Photo 1: Right: My two-story frame house with a permanent wood foundation and basement.

Photo 2: Loft area sleepers showing cross cuts and tube layout. All the scraps were used here but note the ends and tubes don’t line up across the room to facilitate proper nailing.

I continue to get calls about the suitability of wood flooring with radiant heated flooring systems. Many inquiries start with comments such as:  “I know solid wood cannot be used over a heated floor system, BUT my customer wants wood flooring.”

“I know wood flooring should not be used ….”

“I know wide plank is not recommended over radiant heat, but my customer wants to put 6” wide plank over the system.” 

“I know engineered flooring is the only wood product that can go over radiant heat systems, but….”

“The manufacturer does not recommend their product for radiant heating systems…”

“The manufacturer says the water temperature in radiant systems with wood flooring should not exceed 85 degrees.” …and so on...

Let’s start by saying that with proper customer expectations most any wood flooring product can be installed over a properly designed radiant heated flooring system with the normal caveats in practice, i.e. don’t put solid wood below grade; nailers should be 12” o.c. and less; solid flooring and wider widths require a solid subfloor; etc.

As you see, most of the items deal with solid wood. However, engineered flooring can also be a good choice over radiant heating. Since most engineered flooring is proprietary and general directives may not apply to the specific product, the manufacturer’s directives should absolutely be followed. In any case, if the manufacturer does not recommend or warrant their product over radiant systems, DON’T INSTALL THEIR PRODUCT in such an application.

Photo 3: The finished loft flooring in winter, showing very few gaps.

Some years ago, in 2002-3, when I built my “last” house, I installed solid wood over radiant heating systems. Thanks to The Radiant Panel Association, Maxxon, Danfoss, and “Hot Rod” Rohr, I was able to install a radiant heating system in order to monitor how wood flooring reacted to different conditions. I did some things right and some things wrong but have learned that solid wood flooring and radiant heating can be a good combination. My home is a wood frame two-story over an unfinished basement built with permanent wood foundation walls. (Photo 1)

The second floor system - I joists 16” o.c.; 3/4” square edged plywood  subflooring; 2” x 4” SYP sleepers 12” o.c. parallel to joist direction glued and screwed to plywood; between sleepers-Maxxon tubing 9” loops imbedded in Maxxon gypcrete to top of sleepers. The flooring-- one bedroom with 4” cherry plank, one bedroom with 2 1/4” ash strip flooring, and the loft area with 2 1/4” quartered white oak strip flooring. The ash flooring was finished with a water-based urethane. Other floors were finished with a hand rubbed sealer and wax.  

Photo 4: Upstairs bedroom with ash flooring. The coins illustrate the permanent gaps from panelization taken during the winter. These do close some in the summer but remain prominent in the light colored floor.

I installed the flooring sleepers parallel to the joist since this was the shortest direction of the rooms and the flooring would be oriented in the long direction. I found that the sleepers needed to be straightened so cross cuts 1/2 the thickness were made as necessary across both the face side and backside to relieve the warp and twist allowing them to lay flat before screwing and gluing to the subfloor. Any sleeper end joints were spaced 1/8” and all joints were staggered at least across two sleepers. I used mainly 12 foot sleepers so had mostly one end joint per run. Sleepers were placed at all critical points, along the end walls to support board ends, across door jambs and door framing, and at the top step nosing. In my case, the tubing layout was placed before the sleepers so the sleepers were notched to fall over the tubing.  The tube intersections were marked with magic marker to avoid nailing at intersections. The tube layout was also critical so that no sleeper intersection is at the same place across the room.  You may not be able to nail a board run if this happens. Since I was both the flooring and heating contractor, I think I had good communication between the two but I still had to reposition some of the tubes. (Photo 2)

As a flooring contractor you will likely either place the sleepers before the tubing or draw out the sleeper location before the tubing is placed. In either case, make a list of items the heating contractor should follow for proper tubing location for the flooring. You don’t want a tube to be placed where you absolutely have to fasten the flooring. If you don’t communicate and accommodate each other’s requirements a problem floor is likely. The flooring contractor is the last one there, so the blame can easily point to you. With the sleepers down, the gypcrete was poured.  Before the flooring installation began, some of the gypcrete was higher than the sleepers and had to be disc sanded with 24 and 30 grit to a flatter condition.

Photo 5: This is the transition from great room, 2 ¼” and 3 ¼” flooring, to the entry, 2 ½” quartered flooring, taken during the winter. Note the gaps are more numerous in the great room and some are permanent. The entry has fewer gaps that generally close in the summer.

I regularly checked the sleepers for moisture for the next three fall months. The heating was not turned on and the moisture remained above 11% while other wood framing members in the house were 8 to 10%. This confirmed the reports that 1 1/2” gypcrete can take a long time to dry. Flooring installation began that spring. The ash flooring had been stored for some time in a storage building and the quartered white oak strip and 4” cherry plank were delivered from a local distributor. About six to eight weeks after installation the floors were finished. Again the strip white oak and cherry rooms were stained and finished with two coats of sealer and a hand rubbed sealer urethane blend followed by waxing. (Photo 3) The ash flooring was finished with a water based sealer and two water-based top coats. (Photo 4) The heating system was turned on that fall just before move in.

The main floor system -  This is a staple up system with the tubes attached to clips on the underside of the plywood subflooring with insulation between joists.  This flooring was installed during the spring right after the upstairs was completed. The great room is 2 1/4” and 3 1/4” alternating white oak strip; the kitchen and breakfast areas are 2 1/4” hickory/pecan strip; the master bedroom is 5” white oak plank; the entry is 2 1/4” quartered white oak strip; and the dining room is an octagon patterned floor of hickory/pecan strip and 4” plank. I used 1 3/4” cleats for the flooring installation to assure no heating tubes could be punctured. I did not use staples since noticing that staple legs sometimes run out of the subflooring at odd angles and might hit a tube.  I used felt paper only under the great room, entry and master bedroom.

Photo 6:  A close-up of the great room floor with the frequent gaps.

All the strip flooring except the quartered white oak had been stored for some months in the storage building before installation. The stored strip was 8% to 10+%; the quartered flooring was 7% to 9%; and the plank was also 7% to 9% at installation.  The 5” plank was delivered from a distributor’s warehouse. It was installed last after racking out and back-sealing. These main level floors were finished the same time as the upstairs about 6-8 weeks after installation. During sanding and finishing the great room flooring had already developed gaps that were filled. The kitchen and breakfast area were finished with oil modified urethane. The rest were all sealed and waxed.

Over the last five years I have observed the following: the upstairs floors - these rooms are generally more consistently warm during heating season; the rooms also have lower ceilings which likely contributes to the comfort. The sealed and waxed quartered strip flooring and 4”cherry plank show some gapping during the winter. The gaps are not prominent and generally close during the summer. The ash floors developed some noticeable permanent gaps that exhibit panelization about every 5 to 6 boards. (Photo 4) This was the only floor finished with a water-based finish and the only one showing panelization. The white ash coloration also shows the gaps more readily than any of the other floors. Some of this gapping can be attributed to installing the flooring at a somewhat elevated moisture content. I do notice a slight crowning each year when the heating system is first turned on but this flattens after about two weeks. During the winter the floors average 6 1/2% to 7% mc and 8% to 9% during the summer.

Photo 7: The dining room patterned floor. The 4” hickory/pecan shows the permanent gaps of summer. A remedy would be to place slivers in the larger gaps. The angled end gaps will likely remain.

The down stairs floors - these rooms seem to require more heating than the upstairs since I notice the boiler cycles on more often for these rooms. They also seem to cool off more quickly. However, neither system shows flooring temps above 85 degrees F when heating is called for. I have noticed no unusual odors as a result of the felt paper underlayment. I have heard that some sensitive people can detect its presence. I, therefore, consider the felt an option for installation. All the downstairs floors have some permanent gapping. The great room (Photos 5 and 6) and kitchen (Photo 9) are the worst. Most of the gapping can be attributed to the elevated moisture of the flooring at installation. The 5” plank flooring has minimal permanent gaps as it was at a lower moisture content and was back sealed at installation. This plank flooring remains very flat seasonally with only a slight crown when the heat first comes on and a slight “lip” in the spring when the windows are opened during pleasant days. (Photo 10)

Photo 8: An overview of the dining room. The gapping, as in Photo 7, is similar throughout. The darker heartwood does not show gapping as much as the sapwood sections.

With the plank flooring, winter gapping is also minimal and I would think most any discerning consumer would accept.  The quartered entry strip also has very few permanent gaps and the winter gaps that develop are small and not prominent.  The patterned hickory/pecan floor has some noticeable seasonal gapping along with some permanent gapping. Some of the units are from 4” plank, and since there are many angled cuts, gaps at the ends are present. (Photo 7 and 8) We know that hickory is one of the more dynamic woods that has the potential to expand and shrink more than most other species. So this is the nature of this floor and some consumers might object to this amount of winter gapping.  A repair for this floor would be to place some slivers in the most prominent permanent gaps and they would not be noticeable. This operation will be photographed and recorded when I next refinish the floors.

Photo 9: The kitchen floor with hickory/pecan strip, finished with oil modified urethane. Some permanent gapping exists but is generally not prominent with the color variation of the # 1 common grade.

The one item I regret not doing is having the heating system on before any flooring was installed. This would have preconditioned the system to a drier condition and the overall high moisture of the stored flooring would have been reduced and fewer and smaller permanent gaps would be present. For the ultimate performance of the finished flooring, I believe having the heating actively on at least two weeks before installation is essential. I would NOT have the flooring present when this is occurring as it may over-dry any unfinished flooring. Only place flooring in the heated condition if you have determined the need to dry it out before installation. Also, my flooring system averages about 1/2% lower in moisture content than non heated systems, so I would advise a slightly lower than normal average moisture content at installation.

Photo 10:  The master bedroom floor of 5” white oak plank in early winter, January 1. Note that the gapping is small. Even in late March it is not much wider. The key was installation at an average 7 to 7 ½% mc. and back-sealing all the planks. Also the bedroom is somewhat cooler in the winter than other areas.

In all many species and different sizes of flooring can perform very well over radiant heating systems with thought to proper acclimation and installation procedures. If the customer wants wider widths or the more dynamic species, they must be warned of the potential for extra gapping and or associated cupping as the seasons change. Engineered flooring can also be a good choice, but will be subject to the same issues as solid wood, particularly the slight cupping and or crowning as the heat is turned on or off.

Also, with engineered flooring length shrinkage or end gaps may occur. I have noticed some manufacturers require that the relative humidity be maintained above 30 to 35% at all times for their product to perform properly. This may be a difficult requirement where extended heating is required.  In addition, again if the manufacturer does not recommend their product over such a system don’t use that product. I am really pleased with my floors, and the subtle warmth without blowing forced air is a pleasure.