Countersink the sub floor ?
I am renovating an 1890’s home and in order to free up some valuable space in the already tiny kitchen, I wanted to remove some radiators and replace it with hydronic baseboard or radiant floor. I expect to tile the floor.
I didn’t really want to deal with the “step up” from surrounding areas that radiant would require, but also felt that the copper-fin baseboard isn’t very good at heating. (I live in Toronto,Canada – heating is an issue !) My boiler guy suggested countersinking the radiant by hanging a plywood sub-floor in effect 3″ below the present surface of the joists – which sounds like a great idea !
But is it ? ?
My joists are full 2x10s (edited – initially reported as 2×8’s until re-measured) – but old. I would propose to fasten 1″ thick plywood on either side of the joists to sandwich the, from the bottom of the joists up 7″ to 3″ below the top of the joist on either side(to form a ledge to rest the subfloor on), and bond the plywood to the joists with screws and glue (appropriate construction mastic).
Then I would use a 1″ piece of ply between each joist as a sub floor (top of sub floor would now be 2″ below joist level), staple the pex (I assume 7/8″ OD) in a “U” shaped loop through each bay, and then fill with some form of concrete / cement up to the top of the joists and a little higher, and then put a tile floor on top.
Questions:
1. Do I lose too much strength on the joists by doing this – particularly as I am now adding load with cement and tile replacing about 1.5″ of mostly wood floor ?
2. What concrete / cement product should I use to surround the pex ?
3. How deep should it be ? Minimum
4. Should I move the pex from one joist bay to the next by running the pex down and under the joists and back up (is that too sharp a bend for the pex ?)- or can I safely notch – or drill a 1″ hole and run the pex between the bays that way
5. Given that the floor level for the surrounding floor will be 1.5″ above the joists, could I not run the pex only an inch below the level of the current joist tops – cover with concrete / cement (answer to 2 above) to .75″ above the joists, and then my tile set and tiles to bring the level back to 1.5″ above? That should give me a full 2×6 joist, reinforced with 1″ ply, with some additional strength from the 2 inches above the plywood hangers.
BTW – the use of 1″ ply may be overkill – but I just grabbed 45 sheets of 1″, G1S birch for less than $20. USD / sheet at auction – and so while it is a sacrilege to use this stuff as subfloor, it’s costing me a fraction of what I would pay for 3/4″ subfloor grade.
Thanks for any suggestions.
Gavin
Edited 3/31/2003 8:24:52 AM ET by Gavin
Replies
Can you not get to this from underneath? It would be a lot easier to just staple up the radiant to the bottom of the subfloor with AL flange and then fit foam board or reflectix up under it.
Excellence is its own reward!
The existing subfloor has some holes, inconsistencies in the state of the boards (112 year old tongue and groove) and enough squeaks that it would be a good idea to replace it. Removal of the existing finish floor on its own would also likely take longer (multiple layers of lino, tile, who knows what) on its own than just taking out the whole thing. Also, when the rads were put in (who knows when) the many pipes crossing below the joists make fitting the pex and aluminum from below problematic to say the least.
That said, I am also told that the radiant is much more efficient if encased in concrete or cement rather than relying upon AL to reflect the heat - and that the concrete will form a suitable mass for good heat retention.
Well, you certainly don't have enough strength in the joists for the added weight of the mass.
The mass does help the efficiency of the system but the reason for the AL flange is not to reflect the heat upward. The underlying insulation is to do that. The flange is to help transfer heat by conduction from the tube to the subfloor. The subfloor then becomes the radiant surface. If you area of transfer is onbly a couple inches wide, you have less radiation. by using a 12" or fourteen inch flange, you increase the radiant surface dramaticaly.
With concrete, you heat the entire mass by conduction from the tube to the mass and then you have a flywheel effect, constantly radiating from the heated mass..
Excellence is its own reward!
Thanks for the feedback.
Should have noted: These are full size 2x8s spanning only 11'6" resting on minimum 4" at either end, (i.e. unsupported span would be 10'10") and the tile would extend only over a maximum of 9'2" of unsupported joist on any span (usually less due to counter / cupboard positioning) at the worst point
How big would the joists have to be to support the countersunk approach ? Especially if sandwiched between two pieces of 1" ply for their bottom 5 inches ?
Good idea - but to get to the subfloor with the aluminum, I would have to position it - and the insulation board before putting the new subfloor down because of those damn underlying pipes - which may be close to impossible. Fastening the pex would be feasible but difficult. Also, I would then be back to not having the kitchen floor flush with the dining room and hall.
Edited 3/29/2003 2:33:33 PM ET by Gavin
Edited 3/29/2003 2:37:09 PM ET by Gavin
You are in Canada. I might know nothing about your codes.
1) Unless your plywood is stress graded I would not use it.
2) The joists seem a bit small for that span (I don't have my code book here) even without the cement.
3) If you don't put enough cement around the water lines, the cement will crack near the water lines and your tile floor will crack.
4) The proper way to put a "mud bed" in is to set the joists low and build up.
5) There are lots of good ways to do what you want, but we are all to far to suggest them.
I am a fool. I measured from below and must have hit one of the 2X2s that is nailed to the side of the joists in a few spots to support the subfloor and then wasn't thinking. But Piffin's comments got me thinking (better late than never...) and this morning I went and remeasured. The joists are in fact, a full 10" by a full 2".
Does that change your thinking, Piffin ?
And can anyone suggest what kind of product should be used to surround the pex ?
Sorry for being such an idiot.Gavin Pitchford