Help Planning Radiant Garage Slab...

Just as important as the tubing is the underslab and edge insulation. If you dont insulate correctly, it will take a lot longer to bring up the temp in the garage.

radiant heat, it's not cheap and may be more then you want to pay for a garage. my method. excavate/remove all organics to 4', drain perimeter to daylight with drain pipe, use crushed stone or good gravel, if your digging in hard-pan and gravel leave this intact inside the perimeter. if not bring in 1"- gravel and compact, haunches or 2' outer perimeter is thicker 12"+- concrete. use 2X12 form boards staked, kicked, and nailed with cleats, plumbed and straighten with string, snap a grade with a red chalk line by transit or laser. insulate with 2" blue dow Styrofoam, insulate around the outside with 2' of 2" foam and then the slab with same. some use a plastic vapor barrier, some don't. others use different insulation's or a new sheet product that IMHO doesn't work and is way to expensive. staple pex 1' on center to foam boards. tie #4 steel bars 24" OC elevate steel with bricks, pour 3200 psi or better concrete, use expansion joints or saw, some use fiberglass in the mix, prevent cracks, have a center drain or several drains, caution where the pex comes up to the manifold. 1/2" anchor bolts about 4" in 2' OC, burnt concrete is slippery in the winter. their are many threads in this forum that will help. sweetheat

Please refer to my earlier post. https://www.hearth.com/econtent/index.php/forums/viewthread/35851/P15/#382293
You'll see that it's all in the planning stage because once you pour the slab, it's a done deal. I prefer the Pex-Al-Pex for the oxygen barrier qualities plus it's easier to manage.

Peter B. said:

Thanks much for the replies...


I might ask again though... provided the install is proper and decently insulated, is there any (very rough) rule of thumb as to how long the slab might take to warm from (say) 35* to 60*?

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My 25' X 32' shop with 3 X 250 runs of 1/2" pex will come up to 60 degrees from 40 degrees in just a few hours in cold weather. I was really surprised at how quickly it came up in temperature. I was also pleased with how long the slab held the heat. I have learned to make use of the slab to augment water storage. I really like the in floor radiant heat.

Pex is cheap, and cannot be added to the slab after it has cured. Insulation is expensive but pales in comparison to the cost of heating the earth surrounding your garage for the full life of the garage.

Install the pex and super-insulate now, while it is easy.

Floor drains may not be allowed as they can introduce petroleum products into the groundwater. Next time I'll consider pitching the floor toward the doors to drain melt water; 4" over 30' seems like it would work without being awkward to live with.

Have fun!

DC

Pete: This is always a tough call, depending on how much you (or the next owner) will be out there, how cold it is where you live, whether or not you like to wear Sorels, etc. Whatever you do, insulate the perimeter! Use 3" to 4" of blue foam outside the slab edge (vertically) for 2' down, then 2" to the footer. As mentioned, at the footer, go 2' horizontally w/ 2" foam, too. Inside the foundation wall, under the slab, come in at least 8' w/ 2" blue foam, and 1" thereafter, OR 2" everywhere if it is cold where you are. A good vapor barrier under the blue foam is a MUST, from everything I read. Besides, it is dirt cheap and won't hurt one iota if you don't need it. My garage has been ambient for 35 yrs, save for a rare Master Heater session (gag...cough....), but I will have a heatable garage when I build next year (no in-floor stuff). Working much below zero is getting pretty old, and zero does not bring a glaze of joy to my eyes, either. Sorels and Refridgeware go a long way toward heating.... but getting to be a shorter way as I age. Good luck w/ your decision.

I drive truck for a living, and have a 35x75 garage that I park the truck in at night. About 9 yrs ago, when I was building it I debated about radiant. Figured it wouldn't serve my needs. I only heat the garage up when I need it. Most nights park truck, do a quick walk around, and I'm done. Other nights do an hour or so's work, turn the oil fired unit heater on, shut it off when I'm done. This will keep the garage above 30 degrees by morning. Holds heat pretty well. Some times it's heated for a few hours, occasionally, I'll leave the heat on overnight, but turned down to 50. This is done on the subzero nights, 20 below or more. I usually only burn about 150 gals year, once in a while 200 gals. Couldn't justify the extra expense, and radiant wouldn't do what I needed.

jk and flying make good points, but when fuel oil is $10/gal (which will certainly happen within the lifespan of your shop) then the insulation in place may make the difference between a heatable structure or one that you or the future owner cannot afford to heat...it's always a balance between the current finances vs. future finances (and how you plan to use the shop, and how the shop actually is used).

My 900 sq ft garage, is 2/3 below grade, with 4" foam insulated walls (R24), 10" fiberglass ceiling and 2" of foam below the slab (with VB!) takes less than 180 gallons of fuel oil to heat to 50* all winter.

I've seen various posts suggesting that if laying a slab, then it is good to put in PEX, as it doesn't add a lot to the expense, but does give more options for later. At the same time, it was suggested that for intermittent use, a Unit heater / Modine unit can be a better option - if you want to run off the hydronic, they make units that are essentially automotive style radiators w/ fans, which will put out a lot of heat and get you to temp in a much shorter period of time.

I have seen a fair bit advising AGAINST Pex-Al-Pex in a garage or other potential freezing zone - regular O2 barrier pex is supposed to be more tolerant of freezing, and much lower cost (Fred Seton of Radiant Design, says it is a vanishing type of tube, mostly used by old-time plumbers because it feels like the copper tube they are used to....) If Pex-al-pex freezes, it supposedly changes size so that one can no longer put fittings on it, while regular Pex will shrink back down to it's original dimensions, or close enough.

Gooserider

Peter,

I'm building my first house this summer.... and hopefully my shop next year. Even If I haven't planned a wood boiler yet. I will insulate / run pex in my basement slab and in my garage slab. It will be there. And here electricity is so cheap that I may consider an electric boiler to warm the slab. (mais heat will be from a wood stove). So go for it.

I had a quote done by radiantcompany.com it seems resaonnable for pricing. And I went through there web site many time. I learn a lot there, they have plenty of good information for the radiant heat dummy like me. But with the $ exhange rate, I will look locally if I can get same stuff cheaper in Canadian $$.

Gooserider said:

I have seen a fair bit advising AGAINST Pex-Al-Pex in a garage or other potential freezing zone - regular O2 barrier pex is supposed to be more tolerant of freezing, and much lower cost (Fred Seton of Radiant Design, says it is a vanishing type of tube, mostly used by old-time plumbers because it feels like the copper tube they are used to....) If Pex-al-pex freezes, it supposedly changes size so that one can no longer put fittings on it, while regular Pex will shrink back down to it's original dimensions, or close enough.

Gooserider

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From what I have read pex-al-pex has one thing going for it, that is a low thermal expansion rate as compared to regular pex. This can be a benefit in staple up applications with heat plates. Regular pex can be noisy as the tubing expands and contracts through the plates. I plan on using it my radiant ceiling.

Jackpine Savage said:

Gooserider said:

I have seen a fair bit advising AGAINST Pex-Al-Pex in a garage or other potential freezing zone - regular O2 barrier pex is supposed to be more tolerant of freezing, and much lower cost (Fred Seton of Radiant Design, says it is a vanishing type of tube, mostly used by old-time plumbers because it feels like the copper tube they are used to....) If Pex-al-pex freezes, it supposedly changes size so that one can no longer put fittings on it, while regular Pex will shrink back down to it's original dimensions, or close enough.

Gooserider

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From what I have read pex-al-pex has one thing going for it, that is a low thermal expansion rate as compared to regular pex. This can be a benefit in staple up applications with heat plates. Regular pex can be noisy as the tubing expands and contracts through the plates. I plan on using it my radiant ceiling.

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Hmmm... I won't claim huge expertise, and this definitely seems like an "opinion" type question, but your approach seems opposite of what Fred Seton Says on his Radiant Design Institute website...

The site as a whole is worth reading, lots of good info, but the two pages I thought particularly relevant to this were (with partial quotes and added emphasis):

On Pex-Al-Pex -

I do not recommend PEXc it is simply PE pipe ran through an E-Beam to give it a partial cross linking. The multi-layer Pex, called Pex-Al-Pex some times is actually not Pex but simply two layers of PE tubing with a layer of aluminum between the layers. It is much cheaper to manufacture. It was developed to satisfy the plumbing trade, because of their resistance to plastic pipe, it bends and feels like soft copper. The multi-layer tubing has two problems, if it freezes it will not spring back like Pex does, it becomes over sized and will no longer accept fittings, if frozen more than once it will break the aluminum layer. It will also separate the layers when pulling through the holes in the floor joists on long loops where it must be looped through many holes.

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On emission plates -

Few radiant floor heating systems could benefit from using emission plates. Radiant thermostats are very sensitive, as the temperature goes up during the day it shuts the radiant off. In the evenings when the temperature drops one degree below the comfort setting the radiant system is turned on. It only needs to "ramp-up one degree to satisfy the thermostat.
By using the 3/4" pex staple from Peter Mangone Inc. on 1/2" pex, on 8" centers, with 120 to 130 degree water, and using 500 ft. loops, I have designed thousands of radiant systems in the last twenty years (many in Alaska) that are quite, affordable and efficient, with nothing more than staples.

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and (in response to a question about a study that claimed faster floor response w/ plates)

There is still no overall change in the efficiency of the radiant system. They are talking about percentage of change in the speed of heat transfer. A very sensitive thermostat (one degree differential) causes short cycles of heat input to the bottom of the floor and a nearly constant out put to the heated area. As the testing shows, an air gap does improve spreading the heat output. I have found that using a very small air gap (1/8") allows a rapid response, an even output without streaking and more than twenty Btu's per Sq. Ft. Radiant systems are rarely allowed to cool down and seldom need to have high rates of increased transfer output, in that case plates or floor contact would increase the response time. If the thermostat has a one degree differential, the system never needs a rapid increase of temperature. The bottom line is that the staple up system that I design with, does maintain the one degree differential and it does have the same efficiency as other systems including Plated, grooved subfloor or Gypsum. Any change of efficiency must answer the question of where did the heat go? The only place you can loose efficiency is through the envelope or out the chimney.

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I'm not sure I buy Fred's arguments in favor of running a heftier pump and 500' loops instead of the 300' loops that most other sources seem to reccomend, but his claims about emmission plates make a lot of sense to me - after all, as he asks, where else will the heat go? Looks to me like the only time you would see much advantage to the plates is a slightly faster warmup when you first turn the floors on every season - but otherwise if the floor is at a constant temp I don't see that the plates do anything for you. (It would take a fair bit of convincing to get me to use them at this point, as they make the installation harder, and look like the cost of the plates would be more than the Pex...

Gooserider

I'm no expert either. From what I've read, I agree there would be no reason to use pex-al-pex in the OP's garage slab. You also wouldn't want to pull it through holes in joists, if nothing else for the difficulty factor. But it is a fact that it expands and contracts much less than regular pex, and I highly doubt that it was developed just for old plumbers.

I've read a few comments about Fred's approach on the RPAnet forum . To me running a 'heftier' pump equals higher operating cost. I don't remember what kind of water temperatures his systems run, but the lower the temperature the more efficient and more compatible with a storage system. I have read a few unhappy stories of plateless installs. Here is one guy that I trust more than Fred:

Plateless In Radiantville

Here is one thread on RPAnet that I think is worth a read:

Best radiant method ....

I’m not sure I buy Fred’s arguments in favor of running a heftier pump and 500’ loops instead of the 300’ loops that most other sources seem to reccomend, but his claims about emmission plates make a lot of sense to me - after all, as he asks, where else will the heat go? Looks to me like the only time you would see much advantage to the plates is a slightly faster warmup when you first turn the floors on every season - but otherwise if the floor is at a constant temp I don’t see that the plates do anything for you. (It would take a fair bit of convincing to get me to use them at this point, as they make the installation harder, and look like the cost of the plates would be more than the Pex…

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Saying you don't need heat spreader plates because "where else can the heat go?" is like saying you don't need those big exhaust pipes on your motorcycle when 1/2" tubing is so much cheaper because "where else can the exhaust go?" You might get it to idle but there will be a limit to how much exhaust you can push through those little tubes and at some point you just can't go any faster.
It's not about losing the heat, it's all about heat exchange rates. The floor will only be at a constant temperature if it can get heat from the tubes as fast as it is losing it. Read that Siegenthaler link.

The website you're referring to is a notorious example of why you shouldn't believe everything you read on the web. Even if it is stated in a very authoritative manner. I'm sure some of his designs have worked for some folks but I've read some pretty disappointed postings on radiant forums from people who followed some of his "opinions".