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Old 04-15-2019, 04:36 PM   #1
topwobbler
 
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Lightbulb Thinsulate floor - securing problem

I am building my floor using Thinsulate and plywood and have ran into some problems attaching the floor to the van. I know that floating floors are one option but read the final paragraph for why I don’t like that solution.


I have laid down PVC firing strips on the van floor and attached w/ 3M VHB tape. Above that is a layer of AU4002 Thinsulate, and then finally there’s a sheet of 3/8” plywood. I have run into some problems trying to drive screws through the plywood into the PVC firring strips. The screws that I put in are penetrating the PVC fine (90% of the way through) however they are being pulled back out of the screw hole! I tried several screw types and lengths but none of them remained screwed into the PVC!

I believe that this might be because the plywood layer is creating an upward force that’s fighting the screw. The PVC firring is slightly lower than the highest part of the metal van floor (about 1/16” lower, see attached image). I think that the flex in the plywood is not enough to let the floor sink 1/16” above the firrings and therefore the screw is being pulled out of the hole. This is just my guess. The Thinsulate adds an additional level of uncertainty about what’s going on. Is the Thinsulate also trying to push the screw up? Has anyone else tried a similar floor construction? Any advice is appreciated.


Why not a floating floor? The Thinsulate compresses A LOT when you step on it. My floor is made from 4 sheets of plywood. If you step on the edge of one sheet it will actually sink around 1/2” due to the insulation below it compressing. If the floor also shifts a little (as I’ve seen happen) then it will end up below the neighboring piece of plywood. When you lift your foot up then you now have one piece of floor lifting up the other which is very likely to crack the plywood. Perhaps I am missing a critical but 4 separate floating pieces of plywood feels like a bad solution.
Attached Images
File Type: jpg Side View 1.jpg (52.9 KB, 152 views)
File Type: jpg Side View 2.jpg (73.5 KB, 143 views)
File Type: jpg Firrings.jpg (89.4 KB, 139 views)
File Type: jpg Thinsulate Laid.jpg (156.9 KB, 143 views)
File Type: jpg Firring Gap.jpg (57.7 KB, 141 views)
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Old 04-15-2019, 06:05 PM   #2
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Default Re: Thinsulate floor - securing problem

First of all, pvc holds screws terribly - it’s just too soft. If you want to screw into it, I would suggest putting T-nuts into it from below and bolting to that. Alternately, screw directly into the van floor.

3/8 plywood is also very thin to span those gaps without significant deflection. And it’s hard to find quality stable plywood in that thickness. I would suggest going to at least 1/2”.

You will need to provide some method of joining those plywood edges. I used a router to cut a 1/4” dado for a 1/4” x 3/4” plywood spline glued in. There are aluminum channels that will accomplish the same thing, or biscuits.

And make sure that you have solid pvc under wherever you will have cabinetry or any weight so that they have solid support.


Cheers,
Greg
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Old 04-15-2019, 07:17 PM   #3
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Default Re: Thinsulate floor - securing problem

Prolly too late for you, but for others

this is way too complex and heavy a floor system, and for what?

save Thinsulate for walls and ceiling

Polyiso straight on the floor, no plywood needed plenty of compression strength, just a final layer for cosmetics and puncture resistance in exposed traffic areas

Thickness as per R-value desired, same as walls and ceiling if for both hot and cold, thicker only if for aircon & sound.
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Old 04-15-2019, 11:53 PM   #4
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Default Re: Thinsulate floor - securing problem

First off, you need a better quality plywood than AC. Then bond or splice the pieces together it into one continuous sheet. The splices (strips of additional plywood attached to the bottom of the plywood) could make up the distance between furring strips and the bottom of the floor. If not then the furring strips need to be thick enough to directly support the bottom of the plywood where the joints are. Then you can screw the sheets down to the furring strips. Are you securing the furring strips with VHB tape? Either that or you need to return the Thinsulate(TM) and use our Minicell kit. Sorry that you are having trouble and frustration. Please call and I can go over the details and help you solve the problems you are experiencing.

All the best,
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Old 04-16-2019, 01:53 AM   #5
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Default Re: Thinsulate floor - securing problem

topwobbler

Extruded PVC planks have no fiber or grain normally found in real wood.

Is screw threaded the entire shank?

"....when a screw that is fully threaded is driven into wood, this screw can connect two pieces of material together but it will not pull the two pieces against each other; once the head reaches the material, the screw will stop spinning.

Having an unthreaded shank at the top allows the tip of a wood screw to pull the screw into the wood just as a regular screw would. The difference is that the shoulder portion of the screw will actually slide through the first layer of wood and pull it against the head. This causes compression from the head to the threads. When installing two pieces of wood together then the first will be pulled tightly against the second one. The threads can continue to pull forward as long as enough torque is applied. Coincidentally, this can also make the removal process much easier than trying to remove a fully threaded screw
"

Last edited by dynaco1; 04-16-2019 at 02:10 AM.
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Old 04-16-2019, 04:56 AM   #6
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Default Re: Thinsulate floor - securing problem

Quote:
Originally Posted by john61ct View Post
Prolly too late for you, but for others

this is way too complex and heavy a floor system, and for what?

save Thinsulate for walls and ceiling

Polyiso straight on the floor, no plywood needed plenty of compression strength, just a final layer for cosmetics and puncture resistance in exposed traffic areas

Thickness as per R-value desired, same as walls and ceiling if for both hot and cold, thicker only if for aircon & sound.
Polyiso raises the floor too much for us tall people (6'1 and taller) and we'd hit our head even using 1/2" poly.
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Old 04-16-2019, 05:09 AM   #7
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Default Re: Thinsulate floor - securing problem

Quote:
Originally Posted by dynaco1 View Post
topwobbler

Extruded PVC planks have no fiber or grain normally found in real wood.

Is screw threaded the entire shank?

"....when a screw that is fully threaded is driven into wood, this screw can connect two pieces of material together but it will not pull the two pieces against each other; once the head reaches the material, the screw will stop spinning.

Having an unthreaded shank at the top allows the tip of a wood screw to pull the screw into the wood just as a regular screw would. The difference is that the shoulder portion of the screw will actually slide through the first layer of wood and pull it against the head. This causes compression from the head to the threads. When installing two pieces of wood together then the first will be pulled tightly against the second one. The threads can continue to pull forward as long as enough torque is applied. Coincidentally, this can also make the removal process much easier than trying to remove a fully threaded screw
"


Screws are just clamps, and they need to grab both sides to work. A smooth upper shank on a screw allows it to pull that material towards the one the threads are engaging. Pvc doesn’t give the threads much of a gripping medium, so even if the screw doesn’t bottom out, they won’t work very well - just the heat from driving the screw softens the pvc to the point of easy failure.


Cheers,
Greg
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Old 04-16-2019, 05:27 AM   #8
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Default Re: Thinsulate floor - securing problem

Quote:
Originally Posted by MotoXPress View Post
Polyiso raises the floor too much for us tall people (6'1 and taller) and we'd hit our head even using 1/2" poly.
Foregoing the redundant plywood, 95% of the total thickness being the highest possible R-value insulation is exactly what optimizes that issue.
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Old 04-16-2019, 03:41 PM   #9
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Default Re: Thinsulate floor - securing problem

I did 3/4" MDO double sided(bisquit jointed the edges of the 3 pieces it took) on top of thinsulate, and through bolted with rubber sealed washers on the outside. Countersunk the heads on the bolts and epoxied it to make it flush with the floor. We are installing a Marmoleum floor on top.
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Old 04-17-2019, 11:00 AM   #10
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Default Re: Thinsulate floor - securing problem

Home construction requires sheathing clips for roof decks to prevent the plywood sheets from shifting vertically just as you described. Those might help, but I don't know if they make them as thin as you need.

I have some questions:
- Why PVC firing strips instead of wood?
- Why not use thicker firing strips that actually support the floor?
- Why put insulation under the floor if you are compressing it to the point of uselessness?


Quote:
Originally Posted by MotoXPress View Post
Polyiso raises the floor too much for us tall people (6'1 and taller) and we'd hit our head even using 1/2" poly.
The raw interior of the high roof Sprinters is about 76" from top of floor ribs to bottom of roof ribs, which leaves 5" of headroom for a 6'1" tall person before finish. You are telling us that 5" additional space doesn't allow for even 1/2" of insulation? Not trying to be snippy, but it would seem to me that a total of 1" at the floor (1/2" insulation and 1/2" of sub-floor / finish floor) and 1" at the roof (strapping and finish) would still allow a 6'4" person to stand up, admittedly though probably not comfortably. But a 6'1" person should have no issues other than not being able to wear your Fedora hat inside.
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