Foamboard use in subfloor

Started by retiredmarine, August 19, 2016, 08:34:03 AM

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retiredmarine

I ran across a cabin build that uses a layer of .75 foam-board in the sub-floor.  It's the first layer on the joists, then a layer of plywood.  Thoughts?  I was thinking of starting with a layer of the rolled foil insulation, then the .75 foam-board, then the plywood sub-floor.  thoughts?

Redoverfarm

Quote from: retiredmarine on August 19, 2016, 08:34:03 AM
I ran across a cabin build that uses a layer of .75 foam-board in the sub-floor.  It's the first layer on the joists, then a layer of plywood.  Thoughts?  I was thinking of starting with a layer of the rolled foil insulation, then the .75 foam-board, then the plywood sub-floor.  thoughts?

Although there are some that use foam board on floors I am not sure that application would be correct.  I would be worried about compression over the joist where as laid flat on it's entire surface might be ok.  Might end up having an uneven floor once weight is applied.   ???


MountainDon

I'd be concerned about compressing the foam over the joists too. However a layer of foam between two layers of sheathing / subfloor would spread the weight out. That is what we have done in the floor of our cargo trailer, converted for travel trailer use.
Just because something has been done and has not failed, doesn't mean it is good design.

NathanS

Foil does not act as a radiant barrier without an air space. It is a complete waste of money to sandwich a radiant barrier against a surface. Manufacturers of these products have gotten in a lot of trouble for lying, but they continue to exist. Not only is it cheaper to put down foam, but it actually works. The commonly available foam insulations usually have compressive strength that is greater than undisturbed earth but do your homework.


Also about the radiant barrier, think about putting that foil against a stove top. If it's sitting against the burner, it is just acting like a frying pan. If you lift it off the burner by about an inch, you can hold your hand a lot closer to the burner than if the foil wasn't there.

Also if dust ever accumulates on the radiant barrier it stops working.

retiredmarine

That compression was what I was worried about too...  I wonder if the foam-board and foil backed roll sandwiched between two layers of sheathing would provide enough insulation for a Southern Missouri winter?

As for the foil not being a radiant barrier without an air gap would the bubbles in the rolled foil provide that? 

Sheathing - foam board - rolled foil bubble insulation - sheathing... 


ChugiakTinkerer

Rigid foam insulation on the floor sounds like a great way to get some insulation down without having to crawl under a building to do it.  Works great on slabs too.  I can't imagine you would want rolled bubble anything in a compressive load situation.  The rigid foam board does have some very impressive compressive strength, but it varies by type and manufacture.  The local box store has Owens Corning Foamular 250 in inventory.  It is extruded polystyrene and provide R-5 insulation per inch of foam.  Compressive strength for it is 25 psi.  That equates to 3,600 pounds per square foot, which as NathanS says is better than most soils.

My first thought is to agree with MountainDon that putting it directly on the joists might put too much stress directly on the foam.  Doing a mind exercise, let's say a 300 lb person has a footprint twelve inches long and 4 inches wide.  That works out to 300 lbs divided by 48 square inches, or 6.25 psi.  Allowing for a 4x safety factor, you're probably OK.  But if you put a 500 lb wood stove on that floor, that only has 4 small feet, the point pressure could jump up dramatically.  On top of that you have dynamic stresses of people walking, hopping, etc. and it wouldn't surprise me if the foam yields over time, compressing down and resulting in a floor that settles unevenly.  If that same foam were sandwiched between underlayment and subfloor like MountainDon suggests, you wouldn't get creases in the foam from the joists.
My cabin build thread: Alaskan remote 16x28 1.5 story

NathanS

Quote from: retiredmarine on August 19, 2016, 09:31:03 AM
That compression was what I was worried about too...  I wonder if the foam-board and foil backed roll sandwiched between two layers of sheathing would provide enough insulation for a Southern Missouri winter?

As for the foil not being a radiant barrier without an air gap would the bubbles in the rolled foil provide that? 

Sheathing - foam board - rolled foil bubble insulation - sheathing...

Martin Holladay has written a few articles on the scam artists that have tried to peddle radiant barriers. He is a highly respected building advisor.

http://www.greenbuildingadvisor.com/blogs/dept/musings/stay-away-foil-faced-bubble-wrap

"Most brands of foil-faced bubble wrap are only 3/8 inch thick or less, and have an R-value of only 1.0 or 1.1. Since the product often costs more per square foot than 1-inch thick rigid foam rated at R-5, why would anyone use bubble wrap as insulation?

Since the main benefit from foil-faced bubble wrap is due to its radiant-barrier facing, the product is basically worthless unless it faces an air space. A decade ago, when I was the editor of Energy Design Update, I noticed that many manufacturers of foil-faced bubble wrap were promoting their products for use under concrete slabs on grade. In this application, the shiny foil is clearly not facing an air space, so the exaggerated R-value claims made by bubble-wrap manufacturers were particularly outrageous. My article exposing the bubble-wrap scammers appeared in the September 2003 issue of EDU.
"

http://www.greenbuildingadvisor.com/blogs/dept/musings/radiant-barriers-solution-search-problem

Foam is a really fantastic insulator on the other hand. If you put it down on the floor you also get rid of the thermal bridging from the wood joists.

Don_P

I agree, generally speaking the radiant bubble wrap is great packaging material. I helped install a radiant slab over that stuff, the salespeople who sold that as insulation under a slab should be behind bars. I've built up loft floors using 2x6 T&G, 3/4" foamboard and 5/8" osb. This was for sound control and worked well. The foam and 5/8 did not run under the walls, the foam would be outside of code there. On the 25 psi foamboard, the regular density 10 psi stuff is good for 1440 lbs per square foot. A wood floor is typically designed for 50 psf. I agree with the others, I wouldn't use it on just joists though, nor would I run it under load bearing walls.
QuoteR601.2.1 Compressible floor-covering materials.
Compressible floor-covering materials that compress more than 1/32 inch (0.8 mm) when subjected to 50 pounds (23 kg) applied over 1 inch square (645 mm) of material and are greater than 1/8 inch (3 mm) in thickness in the uncompressed state shall not extend beneath walls, partitions or columns, which are fastened to the floor. 

JRR



ChugiakTinkerer

Quote from: JRR on August 20, 2016, 09:53:06 AM
Bet this 100 psi stuff costs a bundle:
http://commercial.owenscorning.com/products/foam/foamular-1000/

Yowza!  Can't find online prices very easily but compare 100 psi to 25 psi at Insulation Supply Co.  Four times the strength costs 5.5 times as much.
My cabin build thread: Alaskan remote 16x28 1.5 story