bottom. 1/2" treated plywood on top of joist and nailed to joist and blocking. Layer of 30 pound felt paper on top of plywood. We may have used two layer's, ice and water product may be a better choice. A 2x6 with a rip of 1/2" osb ( to give me 2") was laid flat on the felt around the perimeter, exterior wall's will set on this. We then laid 2x4 with a rip of osb perpendicular to the joist 2' on centers. we installed 2" inches of foam board. ( R ?). We ran another layer of 30# felt on top of felt. Then installed 3/4 T&G plywood.
*We went with plywood because we painted the floor with porch floor paint (yes battleship gray). Dad did the painting, I'm guessing it was oil based. Our place is mainly a hunting camp and we do not take our boots off coming inside! With it being sandy we don't have any mud, wet boots and snow yes.
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Full Choke
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# Posted: 28 Jan 2017 21:39
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A little more info on my previous post. 20x32 with loft. Wood heat. Free labor and dad paying the bills. 650 miles from home. Getting as much done ("dried in") as fast as possible a tad more important than cost. Crew was one carpenter, sometimes two, and three strong laborers.
Could of eliminated the 2x6 by using longer studs or being ok with lower ceilings. Could of eliminated the osb rips by using 1 1/2 foam. For that matter we could of used two layer's of sleeper if wanting 3" foam...
We don't notice floors being cold.
How we actually did framing; installed joist, blocking and 1/2" treated ply. 12" rip of felt on top of ply all around the premiter with a 2x6 installed around prem also. * no sleepers or osb rips at this time* We framed and stood and braced exterior walls. Framed, sheathed and felt the roof. Dried In !
Worked on sleepers, felt, foam insulation and T&G in the evenings with have to worry about the weather.
** We did the same sorta idea for the roof.
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creeky
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# Posted: 29 Jan 2017 12:35
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I ran chicken wire under one of my buildings. holds the insulation in. keeps the critters out. working so far. 6 years.
I used Roxul insulation. It's hydrophobic (sheds water). I put silverboard up against the waterproof ply subfloor. again. its worked. not sure it was the best idea tho.
I did an overhaul of a buddies place. he had pink insulation in his floor joists. then plastic air barrier. the plastic was bowed/pulling away from the joists. big puddles of water. We were both very surprised by the amount of water trapped there.
whatever barrier you use. i would be careful about breathing. buildingscience.com might have some better ideas.
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