preheating slab before installing floor
Hi,
I’m ready to install a wood floor over a concrete slab that will eventually have radiant heat. Do I need to heat up the slab in order to drive out residual moisture in the concrete? Here’s details:
The slab was poured in August and has been in a dry, warm, enclosed space since September. Beneath the slab I installed three overlapping layers of 6-mil poly, then 2-inch blueboard (both of which extend around the perimeter of the 5-inch thick slab). There is a frostwall around the perimeter, with drain tile running next to the footing. Fill beneath the slab is sand.
I installed pex tubing in the slab for future radiant heat, but we don’t have money now for the heating apparatus (would eventually like to try rigging a solar-heated system). The tubes simply come up out of the slab in the utility room next to the wall where we’ll put the manifold.
I taped a piece of poly down on the concrete a couple months ago and checked for moisture, but didn’t see any. I believe the concrete is probably dry enough to install the floor, but am not sure and am new to this process.
Flooring is mostly ash strip flooring, which will be fastened to 1×4 sleepers over the concrete.
So. . .can I proceed without running hot water through the pex, or am I asking for trouble? Obviously money is the issue here, but I don’t want to be stupid about it.
Thanks,
Ed
Replies
Hi Ed:
I have a similar situation...a floor with tubes and no heat. before I cover it with my flooring, I am going to warm it up. Exactly like you are saying...to remove moisture and also to get it to a "equilibrium" type temperature. It's probably about 40 degrees now...it has been so cold...and the woodstove is hard to keep blasting when I'm not yet living there!
Also, sounds like you haven't performed any pressure tests on your pex? I hope this is not true...but if it is, much better to find a problem now then before it is covered with that beautiful Ash floor!
Even if you don't install hot water (maybe just an electric water heater and pump to get it warm...I wish I had done that a couple of months ago now) I'd really make sure to do the pressure test. You can just apply fittings on the ends and set yourself up with a schrader valve and pressure guage...quick and dirty. I've tested a number of my AquaPEX lines this way, as they are not all tied in to the supply manifolds yet. If you went WIRSBO, they have a cheap little setup...and with the expansion tool, takes no time at all.
My thoughts...not a professional, just DIYer.
Actually we did pump some air in the tubes and monitored with a pressure gauge during the concrete pour. Didn't see the pressure drop at all.
As for slab temperature, I'd guess ours is a bit warmer than 40, probably closer to 50. Like you, we've been running a woodstove, plus our house is extremely well-insulated and has a good passive solar design.
We already purchased a tankless hot water heater--but it would require another thousand dollars to get the rest of the components (manifold, pressure tank, etc.) to pump hot water through the tubing. That's a thousand dollars we really can't afford at the moment.
My gut sense is that the slab is probably dry enough--the architect who helped with the design thinks so too--but it seems like I keep hearing "whatever you do, heat the slab before installing wood flooring."
If your test patch of poly sheeting didn't show any water, you're probably good to go.
But as said, a cheap electric HWH and a pump will get it up to temp cheaply. Note that a HWH will work fine on 120 volts, just at 1/4 the wattage (if you don't have a big enough 240-volt circuit). I've rigged a sump pump in a 5-gallon bucket and suspended it from a rafter. Used washing machine hoses to connect the pump to the HWH, the HWH to the PEX, and the PEX back to the bucket. The bucket serves as your air bleed and as your expansion tank. The HWH must be set low, like near "vacation" setting to not get the floor too hot.
The bucket must be above everything else. If not, the first time the power stops the pump from pumping, you've got 30 gallons of water on the floor.David Thomas Overlooking Cook Inlet in Kenai, Alaska
The rule of thumb for a concrete slab drying is one month every 1" of thickness, so you should just squeak in --- BUT, it's normal for a hardwood floor over a heated slab for the hardwood to be spread out over the slab for a week to 10 days while the heat is on to acclimatise the timber.Obviously you can't do this but the moisture content of the timber does need to be lower than you can accept for an unheated slab -- about 9% is right for European Oak, if that's a guide -- so you may have to talk with the supplier.
If the Relative Humidity of the timber is too high you'll have bad shrinkage problems when you do put the heat on.Your floor finisher should be told also that the slab will be heated because there are precautions he should take against future 'panelisation' when he applies the finish.IanDG
Thanks for the tips. I hate to buy an electric hot water heater I won't use, but maybe I can rig up a similar type of system with my propane-fired tankless unit.
I'll check with the flooring supplier on the moisture content--although I'm pretty sure they said about 9 percent. Right now the stuff is stacked in piles on the sleepers over the concrete. Sounds like I ought to spread it out as much as possible.
By the way, what's the trick you refer to in finishing the floor?
With a heated slab the flooring will move far more than normal. Any film forming coating, when applied, can run down into the tongue and groove and act as a glue. Panelisation occurs when the boards shrink and some remain stuck together so that there are no gaps for, say, 4 boards, then one huge gap and so on. It can also cause the top of the groove to split away from the board.The prevention is to use a sealer on the floor that is softer and will separate a lot easier. IanDG
An installation technique I feel is the best way to do your app:
they make thin rubber membranes out of epdm to lay over the concrete. Then your flooring is edge-glued, but not attached to the subfloor in any way allowing it to float. The rubber membrane has small insulative property but not much, certainly not as much as the below-osb-subfloor radiant systems, and allow the wood to expand and contract across the concrete. No splits, noticeable movement, etc. Base trim hides the movement. good luck. Don't fasten the wood down fast to your floor! too much movement with a heated slab.
remodeler