To retrofit an existing floor with staple up, picture the joist bays you want to heat running left/right for their longest direction, in my case 14'. With a circular saw set for 1/8” deeper than your subfloor thickness, cut out 3 strips of subfloor a little over 1 foot wide and as deep as the room about 1 foot away from the left and right walls, plus one at the midpoint of the floor left to right. Now you have access to every joist bay at 3 points-moving from left to right, you have 1' of untouched floor, a 1' opening, 4.5' untouched floor, 1' opening, 4.5' untouched floor, 1' opening, then 1' untouched floor. If your joist bays are longer, add extra openings as needed, spaced so there is not more than 4’ between the deepest you can reach through the openings-the 4’ heat transfer plates will hold the pex to the underside of the subfloor you can’t reach.
Drill 2 holes though each joist via one access opening (let's say the left side). Drill the first ones about 8" away from the leftmost wall, then another set of holes 8" to the right of those. You can now run the supply leg of your pex through the leftmost holes to the far end of the room (presumably the outside wall of the room), then returning back to the near end of the room through the second set of holes. Make U-loops down each joist bay by pulling extra slack down each bay using the middle and rightmost access openings. Keep slack anywhere it loops so you don't kink it, as it's tough to heat and hold still when it's in a joist bay! If you do kink it in a bad place, you can just pull some extra through until it is in an easy place to heat it and remove the kink. Waste the extra you pulled through if needed-you can probably use a short length somewhere.
Once you have pulled all the u-loops through, lay strips of reflective radiant barrier on the bottom of the joist bay or on top of insulation already laying on the bottom of the joist bay. Also make sure your joist bays don’t extend into any areas you don’t want heated and that they are insulated/sealed wherever they terminate. The poly based barrier is much easier to work with than the aluminum woven kind. Have someone help you to guide a 4 ft transfer plate under the pex, lift up against the underside of the subfloor and staple up. You have to have a compact crown stapler that can fit in the joist bay-mine is a Bostich. You can angle the stapler a bit if needed. The plate supports the pex against the part of the subfloor you can't reach between the access openings. Start at the return end of the room and work backwards toward your starting point in case you have to pull some more from the spool or return some to it. Cut 4" sections of foam pipe insulation to place around the pex where it passes through joists to avoid noise as it expands/contracts. If your manifold is far away, you could put a splice in an easier to access place instead of pulling all the return run length through the floor. After pressure testing the loop, screw blocking chunks of 2x4 around the perimeter of your access holes(everywhere you can to avoid future squeaks), lay the removed sections of subfloor back in place, then screw them to the blocking chunks, preferably with a little construction adhesive, then top with your favorite flooring material.
Carpet gets gross underneath even with powerful cleaning, plus it insulates the floor pretty well limiting the amount of heat you can move. I recommend 3/8" to 1/2" thick engineered hardwood of some sort using urethane gluedown methods. We used 5/16" thick 5 ply with real tigerwood for the top veneer because we got a great deal, but you can see slight undulations where the subfloor edges were sanded down from the factory if you look closely. Still, we saved $8k for the whole house compared to anything that was 3/8" thick or better. This is a minor problem and worth the savings. The floor is stunningly beautiful and all our friends and customers ooh and aah everytime they see it! My realtor friend says I have the best looking floor she’s seen and says the warm floors are a “powerhouse” selling feature since extremely few houses have them-mine is the first she has personally encountered.
By the way, don't be afraid to heat pex to repair a kink. Heat it with a heat gun or rapidly moving torch until it starts to turn see-throughish. The kink will magically almost disappear as the molecules try to return to the position they were in when they went through crosslinking. Stabilize until cool enough to hold, and you are done. I was skeptical, so I took a short piece and kinked/twisted/pinched/hammered it WAY beyond anything that can happen accidentally. I then overheated each damaged area until it began to smoke, twisting and kinking it like crazy while hot. It springs back to it's original position like memory metal!!! After it cooled, I clamped tire valve stems in the ends, pressurized it to 120psi, then kinked/twisted/pinched/hammered it while it was under pressure until I was bored, then left it overnight. Even though the outer oxygen barrier layer was demolished, the pipe never gave way or leaked. This pipe will probably outlive all of us. The section you heat and repair will cool to a slightly larger diameter than the rest of the pipe-this is normal.
Blueridgecompany.com has the best pex and best prices I found. They offer many types, but the best is 1000' of USA made Pex-C for $280 shipped. I tried 4 brands of Pex-A, Pex-B and Pex-C from different vendors and theirs was noticably easier to pull and more flexible than the other kinds.
I will take pics and post when we finally do the remaining upstairs rooms, but it will probably be in the fall. We are relocating the manifolds and accoutrements since we made a change to our basement layout. When we do that, I’ll give some detailed pics of the rest of the system.
Craig
The "MMT"
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