Radiant heat and hardwood flooring diagram
It happens more often than you would think.

I was called out to measure a hardwood flooring project for a new custom-built home being constructed by a local builder. The builder wanted over 3000 square feet of solid, unfinished, quarter sawn nail-down, oak flooring installed, sanded and finished.
I reviewed the job site with the builder. He described the sub-floor as being 3/4" standard douglas fir plywood that he had glued and screwed to sleepers. The heating system was hot water radiant heating set in concrete (as shown in the cross section picture). 

I measured the square footage and took my moisture tests.  It showed the plywood was dry at 11 - 12% moisture content. This is exactly what is recommended.  I went back to the office, wrote up my quotation and faxed it to the builder.  Two weeks later, I was awarded the job and asked to go back to the job site to measure an additional room.

It turned out to be a stroke of luck that I had to return to the site.  While I was there, I did another moisture test. The reading was now 18% in the plywood.  I went into the other areas of the house that I had tested two weeks prior, and all were reading 18% plus moisture content. 

What was going on?

The builder had poured the concrete over his radiant heat piping and up to the top of the sleepers in the correct way, but had not allowed the concrete enough time to dry before he installed the plywood. The newly installed plywood was now absorbing the moisture from the concrete raising it to 18 - 20%. Way above the recommended 12%. 

Even with the heat turned on, this situation would take a really long time to correct itself.

The result?  All the plywood had to be removed....an expensive mistake.
Now, with the concrete exposed the heat was turned on and the concrete allowed to dry.  The new 3/4" plywood was then installed. 

At least a bigger disaster was averted...had the hardwood flooring been delivered to the site and allowed to acclimate on the 18% sub-floor, it would have absorbed the excess moisture and been installed in an expanded state. Later, once the heat was turned on, the flooring would have shrunk leaving behind unsightly gaps between the boards.

Simple advice:
  • Just because the concrete is hard and looks dry does not mean that it is dry enough for a hardwood floor installation. Check the concrete moisture content and the moisture content of the 2 x 4 sleepers before you overlay with plywood.
  • Ensure that the home's heating system is operational, the temperature in the home is approximately 20 celsius and humidity levels are between 40% and 60%.
  • If the heating system is not yet hooked up, do not deliver the hardwood floor to site until it is.
  • Under good drying conditions, concrete dries at about 1" per month in a 3" thick pour.  Radiant heated slabs will dry much faster. The thicker the slab the slower it dries (i.e. 4" might take 5 months to dry and so on).

8/23/2012 09:03:44 pm

This page is very informative about plywood subfloor. I appreciated what you have done here. I enjoyed every little bit part of it. I am always searching for informative information like this. Thanks for sharing with us.

9/25/2012 09:13:46 pm

I liked the way you put together everything, there is certainly no need to go any further to look for any additional information. You mentioned each and everything that too with much of ease.

10/2/2012 06:25:26 pm

A fantastic blog with a lot of useful information. I would love to get updates from you. Keep blogging. All the best.

10/3/2012 11:54:02 am

Thanks for your comment, I'm glad you found the blog useful. Let me know if you have a wood flooring topic you would like covered.

7/12/2017 10:59:36 pm

I had an issue with bouncing floors and searched forever. I did find this company called structure Lock that uses metal Ijoists.

Try them out structure lock.com


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