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Old 03-13-2009, 08:29 AM   #1
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subfloor question

I'm building a new house and will be heating with electric radiant heat. The subfloor will be plywood. I will lay the heating wire mat on the subloor using modified thinset mortar. The plywood will have the usual 1/8" gap for expansion on the ends. Do I need to protect those expansion gaps from being filled with mortar?


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Old 03-20-2009, 06:30 AM   #2
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No, don't even think about it. What your going to do is the way to go.


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Old 03-20-2009, 07:21 AM   #3
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Yup, you're method is OK, depending on what the manufacturer of the heat mats this your whole heating system or do you need, where you are, a back-up source of heat - as we do here! Even at 4 cents per KWH, that type of heating is insufficient for us...

Of equal importance is what's going over the mats?
“The average American woman is about 25 pounds heavier than she was in 1960...Recognizing obesity as a disease will help change the way the medical community tackles this complex issue that affects approximately 1 in 3 Americans,”

Last edited by ccarlisle; 03-20-2009 at 07:26 AM.
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Old 03-23-2009, 03:55 PM   #4
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The answer to your question varies.

Some manufacturers say to fill the gaps with one of their products (e.g. Ardex); others say to make sure no mortar clogs the gaps (e.g. Custom Building Products, Laticrete). Taping the gap over with 1-inch "bond-breaking" tape is an easy, fairly inexpensive approach. You can find bond breaking tape for sale on the internet.

The gaps between plywood sheets are to prevent tenting should the plywood expand because of absorption of moisture from the air. When the subfloor is properly fastened, expansion as a result of absorption of atmospheric moisture will be negligible since modern plywoods are dimensionally quite stable. But you might as well make sure you leave some room for a tiny bit of movement.
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