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# Floor heat on pre-cast slab

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Member Posts: 2,398
<i>"ignore all logic and educational advice"... </i>

:)
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

• Member Posts: 18
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Floor heat on pre-cast slab

I am quoting Radiant floor heating in a 3 level + basement building. Approx 6000 sq. ft. in all. Basement will be poured slab. The 3 upper levels will be structural pre-cast core-slab with a 1.5-2" over-pour. I have 2 questions: 1- is it nessessary to insulate either beneath the pre-cast, or between it and the over-pour? Any down-ward heat loss/ migration is still in-side the building, but might it create control issues? each floor would be individually zoned. And 2- should the tubing be installed closer than 12" o.c. for consistant surface temp? My thought would be that if there is insulation between the pre-cast and the over-pour, than yes. If insulation is not nessessary, than the mass of pre-cast will absorb and "even out" surface temp. Any-one with experience in this type of construction?
• Member Posts: 2,981
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1 question.

What are the floor surfaces (respectively) going to be?

That would be my deciding factor. 1 zone for each floor sounds unrealistic if the surfaces will be different from room to room, so don't fall into a trap on the bid that way! Chris
• Member Posts: 2,398
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Insulation = Control

While you can argue that all the heat is within the building, you also have to admit that your floor temperature was designed to the heat loss of the space it serves.

Given that, if your floor is at temperature and then the ceiling above is at temperature, you are in a thermal sandwich.

If your floor is not insulated, you will lose 50% of your heat to the floor below (assuming bare slab top and bottom) and if you have any floor covering as one would hope, you would be making a better radiant ceiling below than you have a radiant floor above.

Unless you can say that 50% of your heat comes from below and the remainder from above, you will not have comfort. Too many variables of which room has what temperature... Keep it in the space is my advice.

Floor covering is insulation. You need at least, in my book, double the R value going down as you have on top, and R-19 if you can make that happen. 'Backing insulation' is the "anvil" against which the opposite flow of radiant heat can push.

The tubing spacing is a function of a number of things, among them:

1) Heat loss of the space versus available floor area to be radiated. Your "density".

2) Depth of tubing within a slab. The deeper it is, the further apart it might be. The longer it takes to get to temperature, sure, but the longer it takes to cool and the more "even" the floor will be. If "shallow", the tubing should be closer together to avoid "striping", the perception of warm and cool floors along and between tubing.

3) The available or desired water temperature. Lower temperature begets narrower spacing but this gives you more even floor temperatures. If your heat loss requires a hotter water temperature, the spacing might be further apart and this in turn might make your floor surface uneven in temperature.

4) Floor covering: A higher floor finish R-value will require a higher water temperature and/or a more dense spacing. However the added R-value will give a more even floor temperature because the heat is forced laterally more than straight up.

Not the be-all/end-all explanation, just a brief interplay of factors to consider.

Buy any of the RPA books, get a Rad-Pad on this site, some of Dan's book "Hydronic Radiant Heating - A Practical Guide for the Nonengineer Installer", will be money well-spent. "http://www.heatinghelp.com/shopcart/product.cfm?category=2-2

Play with the variables for a while- you can see how one affects the other. The Rad Pad is an invaluable tool to get a lot of "what-if" scenarios down on paper and into your head, where you will own it.

There are products such as Roth Panels, grooved insulation with aluminum facing, which may have an application but it does affect construction sequencing. To me the best thing is to insulate below and more importantly, the edges of the slab. Put that mass to work for you!

"If you do not know the answer, say, "I do not know the answer", and you will be correct!"