radiant heat, plywood vs cement board
Dens
| Posted in Energy, Heating & Insulation on
I just finished glueing and screwing 5/8 inch plywood above 3/4 inch plywood as subfloor for tile. Beneath this subflor there is Radiant heat tubing which is run through the trusses below (and stapled to the 3/4 inch plywood subfloor. . Then along comes the heating and cooling guy comes and says we should rip up the 5/8 inch plywood and install 3/8 cement board.. The rest of the house will have composite wood floors above the 3/4 inch plywood. How much stock do I put in his advise…. he says the two finished floors (tile and wood ) will heat at different rates…. suggestions? We do plan to insulate well below the radiant heat tubing with foil faced foam, hopefully .Thanks,
Replies
I'm going though a second floor finish project and we will be using radiant heat. I have 5/8 sub floot over the joist and due to the fact that I will be placing the radiant in a plywood track above the sub floor which is 1/2" I was told that it plus the sbufloor plus the 1/2" wonder board would be enough for the tile. For the bedroom floors I wanted to use 3/4 hard wood but was told to go with the 3/8 engineered for two reasons.
1. Less movement more stable as it will be glued down
2. Less wood that would increase the R value making the heat less effective.
2a. I think the cement board will heat up your floor more evenly than the wood.
Good luck, Turtleboy
ps watch out for the screws/nails
3/8 wood generally is more stable ( read less movement) than 3/4" engineered or not. Usually with proper care and installation of flooring, the difference is undetectable even with RFH.
Then there are other issues associated with engineered flooring, but I will keep to the use of mastic or floor adhesive. Over time, you may find it does not hold as well as advertised. Not to mention the chemical mix you are subjecting the installer and homeowners to.
Wood does have more thermal resistance than cement board or tile. It will take longer to heat up , but it will also hold the heat longer than the tile will when the heat is removed. What the difference is in effectiveness, I can not say.
Because of this increased thermal resistance, wood will appear to heat more unevenly. But my experience is the difference has more to do with the proper sizing and install of your RFH loops. A poorly designed system will telegraph through what ever medium you choose to use.walk good
Your H&C guy is correct in that the two floors will heat at a different rate. Wood has greater thermal resistance than the tile/cement board and will heat slower. The other end of the equation is the wood will hold the heat longer than the tile/cement board after the heat is shut off. However I have never had any problems with this difference in my floors.
The other issue is the better substrate for tile. Cement board has it all over cdx for substrate for tile unless you plan on using mastic instead of thinset.
The other issue is finish floor heigth...5/8" plus 3/8" tile gives you a finished heigth of 1". Wood flooring (including composite) usually is between 3/8" to 3/4". If you are using 3/4" flooring then the finished heigth will be the same (in theory) of using 1/4" cement board, thinset and tile. At least that is what I use for my floors. I thinset and screw the cement board to my 3/4"cdx. subfloor
if it don't work and/or if you ain't happy with your comfort level don't you dare blame him. I am so sick of clients listening to some hack on flooring choices then blaming me if there are problems. I design my systems based on rubber padding. I explain that to the customer. I spec it in my contract. then the flooring guy says "that's all hogwash' and puts in foam! grrrrr.
I finally figured out why customers listen to hacks like that rather than the guy who should know... seriously...
ready?
Whoever saves them the most money is the expert!
I just told the owner on my latest job I don't expect his infloor system to work and I am not responsible and since he and his GC choose to ignore my written and explained specs that I really don't care if it never works. shook him up. I did it tactfully of course. (they jammed the floor insulation tight against the floor on a staple up job.)
The GC did it just to prove HE was right and to try and make me look stupid. Last job I will ever do for him. there will be no lawsuits because the GC and the owner are both strict Mennonites who will not go to law. The GC is the biggest crook in the valley. But he is cheap.
Dens-
Yes, your contractor is right that there are different heating rates, but that can be accomodated by varyig the water temperature from one room to the next (which may already be put into the system based upon the heat load calculations).
The rooms with the thicker finished flooring will require a warmer water temp the the room with tile and thinner subfloor, but with a heat load calc, those new numbers can easily be found.
Another way to accomodate this change is to add heat emmission plates that will lower the supply water temp by up to 40 degree F. With a thick flooring system like this, you may have already passed the maximum water temp in order to heat the floor correctly and need to add the plates. It all depends upon the heat load calcs.
Another way take care of the different rates (don't you just love hydronic heating, so many ways to skin a cat!) is to install outdoor reset control devices. Automatically adjust the water temperature being supplied to your floor based upon the changes occuring outside. As it gets colder outside, the supply water temp to your floor will increase therefor anticipating the larger heating demand. This type of system also gives you want you ultimately want, warm floors all the time!
Hope this helps and enjoy!
Dave H.