Radiant heated floor makes noise after adding additional 1/2" of plywood

Question

I have installed hydronic radiant heat pipes under the subfloor; used aluminum plates for spreading the heat from pex pipes. The house is old, I think the original subfloor is 5/8 Douglas fir. Up to this point, everything was fine, the flow was not making any noise, for several seasons. Later, we decided to renovate upstairs, and decided to add a layer of 1/2" plywood to the existing subfloor, since we want to run engineered hardwood planks in parallel to joists.

We used just regular, 1/2" 4-ply plywood, attached using 1 1/8" regular wood screws, no glue. Now, here is the issue. Whenever heating starts, I hear the floor "talking" - ie expanding, and making a little crackling noise. If at this time, I walk across the room, there is a wave of that "adjustment" happening, which would be fine, if it would be silent, but it's not. Once you walk, or the floor settles changing the temperature, the crackling is gone, and after it's quiet. When cooling-off starts, the floor goes through the same process.

So, anyone had experience with this, and solved this issue in the past?

I can tell it's not pex moving in aluminum tracks, nor aluminum radiators canning/shifting/etc effect, because other rooms that do not have additional layer of 1/2" plywood don't do that. Besides, the issue appeared only after I have installed that additional layer of plywood.

Running ideas:

• 1/2 plywood is cheap, ie not good quality, and makes the noise on its own. I could unscrew several plywood sheets, and see whether they will make any noise afterward.
• Since the glue wasn't used, and those plywood sheets are from different wood, they expand at different speed, thus rubbing surfaces, thus making the noise. Course of action: use glue (the question would be: pl400: rubbery vs mapei wpa:acrylic vs mapei 975:polyurethane)

Answer 1

You are going to need a lot of adhesive between the 2 layers.This is because of the sag between the joists with the original sub floor. I just removed the sub floor in my kitchen, dining and living areas. 1/2" ply with 16" OC joists. There was a lot of sag between the joists and the floor had 3/4" thick white oak on it too and there was still a huge amount of sag. Lots of construction adhesive and no less than 35 screws in each sheet.

Here is an image of how it should be done.

The red lines is the original sub floor and the dotted lines are the joists, blue lines are the new sub floor and the green dots are the screwing pattern. The new sub floor should be laid down rotated 90° and overlapping the old sub floor like what you see in the picture. Now I know you have the piping and aluminum plates so you are going to have to use a 3/4" screw. I suggest using a sheet rock bit to drive the screws in this way you know you will not be going in too deep. You need to pull the areas that are between the joists together . I would also use a construction adhesive liberally between the sheets. Probably going to be close to one large tube per sheet the large tube (28oz) is good for about 85 or so linear feet. You are going to want to go up and down the length of the sheet 8ish times that's 64 linear feet right there.

Answer 2

I've found the issue, and it's contradicting my initial "nor aluminum radiators canning/shifting/etc effect, because other rooms that do not have additional layer of 1/2" plywood don't do that." statement.

I have loosened up screws used to hold aluminum plates and the noise is pretty much gone in the area I've done this adjustment. In order to loosen up, I unscrewed by half turn, until the screw feels loose, and then screwed back in so that the plate just barely touches the plywood, without putting any tension on the screw.

I've made a mistake and used #8 instead of #6., i.e. too big for the holes in aluminum plates, thus the plates had no room to move and are installed too tight. Once they start warming up, they would try to expand, putting pressure on the screw, which made those crackling noises.

Interestingly enough, I heard the same crackling noise as I was releasing tension on some screws.

Morale: always follow installation manuals to the letter, trying to go "far and beyond" might have a negative affect.