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My wife and I plan on putting up a tub surround.

I have researched a bunch of these and on a few of the reviews I have read that the shelves are flimsy.

The surround she likes (therefore the one we are going with) will probably be this one.

I was wondering could I fill the void in the back of the shelves with great stuff for added strength?

This would firm up the shelves without adding undo weight.

Would the expanding great stuff push to much and possibly crack the shelf void?

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Blue Great Stuff (used for windows) exerts far less pressure than red or black during expansion. This is because they don't want to distort the windows and cause issues when the foam expands. Blue Great stuff should not cause problems. see here greatstuff.dow.com/products/window-and-door –  Matt Johnson Jun 27 '13 at 4:33

3 Answers 3

up vote 2 down vote accepted

I can share some experience and advice here...

  • don't put anything hard in the shelves - like a bar or wood in the shelving void. The house will move, the tub will move and this will move any type of insulation you use in addition. Pretty soon you will have a shelf that is awkwardly hard in some area and soft in other and possible bulges.
  • do put insulation in the shelves
  • do put insulation in all of the other voids in the tub surround

Great Stuff is actually the exact choice you want if you are going the spray foam route. You will want to tape the back of the shelf section (duct tape) after. That is to ensure the spray foam doesn't move too much. Also you may want to quickly sand the inside with a rough grit to allow better bonding. If your fiberglass is thin you want to go spray foam. The drawback of the spray foam is that under heavy stress and time the spray foam can break off from the shelf and then potential move a lot. Shelf will still be sturdier but may make a little noise. And that is really the negative with spray foam - you have the chance of hearing a rubbing noise after wear.

I really really suggest filling large (non-shelf) voids with fiberglass batts. I lay the fiberglass all around the tub, I pack it in huge voids, and fill (pack) shelves with it... I fill shelves if I believe that particular fiberglass is thick enough to handle the extra weight (I know it isn't much). I will generally try to use fiberglass because it isn't noisy and gives the fiberglass a soft marshmallow feeling. I would also tape the back of the shelves with the fiberglass so it doesn't fall out.

IMO this is a huge upgrade done for $40.

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Actually, the "Big Gap" version is the version you want. The window version is too soft and may collapse in large 'pours'.

Lay the unit face down, such that the shelves make 'cups'. Wipe down the inside with a wet cloth (but not dripping) to accelerate the 'cure'. Fill each about 1/2 full and wait about an hour. Top off as needed. The open 'top' of the shelves will allow the foam to escape as needed, so overpressure is not a concern.

Once fully hard (4+ hours), use a hand saw to flatten any extra 'bumps'

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Yes, this will probably add some stiffness. You can also find some foams that are designed to expand less than others, so you might as well choose one of them. But as long as the foam has someplace to expand freely into, it will generally not cause problems here. Definitely spray in the foam in layers, letting it expand as much as it wants to expand before the next layer is laid down.

As pointed out by Michael, you can also lay in some light bar stock, in the form of aluminum, or even balsa wood, which is cheap enough and readily available in stock sizes. Just lay these stiffeners into the void, then fill with foam around it.

When all done, any foam that has expanded too far out of the shell can be cut away with a knife or hand saw.

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I wonder if it makes sense to embed some aluminum bar stock along the length of the shelf void and let the foam fill in around it. –  Michael Karas Jun 27 '13 at 3:53

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