Help planning my garage radiant slab

[WoodNotOil]

Hi guys! I've been away for a bit occupied with other things, but find myself once again in need of the great collective wisdom of the boiler room!

My garage attached to the house is really a small car port (11x21 floor). I am converting it into a home office and plan to take up the 60 year old frost heaved floor and put in a heated slab. I was looking at this page as a reference for how to do it. I would like to know from peoples experience if this is the right approach. I live in northern Vermont so I assume I need 2" of the Extruded Polystyrene foam insulation under it and on the sides. The floor will be 231 sq. ft. and require 2.8 cubic yards for a 4" slab (I think). I have 70 feet of 1/2" pex tubing left over from my staple up radiant floor project. The directions say I will need to put this in with 8" spacing with 1/2" pex, so I will likely need another role of tubing. I estimate I will need to make 14 to 16 runs of 21 feet to get the 8" spacing. That means 294-336 feet of tubing plus the amount needed to make connections in the basement. Does this all sound right?

Lastly, if any northern Vermonters have left over materials (insulation, pex, etc.) from their own slab projects, I will gladly take it off your hands. I also need fiberglass batting for the walls if you have any of that laying around taking up space. PM me and I will see if picking up the materials is feasable and cost effective. Thanks for the help - WNO

 

[in hot water]

I agree with 2" under and around the edges. Some installers bevel the top edge or hold it down an inch or so so you don't have that edge of foam showing.

The trickest detail is the garage slab to drive way insulation detail. I used a 1/2" foam with a plastic H shaped plastic, then filled the gap with a driveway urethane caulk. This gives you a water proof and flexible joint. But not a lot of r-value.

Try not to exceed 300 feet on one loop with 1/2". Either two short loops, maybe 175', or go to 5/8" tube and a 400' loop.

The shorter loops will be a bit more even heat. 300 footers get fairly cool at the tail end.

Be sure to map the layout and take pictures should you ever want to bolt something down to the slab.

hr

 

[WoodNotOil]

Thanks for the feedback hr. I am taking the garage door entirely out and putting a wall with a door and window in since this is going to be a home office now and not a garage. I will have to consider what to do with that edge of the slab to make sure it can become a garage again if it needs to be at some point in the future, but for now it simplifies the transition to driveway issue. Thanks for the tip on loops as well.

WNO

 

[Gooserider]

A lot of good info on that page, some bad ideas though as well...

Biggest thing was their idea of leaving the center of the floor un-insulated - VERY BAD Unless you are on an incredibly dry site, w/ no water table, dry soil, etc. you will find that is a major heat loss - insulate the ENTIRE area, preferably w/ at least 2" foam...

As a minor detail, I found an article by Sigenthaler that suggested there are some minor performance benefits by lifting the tubing to be an average of 2" below the slab surface - this is supposed to give the optimal balance between fast response and uniform floor temps, and minimizes the water temperature needed to get the required heat out of the slab...

Otherwise it looked pretty reasonable as a starting point.

Gooserider

 

[Greenfield Dave]

Another suggestion for you...When you are ready to pour the slab, either fill the tubes with water, or insure you use a thousand or so screw clips to hold down the pex. I put 1500' of 1/2 pex in a 36 x 48 garage, and the pex floated on the ends of the loops! I used clips at the beginning and end of the turns, but not in the middle.. We did get them pushed under, but what a pain... Also, i do agree that leaving the middle uninsulated is insane!!

Dave

 

[carbon neutral]

You might want to consult with your neighbors at radiantec. I think they are located in lyndonville?, VT. They offer free design consultation and since they are in Vermont they would understand your climate well.

 

[pybyr]

Welcome back WNO-

If the existing floor is old enough and was un-reinforced enough that it has gone to pieces-- do you have reason to be confident that the concrete side walls and footings on which the walls rest go deep enough that they won't be heaving and/or shifting as seasons change? Right now, with unheated and unfinished space, they may just rise and fall with the seasons, which is no big deal-- but may become a whole different situated if you create finished interior walls and a structurally-separate and separately-insulated heated floor slab within. If you're not sure, you may want to check- it'd be a shame to have a great new radiant slab and finished space, only to find that the walls and slab are moving separately because the footings were done in an outdated and/ or low cost way-wracking the inside finish work. Perhaps you've already checked this.

On insulation- consider doing DIY dense-pack cellulose in the wall cavities with a rented insulation blower. Not only is it relatively inexpensive, in my opinion/ experience, it is better at minimizing any air leaks/ convection losses within the wall in a cold climate like VT.

 

[WoodNotOil]

I finally finished the prep work for the floor to be poured. I tied the crap out of the tubing as suggested and the concrete contractor said it looked great. I also pressure tested the lines to make sure the couplers I used were crimped well. Then I wrapped the couplers with vinyl tape as suggested on the page I linked to above. The tubing is run in two loops. Most of the tubing is 3/4" set 12" apart, but part of one run is 1/2" set 8" apart. I used all left over tubing I had and only had to buy 100' of additional 3/4" to finish it off. Here are some pics:

 

[Jim K in PA]

Looking good WNO.

Question: How many pumps are are going to need? What is the rule of thumb for multiple loop systems? Do you just take the published head loss/100' for the piping, muliply it by your total run, and then divide by the sweet spot on the pump curve for the pump(s) you want to use to see how many you need?

 

[WoodNotOil]

Jim

In many radiant applications people are using the Grundfos 3 speed pumps and adjusting the speed once the install is complete to get an adequate temp differential. The multiple loops is just to ensure that the water isn't cold by the time it gets to the end of a long loop.

 

[Jim K in PA]

Jim

In many radiant applications people are using the Grundfos 3 speed pumps and adjusting the speed once the install is complete to get an adequate temp differential. The multiple loops is just to ensure that the water isn't cold by the time it gets to the end of a long loop.

10-4. I was thinking that a single pump should be adequate, but this is new stuff for me. I am helping a neighbor set up a warm slab for his 36x48 shop. He is going to need about 1600+ feet of tubing. Looking like 6 loops of ~275'/loop, going with 12" seperation on 3/4 or 7/8 tube.

 

[Jim K in PA]

gotta question why polyethylene vapor barrier on center of slab wouldnt avert the wet ground concern.
foam can absorb water so the whole slab should have polyplastic under it.
foam should be extended 2' beyond edge of slab to minimize cold penetration & possible frostheave. 2' extension is a guess i'd call local university for local details

Correct - poly sheet on top of sand/gravel and below all 2" blue board.

But, leaving out blue board and just using poly sheet offers no barrier to heat loss into the wet ground. You do not need direct water contact with the slab to lose heat into the water/wet ground.

 

[WoodNotOil]

Jim

In many radiant applications people are using the Grundfos 3 speed pumps and adjusting the speed once the install is complete to get an adequate temp differential. The multiple loops is just to ensure that the water isn't cold by the time it gets to the end of a long loop.

10-4. I was thinking that a single pump should be adequate, but this is new stuff for me. I am helping a neighbor set up a warm slab for his 36x48 shop. He is going to need about 1600+ feet of tubing. Looking like 6 loops of ~275'/loop, going with 12" seperation on 3/4 or 7/8 tube.

12" separation for 3/4" and 16" for 7/8" tubing. Loop lengths sound reasonable. Amount of tubing with vary based on diameter and spacing. Good luck!