In slab hydronics – garage and snow melt system
I planned to put pex in the slab of my new garage, and in enough of the driveway to get snow melted from the garage to the front door. This is climate zone 8. The plan was to do either eps or xps rigid boards on top of a layer of gravel, a vapor barrier in the garage only, and then the slab. The concrete guys i have spoken to said they typically put down a mesh, tie the pex to the mesh, then rebar grid over.
One of the contractors said the rigid insulation under the slab causes the concrete to crack and cure abnormally. And they are worried about the vehicle weight on a slab with rigid insulation under. He suggested bubble insulation (which i think has almost zero effectiveness) or spray foam the ground. I would do the spray foam layer, but the location is remote, and finding a contractor likely will be difficult. And i am not sure spraying foam would resolve concern on the curing issues.
Opinions on the sub slab insulation is appreciated, particularly related to the contractor’s concerns. And while on topic, any opinions of pex type (pex-al-pex, or normal O2 barrier)?
Tracy
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I was on a recent project and we discussed three different ways of keeping a slab from freezing. 1. Pex with heating hot water.
2. Hot air blown through piping.
3. Electric heat tracing.
The electric heat tracing won out and although I am a fan of the warm air idea, I would have to say that the electric heat tracing is likely the best avenue. In order to install the heat tracing conduit was to be (the project was mothballed) placed in the slab with pull string and then the electric heat tracing cables were installed after the fact. There were junction boxes and thermostats placed at appropriate locations since you can only go around so many bends with an electric cable. If I were to do what you are describing, this is the route I would take. If a cable burns up, you just pull it out and replace it but if a pipe bursts, you're sol. The hot air was a neat idea but the pipes tend to get quite large and they have to be sloped to drain which can be tricky and you have a decent pressure drop across them as well depending on the size so. Anyhow, that's my two cents. Good luck.
Tracy,
Not sure what the contractor meant by "the rigid insulation under the slab causes the concrete to crack and cure abnormally." Rigid foam + poly + concrete is done routinely, and if done well should not have issues. I do think you'd want a higher compressive strength in the foam than whatever is typically off the shelf, but someone smarter than me would have to tell you what that compressive strength should be.
Several contractors I work with have started using closed cell spray foam directly on gravel under slabs; they have found that the labor savings offsets the higher cost of CCSF. Although the CCSF would technically (I think) qualify as a vapor barrier at 2" thickness, they always use Stego between the CCSF and the concrete - it's cheap insurance. Again, not sure what sort of compressive strengths you'd need here, or what CCSF clocks in at.
It might be nice to have the garage slab and the exterior driveway portion on different zones so you can turn the exterior snowmelt off if you want to, but still keep the garage heated.
This may be in the plan already, but I'd be using glycol or something other than hot water, to reduce the likelihood of a pipe freezing.
Zone 8 - that's gotta be pretty up there in Alaska, yeah?
I mis-typed - that should have been climate zone 7 (at nearly 10,000 ft in colorado).
Definitely would be on a separate zone, and freeze protected with glycol. I am more sure about wanting a small snow melt area in the driveway, than garage heat. We get over 300" of snow annually. I've had 2 heated garages, and never turned the heat on in either. I have a suspicion concrete contractors don't like pouring over anything but gravel due to longer cure times when poured over foam or vapor barriers.
I have to consider the electric heating route, if for no other reason than to eliminate the need for glycol.
krackadile, what size conduit did you consider? We you concerned with the reduction in heat transfer compared to the electric cables buried directly in the concrete? I like the idea of being to pull the cables to replace if needed, or when the slab needs to be replaced. Concrete guys have told me more than once - there are 2 types of concrete slabs - 'already cracked' and 'will crack'.
The way I've always heard it is slightly different... there are two things you can guarantee with concrete: it's gonna crack, and nobody's gonna steal it.
It's obviously a different aesthetic (ie, you'll see a big dumb heater), but if you're less sure about having the garage space heated you could probably save a big chunk of change by just putting in Hot Dawg heaters and running them when you want.
Generally heated floors aren't a good match for garages. The big achilles heel of heated floors, particularly when concrete is used as the heating element, is that they can't change output quickly. In inhabited spaces, where the comfort of underfoot heating may make the compromise worthwhile, this is an acceptable tradeoff. But garages aren't spaces where you tend to hang out barefoot.
If the heating source is going to be resistance electricity -- like an electric water heater -- there's no efficiency gain from using hydronics rather than just a resistance grid under the floor. Installation cost for the resistance grid is substantially lower.
Contractor is FOS.
Foam under slab with radiant tube is no problem
Been there, done that, got the t shirt
I think other than specific ice melt scenarios, snowmelt is a waste of money and energy
One thing to watch with partial snow melt is where the melt water goes. There are a number of heated driveways here that are very effective at turning a bit of easy to shovel snow into an iceberg that needs to be chipped away.
I also think that contractor is fos.
What cause the concrete to crack is poor substrate prep. As in not enough gravel, and not compacted well enough, and uneven grading. The EPS foam between the gravel and the concrete in inconsequential to the slab moving.
Also note that the EPS at home Depot (durospan 250) is rated for 25psi and is used for foundations and can easily handle the cars. You can get higher rated EPS,.if you really like, but that usually for heavy duty applications. find a local EPS supplier or rep, and discuss with them to get a better sense of the foams deflection and compressive strength needed.
As for the snow melt system, personally I'm a fan of hydronic systems because they're cheap to install. The pex tubing doesn't cost much (uponor hepex) and is easy to install without getting heavy into wiring and stuff, but admittedly, a hydronic system adds more complexity with pumps and controllers.
BUT, here are my lessons learned: you gotta decrease the pex tube spacing, like 6 inches apart. This means more zones and more tubing. You also need the tubing near the bottom of the slab (as the concrete contractor suggested). Keeping it too close to the top (like I used to do for a more reactive system) ended up with distinct lines in the snow of where the tube is, and wouldn't heat the slab evenly and so it wouldn't melt the entire walkway or driveway. Water runoff and draining is also important as akos said because the standing water will refreeze and cause more of a hazard than just straight snow.
Jamie
Echoing that contractor is FOS. Rigid under a slab is a super common detail and pretty easy to do. Poor curing comes with poor prep or improper install / weather considerations or a bad batch. Also, at some point either 1, 10, or 100 years down the road, the slab will crack no matter what. That's why there's typically control joints.
Also echoing a few users in that hydronic is less preferable IMO because you have liquid in tubes that can bust or leak and then youre ripping up the slab to repair. I have used alot of the Thermosoft systems lately - theyre pretty cost effective, easy to install, and have sensors that can kick on at a certain temp. You can also zone them separately from garage to driveway so that they can be controlled independently.
https://us.thermosoft.com/snow-melting/systems/driveway-walkway-mats
Also attaching a resource regarding depth of heat trace/pex piping. Sometimes if it's tied to the mesh, it ends up dumping all the heat into the ground and the slab even if it's insulated. I assume you have a 4" or 5" deep slab so keeping the trace 2" from the surface w/ rebar chairs or something would be ideal. Just don't let it be at the bottom. I am sure Thermosoft has this is in their install details also.
I appreciate all the feedback on this. The point of the melted snow re-freezing is one i had not really thought of. The driveway has a shallow grade (less than 3%), so the ice would defiantly be a problem.
I have a few weeks to make this decision. I'll look into the thermosoft system. The entire driveway is about 2500 sq ft, so the cost to do the whole driveway is likely way more than i want to budget. Potentially, i scrap the garage in slab heat and the snow melt system, and invest in a good, electric snow blower. Or put the money toward an electric ATV with a plow. The ATV could be a fun summertime toy as well.
Makes sense in garage if you plan to work in there during cold seasons but I agree that for the driveway it probably doesnt make sense, and a snowblower or plow would do you better.
Good luck!
For the projects I typically work on (luxury residential, typically in the mountains, CZ6) we very rarely snowmelt anything more than garage aprons and stone entry paths. Everything else gets plowed / snow blown / shoveled.
Granted, it's always a caretaker or snow removal company doing that work, not the owner - it's just cheaper to pay a snow removal company than to install and run that much snowmelt (also I have to imagine the boiler for that much snowmelt would be, like, a nuclear reactor or something). Your ATV + plow idea sounds like a great way to go.
As one of the commenters that suggested a snowmelt. I will also agree that a plow is the better solution.
Snow melt systems are hard to do well, can ridiculous to repair if broken, costly to install and super energy intensive to run.
So maybe minimizing the scope to the porch, steps, walkway & otherwise places where a plow can't get to or would destroy is a better solution. At that point since it's a smaller system, an electric heat cable system might be a more viable/easier solution that you don't have to get a new electrical service or anything crazy for and If you have access to cheaper electricity.
And you can spend the difference on getting a Yarbo snowblower robot for the driveway.