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xinull
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# Posted: 17 Sep 2020 09:51pm
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OK, so i'm planning my floor today and have a few question.
first, insulation. I have a few ideas, just wondering what you're thoughts are on the subject
One design for floor insulation i've seen done. Is just to place rigid foam of 1" or 2" on top of your subfloor and then place another layer of plywood/osb on top of that. This seems like the simplest solution to me. This way, i can actually place the extra layers at any time after the frame is built (as long as I plan for the doors/windows/ceiling accordingly ahead of time. For now i'm planning my walls being 10', so i would have room to spare, but that is still being debated with the recent cost of wood going up again. I guess i'm looking for comparison feedback between this design and the second. See if the compare.
Second design would be to place either bat insulation in between the floor joist and screw 1/2" or 1/4" plywood under the floor joist to prevent critters from getting in there. It looks like harder work to get done. I will have only a little over 2' of clearance under my floor joists. And I do have 2 questions about this design.
1. can i use 1/4" plywood for under the floor joist
2. do i need to put any kind of vapor barrier between the floor joists and my subfloor? I heard different opinion in these forums about this.
Next, about the floor structure. I'll be doing 20'x28' (well, not exactly 20' because of the 1/2" i'll be loosing to the tongue and groove subflooring), with 3 girders. 2"x10"x10' floor joist butt-ended and scabbed.(the reason i dont offset my joist is because i want my studs and my rafters, which are 24" OC to line up with my joists which are 12" OC) So my span should be a about 9 feet between the outer girders and the middle girder.
1. Do i need blocking between the joist, or will the subflooring be enough to keep everything nice and tight?
2. Plywood or OSB 
thanks guys
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Brettny
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# Posted: 18 Sep 2020 08:05am
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I would tend to shy away from the foam board over sub floor then plywood over it. It would just be to easy for the foam to crush in high traffic spots and then you will have a piece of plywood squeaking against the nails/screws.
Have you priced out a diy spray foam kit vs foam board and 3/4in plywood to cover?
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ICC
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# Posted: 18 Sep 2020 10:12am
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The best subfloor is made with Advantech sheets from huberwood.com
The foam sandwich will not crush in a sandwich like that. It is commonly used under concrete. The std XPS can support 150 PSF. The question to me is more, how are the walls attached to the floor assembly.
Personally I do like the use of spray foam underneath, but also like the way a foam sandwich bridges the heat path of the joists.
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gwindhurst
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# Posted: 9 Oct 2020 01:48pm
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I too plan to use 1-1/2" rigid Owens Corning Foamular over my Advantech subfloor, then a layer of plywood screwed down over that, then final floating laminate floor covering.
My question is do I need a vapor barrier in this scenario and if so where would it be? On top of Advantech subfloor under rigid insulation or on top rigid insulation underneath underlayment?
I'm unsure as the closed cell foam may already serve as a vapor barrier... Any thoughts?
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rockies
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# Posted: 9 Oct 2020 08:16pm
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There's this article (you can download it in PDF format).
https://www.finehomebuilding.com/2012/03/08/how-to-insulate-a-cold-floor
This site you can join for free and download the articles.
https://www.jlconline.com/how-to/framing/raised-floors-for-the-low-country_o
The thing more important than heat retention in a raised floor assembly is moisture transmission. How does moisture get into (and more importantly, get out of) a floor assembly?
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Brettny
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# Posted: 10 Oct 2020 06:46am
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The closed cell foam and even the laminate flooring may act as a vapor barrier. I wouldnt even use the foam underlayment under the flooring.
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rpe
Member
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# Posted: 10 Oct 2020 08:05am
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I used Roxul batts between the floor joists, with a layer of Tyvek stapled under, then covered in 1/4" galvanized steel mesh to keep the rodents out. Just did the other half of the cottage this week. It's a miserable job in a narrow crawl space that varies from 3 ft down to 18".
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gwindhurst
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# Posted: 10 Oct 2020 09:15am - Edited by: gwindhurst
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Rockies, Very interesting articles, thank you. However, both refer to insulating from below the subfloor between joists. I plan on insulating above subfloor with 1-1/2" closed cell foam board, then 1/2' plywood screwed down on top of that and finished with final flooring (I've compensated door threshold heights etc). So as suspected, the closed cell is a vapor barrier in itself and as long as I seal all my edges and joints and sandwich between plywood eliminating air gaps I should be fine. Also, as for water getting out, it would need to get in first which is unlikely in a small off-grid cabin 2-3' above grade without plumbing in the mountains of VT.
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rockies
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# Posted: 10 Oct 2020 06:46pm - Edited by: rockies
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Due to compression I wouldn't put foam board between the subfloor layers unless you install 2x material to help keep the two layers of plywood apart.
This sounds like a lot of work, not just building it but also trekking all the materials out to the build site. The other issue is where do you put the mechanicals (water/sewage pipes, electrical, etc if there are any)?
I would suggest building your floor system out of 6 1/2" thick SIP panels (or thicker). These panels are structure, insulation and vapor barrier in one. Then put 2x material on top of that (inside the exterior walls) to create a false floor where you can run any mechanicals.
Using this method all of your pipes are within the heated cabin interior so no chance of them freezing and you wouldn't have any moisture getting trapped within the floor structure.
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ICC
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# Posted: 10 Oct 2020 08:02pm - Edited by: ICC
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Concrete is poured over both EPS and XPS foam sheets for concrete slabs for pretty much all residential and commercial construction in the past 15 years or more.
I already mentioned this in this same topic or maybe rockies doesn't read or doesn't believe it. Google it if in doubt.
The standard pink or blue XPS foam that you see in home supply big box stores has a compressive strength of 15 PSI. (For example the pink foamular 150 is a 15 PSI material. Foamular 250 is 25 PSI, etc.) The standard EPS foam (the white stuff) is 10 PSI, IIRC, with varieties that offer higher values available too.
That 15 PSI calculates out to 2160 pounds per sq ft. That is a little bit more than the standard of 40 PSF that is used to determine floor joist sizes.
With a layer of some sort of plywood on top of the floor you will have no problems with the foam compressing.
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ICC
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# Posted: 10 Oct 2020 08:25pm
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It is not clear to me if the intent is to construct the wall framing on top of the foam floor sandwich or if the intent is to frame the walls on the first OSB/plywood subfloor layer and then infill between the exterior walls with foam and the top sheet of OSB/plywood.
If using the infill method then you may need to install blocking around the wall bottoms to supply a nailer surface for drywall, or whatever. If constructing the walls on top of the foam and ply sandwich, how are the wall bottom plates being secured through the foam to the joists (floor and rim) to provide a well anchored wall?
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gwindhurst
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# Posted: 10 Oct 2020 08:47pm
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Quoting: ICC That 15 PSI calculates out to 2160 pounds per sq ft. That is a little bit more than the standard of 40 PSF that is used to determine floor joist sizes. With a layer of some sort of plywood on top of the floor you will have no problems with the foam compressing.
That’s been my thought give or take, as I will have nothing of significant weight other than my wood stove (approx 240lb Fisher Baby Bear) which will be placed directly over a pier location with cement board underneath. My floor joists are 2x12x10’ 24†o.c. with 5/8†Advantech subfloor, 1-1/2†Foamular covered with 1/2†- 5/8†plywood then finished floor. My original concern was about a vapor barrier in this scenario. Again this is an off-grid, part-time use cabin in the Green Mountains of VT, no mechanicals. Just your basic 12v solar powered Deer Camp with an outhouse...
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ICC
Member
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# Posted: 10 Oct 2020 08:51pm
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Tape or seal the joints with a compatible sealant and there's your vapor barrier. OR, use two layers of sheets with the joints staggered.
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ICC
Member
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# Posted: 10 Oct 2020 08:52pm
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I would not worry about a post under the stove; the floor joists are robust and with the advantech the weight is distributed well.
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gwindhurst
Member
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# Posted: 10 Oct 2020 09:00pm
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Cabin is dried in, floor insulation will be filled within floor plates and on top of subfloor. Not under wall plate or point loads. Also, I’ve already taken into account for interior floor height increase. Ie: Door thresholds, blocking between studs above floor plate for nailer etc.
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gwindhurst
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# Posted: 10 Oct 2020 09:06pm
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Quoting: ICC Tape or seal the joints with a compatible sealant and there's your vapor barrier.
Over Advantech seams at subfloor level and over Foamular between sheets?
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gwindhurst
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# Posted: 10 Oct 2020 09:09pm
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Quoting: xinull OK, so i'm planning my floor today and have a few question
Sorry, didn’t mean to Hi-Jack your thread. I hope you got some clarification and answers though!
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ICC
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# Posted: 10 Oct 2020 09:17pm
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The high density of the resins in Advantech coupled with the T&G joints make that a pretty good vapor barrier itself. The Advantech is not likely to absorb much moisture from the air. Now, if that floor is over an enclosed crawl space there may be moisture problems if the earth is not properly covered with a water barrier, etc.
So, is there to be a closed crawl space? Ventilated or not?
If the area under the floor assembly is left open sided I would lay the foam on the Advantech, install the top sheeting and leave it at that. If you would want to seal the joints in the foam.
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gwindhurst
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# Posted: 10 Oct 2020 10:01pm
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Quoting: ICC If the area under the floor assembly is left open sided I would lay the foam on the Advantech, install the top sheeting and leave it at that.
Perfect! That has been my thought process and just wanted clarification from knowledgeable member’s such as yourself and so many others. Thank you!
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