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Thermal break beneath tub

ez1980 | Posted in Energy, Heating & Insulation on August 2, 2010 10:15am

I would like to put a thermal break underneath a new fiberglass tub in my son’s house.  Mine was installed over a mortar bed, and is quite strong; however, I would like to avoid the cold bottom (mine and the tub’s) in the winter if possible by installing over rigid foam or some other material.  Is this feasible?

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  1. rdesigns | Aug 02, 2010 12:22pm | #1

    Spray-foam the bottom of the tub before you set it, being careful to trim the cured foam back where needed, so that it doesn't hold the tub too high.

    1. Scott | Aug 02, 2010 12:29pm | #2

      >>>Spray-foam the

      >>>Spray-foam the bottom

      Can you get that stuff flat enough to avoid rocking back and forth when the tub is set?

      1. rdesigns | Aug 02, 2010 03:36pm | #3

        There needs to be clearance for setting the unit in mortar, if needed. Some units do not require a mortar bed, however.

        1. DanH | Aug 02, 2010 07:44pm | #4

          Yeah, if the tub doesn't require support underneath then just space it away from the floor an inch or so -- no real insulation required (air works pretty well), though insulation can be used if desired.  If the tub requires a mortar bed then you're kinda stuck.  If it requires support, but is somewhat "flexible" on the type, one can hold the instructions up to the light and try to divine how much support ... or just call the manufacturer and ask.

          1. rdesigns | Aug 03, 2010 10:13am | #5

            one can hold the instructions up to the light and try to divine how much support

            I like it--you read the white parts.

            We used to used to foam under jet tubs for good support and insulation. You fill the tub with water first and use low-expansion foam; leave the water in till the foam cures. Never had a problem.

          2. ez1980 | Aug 03, 2010 02:08pm | #6

            thermal break beneath tub

            The tub is a Kohler product.  Unlike the last one I set nearly twenty years ago, this one does not say anything about setting it in a mortar bed.  One suggestion I heard was a layer of blue board, rigid insulation, for the tub to rest on.  Any comments?  Will it degrade over the years and cause a problem?  The tub has a tile flange.  It isn't an integral flange, but one that is riveted and siliconed, I guess, fairly thin.  I intend to use 1/2 inch backer board over the tile flange for the tub surround, so I can also put a shower in it.  I was concerned that if the blueboard deteriorated and the tub dropped, it would screw up the tile flange's effectiveness.

          3. DanH | Aug 04, 2010 12:46am | #7

            If you use foam directly below a tub that has any flex and which depends on the foam to support it, the foam will slowly over time become compressed, with each fill/empty cycle, moving around in the tub, etc.

          4. User avater
            xxPaulCPxx | Aug 04, 2010 02:04pm | #8

            I wonder if placing a 1/2"

            I wonder if placing a 1/2" plywood sheet between the tub and foam would distribute the point loads enoung to keep the foam from crushing?

          5. rdesigns | Aug 04, 2010 03:47pm | #9

            Even if the tub were 24"

            Even if the tub were 24" deep, that would come to just over 1lb/sq. in. I have a hard time imagining that small weight would ever crush polyurethene  foam.

            And, as a matter of experience, tubs that I personally foamed over 20 yrs ago have never settled. I know this because of going back to re-caulk some--we always postponed caulking till after the filled tub had been foamed and allowed to cure, and always held the tub flange of a deck-mounted tub up about 1/4" above the tile deck  to ensure that the tub's weight was not being carried by the flange. In the cases of re-caulking years later, the 1/4" gap is still there--no settling. 

          6. DanH | Aug 04, 2010 06:51pm | #10

            Depends on how the tub flexes.  If the tub is solid (CI or heavy FG) then it won't flex and the weight's evenly distributed.  If the tub flexes then the pressure at any point can be ten times you calcs.

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