Kitchen tile floor heating project

[Bea Tyler]

Hi everyone, been a while since I did any tilework. But we're doing a kitchen floor with heat now and I'm a bit overwhelmed by all the choices.

It's about 200 sq ft, on SYP 2x10's, 16" centers, 10' spans. We want to use 6"x36" 3/8" thick porcelain tile. Subfloor is in great shape, real 3/4" plywood, very level. We would like to minimize the height of all layers above the subfloor, to try to match an adjacent tile floor.

I have looked at a lot of options, and would greatly appreciate some feedback on my narrowed-down choices.

Option 1:
-install Warmboard-R (13/16") directly over the subfloor (to accomodate 1/2" pex tubing).
-layer of mod thinset
-ditra membrane
-thin mortar bed
-porcelain tile

Option 2:
-use a staple-up technique to put the pex tubing under the subfloor
-what happens above is much less critical since I would have 13/16" more room to work with

Obviously I want Option 1, because of the greatly increased efficiency of the hydronic system above the subfloor. In fact I'm not even sure I would do it if I had to put it below. Would it be worth it?

Thanks for your interest!

Bea

[PetrH]

Heating the space below your floor works really well. it's done quite often around here. The idea is to put in insulation below the pex and create a 1" air space that distributes the heat evenly in each joist space.

As for efficiency, whatever energy you put into your house stays in your house, so both systems are equally efficient. It's the response time to cranking the heat that varies.

[Kman]

Hello, Bea.

I spent a few minutes reading through the Warmboard installation instructions, and I have to say I'm not impressed with a couple of things. One, the installation of Ditra over aluminum, and two, their fastening schedule for the board.

What is underneath your floor? Is it a crawl space or an unfinished room?

[Bea Tyler]

Heating the space below your floor works really well. it's done quite often around here. The idea is to put in insulation below the pex and create a 1" air space that distributes the heat evenly in each joist space.

As for efficiency, whatever energy you put into your house stays in your house, so both systems are equally efficient. It's the response time to cranking the heat that varies.
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Petr

Hi Petr, thanks for the reply. I guess efficiency wasn't the right word. Cost-effective is better? Commiting to transferring heat through an extra layer of 3/4" plywood just makes me cringe, because of the higher temperature the water needs to deliver the same heat to the tile surface. Plus I have read that this method can lead to "striping" of floors from heat concentration, even with aluminum plates underneath. Can this happen with tile?

Bea

[Bea Tyler]

I spent a few minutes reading through the Warmboard installation instructions, and I have to say I'm not impressed with a couple of things. One, the installation of Ditra over aluminum, and two, their fastening schedule for the board.

What is underneath your floor? Is it a crawl space or an unfinished room?
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Kevin

Hi Kevin, I have a 4' tall crawl space that is completely accessible and clean, nothing much in the way in joist cavities. I wouldn't have to use Warmboard - I'm open to other suggestions that would minimize the total height of all layers above the subfloor. I'm also very interested in opinions about the advisabilty of the staple-up approach. Thanks!

[Steve in Denver]

Based on your discussion it sounds like the floor heat is your primary heat source for the kitchen - is that right? If not, and you are just trying to get some barefoot comfort, have you considered an electric system? It would help on the height issue, but it would most likely be more expensive to operate.

[Bea Tyler]

Steve, thank you for your reply. This is not the main heat source and it would just be to have warm tiles. I have looked at electric systems and do not want the higher operating costs. At 10 watts/sq ft, 200 sq ft uses 2 kw. We pay 12 cents a kw hr so if we operate it 10 hours a day that's $2.40 a day or over $60 a month. And the biggest reason for steering away from electric is we are actually going to do 3 more rooms on the first floor the same way, totalling 800 sq ft.

[Kman]

Bea, if you'll add your geographic location to your user profile, it will help us give you more accurate answers.

I think one of the things that's steering you away from the underfloor system is the heat loss. As mentioned, if you insulate it well, you should have very little heat loss. And once the floor is up to temperature, maintaining it is not especially costly.

But I agree with you regarding the electrical heat, it would cost you much more long term to operate.

[Bea Tyler]

Thanks Kevin, I guess staple-up is my best option. I do have some lingering concerns about "striping" however. I've seen pictures of some nasty damage to wood floors from staple-up hydronics, can this happen to tile?

[cpad007]

Originally Posted by Bea Tyler

I have looked at electric systems and do not want the higher operating costs. At 10 watts/sq ft, 200 sq ft uses 2 kw. We pay 12 cents a kw hr so if we operate it 10 hours a day that's $2.40 a day or over $60 a month. And the biggest reason for steering away from electric is we are actually going to do 3 more rooms on the first floor the same way, totalling 800 sq ft.

Bea,

You're assuming that you will be drawing 2 kw for 10 hours. I assure you that you will not be as once you hit your set temperature, the power required to maintain that temperature goes down. Now I can't say for sure how much but it'll be 'a lot' less that the absolute maximum worse case that you posted above.

That said, heating a large area of floor (tile) with electric is not most cost effective way to go about it.

In my case, my master bathroom is on a second floor so access under it is quite limited unless I want to open up the ceiling in our den. I installed Ditra Heat to take the edge off of chilly tiles but I had less than 40 sq ft of it.

[Steve in Denver]

What is your energy source for your water heater? If it's natural gas, you are probably right about the cost...propane might be closer than you think. Electric - I'll assume you don't have that.

Depending on your set point on the floor temperature, I'd guess you would be in the 25-50% duty cycle, so your energy consumption is likely to be much lower than 20 KW/day. In my bathroom I will hit 100% duty cycle, but it's the primary heat source and I run it at 92 degrees. In milder months (when I ran it at all) 25% duty cycle was common.

[Bea Tyler]

The energy source for the hot water is natural gas. I'm looking at doing an open system as described here: http://www.radiantcompany.com/system/opensystem/.

Also this is my source for the staple-up installation guidelines: http://www.radiantcompany.com/details/methods/joists/

Since there is a very large total sq footage (~800) being planned in this project, I have pretty much committed to using a hydronic heat method rather than electric.

My primary concern of "is it worth it to do under floor hydronic heating?" has been pretty well eased by some of the replies here. I'm still wondering if I need to be worried whether "striping" will occur on tile. I will try to find the picture I saw of its effect on a wood floor.

Thanks everyone!

[Kman]

Bea, the manufacturer of the heat system should have a schedule for running the tubes underneath, that is, the distance the tubes should have between them to reduce the chance of striping.

If you don't see anything in writing, I'd recommend calling them.

[PetrH]

How much height do you have to work with for your total system? You could embed the water pipes directly in a 1" mortar bed and then paint on an antifracture membrane, for a total thickness (including tile) of about 1 and 3/8"

[cx]

Petr, do you know of an anti-fracture membrane manufacturer who actually recommends that installation method? Seems to be a good bit shy of the requirements of any published method with which I'm familiar.