FAQ Installing Hardwood Flooring Over Radiant Tubes In Plywood Panels

Hi Don,

Thank you for your detailed and informative essay on installing wood
flooring over radiant systems. I enjoyed the read a lot.

I was wondering if you might be able to give me some advice concerning whether or not to seal all sides or at least back seal 4" rift & quartered white oak before installing over "warmboard?”

The heating system has been in place and running since December 09' and the AC has been running for 1 week. The white oak was delivered a few days ago and I was planning on giving it 2 weeks to acclimate. Most construction on the house is complete including Sheet rocking tile and paint. Most of the millwork installation remains. The
Project is a complete structural shell renovation of a 3500sf home in Queens New York. All floor, wall framing and windows are new. I have heard various opinions on back sealing and end grain sealing over radiant heat and I’m on the fence as to whether or not to do it or to skip it.

One last notable issue is that we would like to install some of the flooring parallel to the tubing which would place every 4th board directly on top of a line of pexal tubing. Warmboard recommends installing flooring perpendicular to the tubing but tell me that parallel can be done. Is it simply more difficult?

Sorry for sending you such a long email but hopefully you will have a few moments to give me your opinion.

Sincerely,

Max
New York, NY


Hi Max.

I hope you'll excuse my taking some extra time responding to your questions.

Radiant heating systems are near and dear to me. Many years ago before it became vogue to do so, I helped design and install geothermal heating and cooling systems. We used both the heat from the sun and the ground (and from lakes or ponds if they were handy) to warm homes in cool weather. In warm weather, we collected cool air from the ground or from lakes or ponds and pumped it into the homes. Warm air was taken off the outside shell of the homes and deposited back into the ground in place of the cooler air we’d collected. In the days before expensive energy, we cut heating/cooling costs by 90 to 95% with systems that are still running efficiently today.

The tenor and tone of your questions to me on wood flooring over radiant heat suggest a somewhat practiced understanding of their application. I applaud both your attitude and approach. So many architects these days seem in lockstep with the lemmings. Engineered wood flooring and plywood with plastic radiant tubes does indeed work, albeit the inverse of an efficient radiant heat transfer model.

For the peak performance, efficiency and appearance in wood flooring over radiant in-floor heating, quality conscious wood flooring installers around the globe continue to express their preference for solid wood flooring (preferably rift and/or quarter sawn material as you’ve specified) installed by gluing and nailing to a multi-layered plywood system that is set to “float” over a 1-1/2” or thicker cementitious mass. Such a system, when properly acclimated and installed, should perform exceptionally well with only a minimal amount of maintenance for many years.

The plywood panel systems were developed for retrofitting radiant systems into homes where the added weight of a more efficient system could not be supported. While variations of this method can perform adequately as a primary heating source, great care should be taken when comparing the efficiency these designs to those systems that enjoy significantly greater mass. As a rule, the greater the mass the more efficient the radiant system.

Systems operating on the fringes of radiant efficiency can be anticipated to function marginally, not just as heat transfer devices, but in other aspects as well. For example, noteworthy variances in conductivity within the hardwood flooring’s structure will translate to major variances in heat transfer and overall system efficiency. Minor glitches can become major anomalies in the workings of the entire mechanism. Minor to major appearance changes, such as color shades and gaping, are sometimes resultant within the wood flooring. On occasion these appearance changes are striking, depending on the floor-to-tube temperature and overall heat loss within the system.

Utilizing a plywood flooring system (i.e. an engineered wood flooring product) may serve to further stress an already strained radiant design. Many such systems result in a radiant design that employs an inordinately high floor temperature. Knowing this and knowing the results of a too high wood floor temperature, savvy hardwood flooring manufacturers are not guaranteeing their products installed over a radiant system that causes the surface wood flooring temperature to exceed 80 degrees Fahrenheit. Many such manufacturers are requiring temperature sensors be installed in their flooring systems in order to validate their warranties.

Leaving the wood flooring until all wet work is completed is always a good strategy. And, the two week scheduled acclimation period would normally be adequate in a standard scenario after stocking the material in a “dry” structure. Structures with radiant in-floor heating however should command a longer acclimation period. While time is important for acclimation, moisture content (MC) checks are mandatory.

Every good contractor, general or flooring, will own and use a moisture meter. The really good ones own several different types of meters. There are meters that measure concrete and cementitious materials and those that measure wood. There are pinned meters that leave holes in the materials being measured but they measure moisture at different depths. There are pin less meters that take only average overall readings of a material but do not leave holes in the material being measured. Regardless of the type meter the contractor uses, multiple moisture checks should be made of the subflooring, surrounding wood materials and the flooring to be installed.

Wainscoting, trim and base molding should also be measured and acclimated prior to installing on or near a radiant heated floor. These materials, along with the wood flooring, should be stickered in stacks no higher than two feet off the radiant floor with the heat on. Stacks should be arranged to accommodate a good air flow under, between and within the bundles of material. When the MC of the subflooring and the flooring to be installed is within 2% MC of each other, it’s time to install.

Sealing the backs and ends of the wood slats or planks as you propose can substantially reduce the ingress and egress of moisture within individual planks. For any questionable installation, I strongly recommend that a moisture resistant (but not moisture impervious) product be applied to backs (or bottoms) as well as to the ends if flooring is not to be glued to the substrate. There are even some hardwood flooring manufactures who use packaging tape applied to the bottoms of their planking to help resist a rapid ingress of moisture.

Our company began sealing the bottoms and ends of wood flooring boards years ago when we were required to install hardwood in homes or structures left unheated for months on end, or when we were required to install flooring on houseboats, cantilevered structures or buildings built on pier poles.

This extra sealing procedure is mostly redundant when gluing and nailing except for the board ends. As long as the sealant used is resistant and not impervious to moisture migration, it can’t hurt. Impervious sealants are perfect, right up to and until they fail. If boards expand and rupture the impervious seal, the impervious bond begins to work against rather than for protection of the wood. With today’s forest product colleges leaning toward “fast drying” procedures over previously recommended slow air drying prior to artificially drying in kilns, internal stresses can be built up within the wood’s structure causing “artificial” movement or movement without apparent significant changes in moisture content. But that’s another story.

Finally, I never recommend installing strip or plank flooring parallel to seams of any kind. At the very least, these seams can cause the flooring boards directly above them to move differently than those throughout the remainder of the structure. At worst, the boards directly above the seams can flex or move up and down with live loads. Gluing and nailing can help reduce these problems as well as improve the overall “feel” of wood flooring when installed over a radiant plywood panel system. Unfortunately, I’ve found that radiant plywood panel manufactures are somewhat divided on their allowance of glue-down flooring, let alone their allowance of gluing and nailing.

In conclusion, radiant systems are changing constantly, but the laws of physics remain the same. Like my old pappy used to say…you can teach an old dog new tricks but you’ll play hob teaching him to do tricks that don’t work.

I hope this all helps.

Don Bollinger