Flooring Insulation - Layering Foam Board, Cork, and Hardwood

[Sevier]

Hello All -

I havent found a thread dedicated to cork underlayment/subfloor, a few questions here and there that are dated, and didnt seem to conclude in resolution. So, starting one with my questions here and hope it serves as a guide for those in the future!

Cork is more water resistant then plywood (and most certainly more resistant the OSB!); if heat pressed it does not contain adhesives, and so is toxin free all natural; can be sealed with a non toxic sealer; and is also bug and germ resistant. Cork has an R value of about 4 per inch depending, which is more then double that of plywood. I know the point of plywood in a skoolie isnt as an insulator, but rather as a firm base, which brings me to my question about firmness!

Does anyone have any experience with how cork would hold up in a layered system?

One particular cork brand (Thermacork) lists it as:

Density: 7.0-7.5 lbs/ft3 or 100-120 Kg/m3
Perpendicular Face Resistance:60 Kpa or 8.7 PSI
Compression Resistance: 10% at 100u Kpa (14.5 PSI)

The process I am considering (after stripping, sanding/cleaning, prepping and painting w a rust protector) is:
1) 1" XPS foam board as bottom layer (2021 rules will lead to further reductions in toxicity)
2) Then a floating subfloor of 1/2" cork panels that could be various sizes, 2x3 one example, and I can make those T&G, which seems to be what people advise for the plywood subfloor
3) And then a solid 5/16" hardwood T&G floor finished with a non toxic sealant.

I'm just not sure the 3 layer combo is firm enough.

Look forward to hearing your thoughts!

[Jaybz]

I don't know how you would install a traditional hardwood floor with that system. Is that what you're planning or am I missing something? Withdrawal resistance of cork is low, so probably the only thing that would make sense is a click lock floating floor.

[Sevier]

Originally Posted by Jaybz

I don't know how you would install a traditional hardwood floor with that system. Is that what you're planning or am I missing something? Withdrawal resistance of cork is low, so probably the only thing that would make sense is a click lock floating floor.

We were going to glue the T&G hardwood floor to the cork, and were advised by various cork companies that this would work fine. Always researching, hopefully 2 months from now when its time to install, we will feel confident about our plan! We are leaning more towards 2" of cork alone, as it deals with moisture better then foam board (and is not toxic), and a fully floating floor is something to consider - more water resistent then solid wood, not as much expansion issues etc.

[Jaybz]

Interesting. I don't have any experience with glue, but I suppose that if its flexible enough to handle the natural compression of the cork without separating, then sure it could work. I personally would feel a bit better about a engineered/laminate click together type since you can be certain it will handle flex without detaching from the adjacent piece and be not be subject to the expansion issues like you said.

[musigenesis]

Cork is even less compression-resistant than XPS, so you're basically putting 1/4" plywood on top of foam. That will probably not feel very firm to walk on, and you certainly couldn't anchor anything to the plywood. If you wanted to sacrifice some of the insulating benefits of XPS for whatever benefits cork provides without losing any headroom, 1" XPS and 1/4" of cork and 1/2" plywood would probably work OK.

[Sevier]

Originally Posted by musigenesis

Cork is even less compression-resistant than XPS, so you're basically putting 1/4" plywood on top of foam. That will probably not feel very firm to walk on, and you certainly couldn't anchor anything to the plywood. If you wanted to sacrifice some of the insulating benefits of XPS for whatever benefits cork provides without losing any headroom, 1" XPS and 1/4" of cork and 1/2" plywood would probably work OK.

At an R value of 4.2/in, cork is nearly as insulating as XPS, with 14.5 PSI, only .5 PSI less then the most commonly used XPS. I was figuring it would be just about the same? And so we could actually just use 2" of cork, then top with our 5/16" hardwood tongue and groove flooring, skipping plywood and foam board all together. We were going to anchor everything to the ribs of the bus, and the ceiling (which is anchored to the ribs). A slightly soft floor underfoot sounds a lot better to us then a wet and moldy floor underfoot! We could go with 3/4" hardwood instead though.

[musigenesis]

Originally Posted by Sevier

At an R value of 4.2/in, cork is nearly as insulating as XPS, with 14.5 PSI, only .5 PSI less then the most commonly used XPS. I was figuring it would be just about the same? And so we could actually just use 2" of cork, then top with our 5/16" hardwood tongue and groove flooring, skipping plywood and foam board all together. We were going to anchor everything to the ribs of the bus, and the ceiling (which is anchored to the ribs). A slightly soft floor underfoot sounds a lot better to us then a wet and moldy floor underfoot! We could go with 3/4" hardwood instead though.

The XPS I'm mostly using (Kingspan Greenguard) is 25 PSI, but even that is still soft enough that it's easy to deform just from walking or kneeling on it. Try laying out the material you're planning to use on your garage floor and see if it's firm enough for you, before you commit to it for the whole bus.

[Sevier]

Originally Posted by musigenesis

The XPS I'm mostly using (Kingspan Greenguard) is 25 PSI, but even that is still soft enough that it's easy to deform just from walking or kneeling on it. Try laying out the material you're planning to use on your garage floor and see if it's firm enough for you, before you commit to it for the whole bus.

Shoot! Okay, we will continue to experiment. Perhaps a wooden grid, which can support the upper wood floor more.

[musigenesis]

Originally Posted by Sevier

Shoot! Okay, we will continue to experiment. Perhaps a wooden grid, which can support the upper wood floor more.

Floors in houses typically have the joists 16" on center, and 3/4" is really the necessary thickness of plywood needed to resist perceptible bending when you stand in between two joists. The thinner the decking material, the closer the joists need to be, in effect. It's not exactly a linear relationship, but a ballpark figure for 3/8" decking would be joists every 8" or so to get the same floor stiffness. A grid like that would be too much work for me just to save 3/8" headroom.

[Sevier]

Originally Posted by musigenesis

Floors in houses typically have the joists 16" on center, and 3/4" is really the necessary thickness of plywood needed to resist perceptible bending when you stand in between two joists. The thinner the decking material, the closer the joists need to be, in effect. It's not exactly a linear relationship, but a ballpark figure for 3/8" decking would be joists every 8" or so to get the same floor stiffness. A grid like that would be too much work for me just to save 3/8" headroom.

Unfortunately, its about the toxicity of foam board for us, not about the head room :/

Will keep brainstorming, so grateful for your help!

[musigenesis]

Originally Posted by Sevier

Unfortunately, its about the toxicity of foam board for us, not about the head room :/

Will keep brainstorming, so grateful for your help!

You could drill holes through the 2" cork and insert 2" long pieces of dowel, say every 10" or 12" inches in a grid, then lay your thinner plywood on top (similar to how I did my floor only without the screws welded to the sheet metal). I think that plus what compression resistance cork does provide would make your floor adequately stiff even with the 5/16" decking while still leaving your floor mostly the higher-R cork. Just don't park your hog inside.

[Jaybz]

Originally Posted by musigenesis

You could drill holes through the 2" cork and insert 2" long pieces of dowel, say every 10" or 12" inches in a grid, then lay your thinner plywood on top (similar to how I did my floor only without the screws welded to the sheet metal). I think that plus what compression resistance cork does provide would make your floor adequately stiff even with the 5/16" decking while still leaving your floor mostly the higher-R cork. Just don't park your hog inside.

I ran this through my calculator- this method would yield a reduction of R-0.44 as compared to 2" continuous cork. You would wind up with an assembly rated at 7.56 instead of 8. Not bad.

Assumptions: 2" dia dowel, 10" spacing, 2" depth

[Sevier]

Originally Posted by musigenesis

You could drill holes through the 2" cork and insert 2" long pieces of dowel, say every 10" or 12" inches in a grid, then lay your thinner plywood on top (similar to how I did my floor only without the screws welded to the sheet metal). I think that plus what compression resistance cork does provide would make your floor adequately stiff even with the 5/16" decking while still leaving your floor mostly the higher-R cork. Just don't park your hog inside.

And using sheet plywood instead of tongue and groove would be okay? The non toxic plywood doesn’t come in tongue and groove, which is why we were going to skip it and just go straight for the tongue and groove hardwood floor.

I’ve seen people say you don’t want to use sheets of plywood because it will now when you put furniture down on it along the sides of the bus.

Or we could skip the plywood and go for a 3/4” tongue and groove hardwood floor instead of a 5/16”.

[musigenesis]

Originally Posted by Jaybz

I ran this through my calculator- this method would yield a reduction of R-0.44 as compared to 2" continuous cork. You would wind up with an assembly rated at 7.56 instead of 8. Not bad.

Assumptions: 2" dia dowel, 10" spacing, 2" depth

Interesting, what calculator did you use for that? I actually used 1.25" diameter dowel (which is 1.22 square inches) on my floor; 10" spacing means one dowel per 100 square inches, so the floor would be 1.22% wood (R1 per inch) and 98.78% cork (R4 per inch), with a seat-of-my-pants estimate of 7.93. I assume your calculator does more than just simple percentages of materials like that.

[musigenesis]

Originally Posted by Sevier

And using sheet plywood instead of tongue and groove would be okay? The non toxic plywood doesn’t come in tongue and groove, which is why we were going to skip it and just go straight for the tongue and groove hardwood floor.

I’ve seen people say you don’t want to use sheets of plywood because it will now when you put furniture down on it along the sides of the bus.

Or we could skip the plywood and go for a 3/4” tongue and groove hardwood floor instead of a 5/16”.

Hmm, I feel like I just wrote something about this on another post, but it was probably someone/somewhere else.

One of the side benefits of the dowel thing is that as long as you run a row of them under the seams between pieces of plywood, you can screw the plywood into the dowels and this joins the two edges almost as well as the T&G arrangement does - so you can use regular plywood. I used oak for my dowels which is totally overkill from a compression standpoint, but it's a small piece of wood with multiple screws being driven into it and regular pine dowel can split pretty badly in that situation. To economize, I would use oak anywhere two or more screws were going into it (like under the seams between plywood) and pine elsewhere.

T&G hardwood floor directly over cork would probably work fine, too.

[bus-bro]

I'm a 3/4 inch tongue-in-groove plywood on top of 1 inch pink polystyrene kind of guy. And at 15 psi on the foamboard, no sleepers are needed!

That said, a more natural floor wood be an interesting thing to build. A thinner offset two-layer plywood top would be cool. What you put underneath, and how you deal with organic insulation and it's issues would be another challenge.

[Jaybz]

Originally Posted by musigenesis

Interesting, what calculator did you use for that? I actually used 1.25" diameter dowel (which is 1.22 square inches) on my floor; 10" spacing means one dowel per 100 square inches, so the floor would be 1.22% wood (R1 per inch) and 98.78% cork (R4 per inch), with a seat-of-my-pants estimate of 7.93. I assume your calculator does more than just simple percentages of materials like that.

I just built an excel sheet based on the ASHRAE method for calculating the R value of composite wall assemblies.

Here’s a good primer:
https://www.taitem.com/wp-content/up...s-Nov-2008.pdf

Essentially it just accounts for a disproportionate amount of heat loss through the less insulated/thermally bridged areas. I can share the sheet if you’re interested.

[Sevier]

Originally Posted by musigenesis

Hmm, I feel like I just wrote something about this on another post, but it was probably someone/somewhere else.

One of the side benefits of the dowel thing is that as long as you run a row of them under the seams between pieces of plywood, you can screw the plywood into the dowels and this joins the two edges almost as well as the T&G arrangement does - so you can use regular plywood. I used oak for my dowels which is totally overkill from a compression standpoint, but it's a small piece of wood with multiple screws being driven into it and regular pine dowel can split pretty badly in that situation. To economize, I would use oak anywhere two or more screws were going into it (like under the seams between plywood) and pine elsewhere.

T&G hardwood floor directly over cork would probably work fine, too.

Thankyou Musigensis, Bus-bro, Jaybz! So sorry for my delay in thanks, I missed your responses (I dont seem to get notices). I think that is a great solution Musigensis!

We have decided (and I hope this helps others in the future), to go with:

1) A 1" base layer of Thermacork, which has the same R value as foam board, yet none of the toxicity, and also "breathes" better, so will dry if ever wet.

2) We will use Musigensis' brilliant oak dowel system, along the seams. We will seal the dowels with SafeCoat Hard Seal to protect against moisture damage.

3) Then a 1/4" of PureBond (formaldehyde-free), plywood screwed into the oak dowels, also sealed with SafeCoat Hard Seal.

The Eco Supply Center has been great in answering all of our questions about the Thermacork, and they feel confident 1" can take weight, but to be safe, we'll do the plywood anyway. Will also give me a sense of security with anchoring down our objects.

4) Lastly, 5/16" tongue and groove hardwood, sealed with a non toxic European finishing oil.

[musigenesis]

Good luck, my future dowel brother.

[Sevier]

Originally Posted by musigenesis

Good luck, my future dowel brother.

Thankyou! (though I will be your future dowel sister