Radiant flooring on top of old slab
Dear GBA Community,
I just purchased an old 1930’s farm house in Long Island, NY. I removed a stone/thinset floor and have revealed the original slab, which is approximately 3″ thick, directly on the ground (no vapor barrier, no insulation). I would love to install radiant flooring by way of 1/2″ pex tubing surrounded by 3/4″ CDX plywood sleepers and aluminum diffusion fins, ideally finished by hardwood flooring (I am aware of the potential issues here!). The main problem is that I only have 1.5″ of clearance until I run into my doors and the first step in my staircase becomes oddly short.
My main question is if I need to put down a vapor barrier and/or insulation directly on top of my slab (which I have now leveled with LevelQuik RS) before I lay the sleepers and pex. I have room for a vapor barrier (6 mil poly unless you all recommend otherwise), but not for an inch+ of rigid foam, which seems like the ideal solution. It would be great to learn how I can address the potential moisture and heat loss issues. I have seen these rolls of foil radiant barriers, but have read that without an air gap they are useless.
Please let me know if radiant flooring is futile given my circumstance, or if there’s a way to solve this puzzle!
Many thanks in advance 🙂
-Isaac
PS: I assume that in winter this isn’t so definitive, but my slab appears to be moisture free. I duct-taped down a square of poly for 10 days and it was bone dry. While the slab might be dry, it is most certainly not insulated. Will the lack of insulation only result in a slightly more costly bill and longer heat up times or are there larger problems I’m missing? Thanks again.
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Replies
Isaac,
In-floor radiant heat is a very poor choice if you have an uninsulated slab. If you install hot water tubing above such a slab, you'll increase your heating costs compared to other methods of heat delivery (because much of the heat produced by your boiler will be delivered to the soil under your slab).
Regardless of what type of heat delivery method you finally settle on, it would be wise to insulate your slab. The best way would be to install 2 to 4 inches of horizontal rigid foam above your slab, but it sounds like the costs associated with that work (changing the doors and rebuilding the stairs) make that approach unattractive.
One approach that will reduce your energy bills is to install vertical rigid foam at the slab perimeter. You would need to dig a shallow trench at the slab perimeter, deep enough to extend the vertical rigid foam at least 2 feet below grade. The foam should be between 2 and 4 inches thick. You would need to protect the above-grade portion of this rigid foam with a durable material like metal flashing or pressure-treated plywood, and you would need to install Z-flashing between the lowest course of siding and the top of your rigid foam.
For thin above slab retrofits, Roth panels can work OK in L.I. location. Their panels designed for 1/2" tubing are 1" thick, and R4-ish between the tubing, the panels designed for 3/8" tubing are 3/4" thick and R3.5-ish. See:
http://www.roth-usa.com/products_radiant_panelsystem.cfm
and
http://www.roth-usa.com/PDF_Download_Files/Panel_Installation_Manual_June_2009_full.pdf
There will be some losses to the slab, but it's a heluva lot better than a DIY with sleepers & sheet-metal heat spreaders.
A true vapor barrier over the slab is still highly recommended.
+1 on the slab-edge retrofit insulation, since that will be where the greatest loss out the slab will be.
Martin and Dana,
Thank you so much for your help! I will give Roth a call to get some quotes. Seems like a great product.
Martin -- what would be a better way of heating the space? I have hydronic baseboard at the moment and would like to switch to something more efficient and less obtrusive. The costs of changing the doors and the stairs isn't so terrible, its more the fact that I have a low ceiling as is and am trying to get as much headroom out of it as I can! Not to mention I have counters/cabinets in my kitchen and plumbing in my bathroom all constructed decades ago with this 1 1/2" clearance above slab in mind.
However, if I were to insulate with 2" of foam, which make/model do you recommend and how do I incorporate a vapor barrier and subfloor into the equation (order of operation, fasteners etc)? I'm stumped on how to fasten a sublfloor to a slab with a vapor barrier between the two. It seems as though the fasteners (tapcons?) would puncture the vapor barrier no? And glue is out of the question as it won't adhere to poly or any other moisture barrier I've seen.
Many many thanks!
-Isaac
Isaac,
Q. "I have hydronic baseboard at the moment and would like to switch to something more efficient and less obtrusive."
A. There is no reason to change your heating system unless you have a compelling reason. Switching from hydronic baseboard to in-floor tubing will not improve efficiency. To improve efficiency, you would need to look at (a) the efficiency of your boiler or water-heating appliance, and (b) the electrical efficiency of your circulators.
Of course, if you want to go to all of the trouble you are describing because you find hydronic baseboard "obtrusive," then you should go ahead.
Q. "If I were to insulate with 2 inches of foam, which make/model do you recommend?"
A. Either EPS or polyisocyanurate would be more environmentally friendly than XPS. For more information on this topic, see Choosing Rigid Foam.
Q. "How do I incorporate a vapor barrier and subfloor into the equation (order of operation, fasteners etc)?"
A. You would install a layer of polyethylene on the slab, followed by one or more layers of rigid foam, followed by a layer of OSB or plywood. The Tapcon fasteners must be long enough to extend through the rigid foam to the concrete below. Penetrating the polyethylene with fasteners does not affect its performance as a vapor barrier. Even if the poly layer is 5% holes, it will still be 95% as effective a vapor barrier as a layer of poly without any holes.
Radiant floors are a comfort thing, not about efficiency. If the baseboards are obtrusive and a radiant floor loses too much headroom, using radiant ceilings can work. Even 1" of foam under an unheated new floor over the slab will do a lot for the winter-chill too, especially with radiant ceilings. You don't need sleepers between the insulation- there is no flex to a wood floor supported by 1.5lb or higher density foam (a lot less than a 3/4" subfloor supported by 16" o.c. joists), and if it's going to be tile (that needs no fasteners) you can even use half-inch OSB glued to the foam layer (with foam board construction adhesive) for a subfloor if you've already leveled up the slab to near perfection.
But you have to DESIGN this stuff, not just hack it in, starting with a room-by-room Manual-J, and estimating the amount of unobstructed available floor or ceiling that can be used.
BTW: What is currently delivering hot water to the baseboards?
The idea of radiant ceiling panels makes a lot of sense here. You'd have better efficiency compared to floor heat with minimal insulation under it, and you'd leave more room for insulation under the floor without needing to adjust the doors and stairs. You'd presumably still lose a little ceiling height, but that's probably acceptable.
If cost were no object, you could also tear out the slab, insulate, and pour a new slab, but that's disruptive and expensive.