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opinion and topical commentary
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Over Radiant In-Floor Heating
What you may hear – if you haven’t
I know a number of architects, builders and wood flooring contractors (good ones) who’ve had bad experiences and will try as hard as they can to talk clients out of installing wood flooring over radiant in-floor heating systems. On the other hand, when given the opportunity to investigate “problem projects” and the source for their feelings, I’ve almost always located the real cause for the failure(s) and wondered why it wasn’t obvious to them as well. I think it’s just human nature to avoid issues where pain was once involved. It’s been my experience when we try something new and things go wrong, our focus tends to stay more on the perceived (new) and often less on the real (old) cause. But maybe that’s just
Our company has been installing wood flooring over radiant in-floor heating systems for over 35 years. I personally have experience installing hydronic geothermal systems in homes and other structures prior to that. For many years, The Oak Floors of Greenbank, Inc., was one of the few hardwood flooring companies that would agree to install any type of wood floor over a radiant in-floor heating system.
In the late 1980s and early 1990s I wrote and published several articles for various magazines such as Fine Homebuilding Magazine and Hardwood Floors Magazine describing in great detail exactly how we went about this. A number of wood flooring contractors followed us into the field. Some have been more successful at this than
How we care for our clients with in-floor radiant
When potential prospects for wood flooring installations over radiant heat contact one of our two companies, we do our best to educate them as well as any others involved in the project. If our contracting company is contracted to install a wood floor involving radiant in-floor heat, we insist on getting to know (if we don’t already) the architect, builder, radiant heat installer, plumbing contractor as well as the owner, to get everyone on the same page. This is best accomplished very early on during the planning stages of the project. Wood flooring is the most obvious and evident recipient of malfunctions and other misbehaviors of any heating or cooling system in the structure (especially in-floor radiant heating systems). The wood floor and by inference the wood flooring contractor is typically the first (and often the last) one to be blamed for the “effects” that can come about as a direct or indirect result of the misdeeds (or lack of knowledge or awareness) of others. As it turns out, wood flooring is an especially good precursor for malfunctions within heating or cooling systems – especially those that function directly under or over a smooth finished wood
Most types of wood flooring (engineered or solid) can generally be effectively installed by floating, gluing, nailing or gluing & nailing. By minimizing the shifts in relative humidity inside the structure and by reducing the heat load the flooring must undergo as the heat passes through it and is radiated from it throughout the structure, even relatively unstable species in wide plank form can perform well over some systems of radiant heat. Keep in mind, that the wood flooring itself becomes a heat source as it grows in temperature from the tubes, fins, plates or whatever warm beneath it. The keys are proper acclimation, a steady and balanced low level of heat, and very gradual moves when changing temperatures. Installing humidity controls within the structure and maintaining a ±10% to 20% RH will virtually guarantee success. A few wood flooring manufacturers require this or they will offer no guarantee whatsoever for their products over radiant in-floor
Water and wood:
Changes in moisture content (mc) are important with all wood products since changes in moisture content can cause changes in their shape. Consider how wooden doors or drawers will often swell during “moist” times and are prone to “sticking”, while they move with ease during the “dry” times of the year. Changes in the moisture content of wood flooring are especially important because of the accumulated force that develops within the large single plane of material that a wood floor represents. Wood flooring strips or planks will swell during moist times and jamb tighter together. One indication of excessive moisture in a wood floor can be seen as “cupping” or “crowning” or a disfigurement of the normal smooth plane within or between the individual strips or planks. If a floor is flooded or an extreme level of moisture is allowed to penetrate the flooring strips or planks, it will begin to act and move as a single large unit. If there is sufficient room for the boards or planks to expand and move, they will lift and buckle. If not, it will virtually crush anything in its path. The “expansion gap” conscientious wood flooring contractors leave around the perimeter of the floor is to give the strips or planks room to move (even buckle if necessary) as moisture content rises. The pressures expressed by the changes in moisture content within the cellular structure of wood are enormous. They can bend steel and fracture
Radiant in-floor heat and its effects on water in wood and other products:
Radiant in-floor heating exponentially impacts the normal ingress and egress of moisture into and out of the cellular structure of wood items. Even relatively small changes in the moisture levels of wood flooring can be exacerbated by the effects of radiant heat. But even the most stable materials (wood, stone, glass or metal) including radiant-designed products can and will respond poorly if and when they’re pushed beyond their limits by excessively high temperatures of radiant heat passing through
Poor or insufficient planning coupled with the lack of proper acclimation are the chief culprits behind most of the visibly obvious cosmetic affects to wood items (wood flooring, underlayment, base molding or other materials at or near the floor line where the impact is the greatest) in radiant in-floor heated homes. Even where complete failures or total losses of serviceability occur, much of the blame can usually be traced back to a lack of planning or acclimation. Pipes or lines can be broken, pinched or punctured. The radiant mass (be it cementitious, stone, gravel, sand, wood, etc., or a mixture of materials) can be impugned from flooding ground water, sprinklers or broken waterlines that when left unabated will send finished wood flooring or other such items into a state of shock and awe. Still, with proper planning, acclimation and installation of all materials involved together with a common sense method of operation and maintenance can indefinitely defer or virtually eliminate all but the most unforeseeable
I relate installing wood flooring over radiant in-floor heating similarly to driving an automobile at freeway speeds. If you have the right automobile on the right road with conscientious drivers around you – there is little to worry about. But, erase or eliminate one of those key factors and you have a recipe for disaster.
Acclimating the flooring prior to installation:
As the old story goes, you can never have too much acclimation. That applies to any wood floor in any application. The real key is acclimation to what? A structure (in particular a single family residence) will usually experience its most traumatic exposure to moisture during its construction phase – second only to a major remodel or heaven forbid, a complete dousing of man-made or celestial origin. Acclimating the flooring to the ever-changing, often aggravated conditions prevalent during construction can do much more harm than good. To properly acclimate, the wood flooring needs to be surrounded by the average anticipated “lived in” temperature and humidity of its new home. This means that the radiant system must be up and running and for a period long enough for it to properly “acclimate” the other materials in the structure, especially that material over which the new flooring is to be installed – before the new flooring is introduced to the job site.
All of this can be easily verified by checking the moisture content of the flooring to be installed and comparing it to that of other wood products already installed inside the structure. Of paramount importance is the moisture content of the subflooring, underlayment or whatever material over which the flooring is to be installed. On occasion we are asked by a builder to “stock” the new wood flooring in a home while things are still “drying out”. This is too soon. In such a case, the flooring will try to adapt or acclimate to its surrounds by picking up moisture and swelling. At best, this means it must now “give off” this extra weight prior to its installation if things are to go well. It could mean the flooring will swell, twist and bend out of shape, making it more difficult to install, sand and finish. Unfortunately, this could also result in the finished product exhibiting splits, fractures and gaping at some later date. Wood flooring should never be introduced into a structure before the HVAC system has had the opportunity to “condition” the rest of the structure – especially those with radiant in-flooring heating
We always recommend utilizing acknowledged stable wood species made from quarter sawn material or a mixture of rift cut and quarter sawn. Using quarter sawn or rift/quartered material will substantially improve the stability of the flooring and in most cases the hardness (durability) as well. Vertical grain flooring made from nearly all wood species exhibits far less horizontal movement compared to flat or plain sawn cuts when moisture is forced into or out of it over time. The increased stability with this type of material will help reduce the “walking” or cupping, crowning or curling often seen in wood flooring boards undergoing “stresses” from pressures within the flooring system as a result of increased moisture levels.
We also recommend using planks or strip with a more narrow gauge profile (typically a maximum 5-inch-wide plank) unless adequate humidity controls are in place. The narrower the plank or strip, the more joints or seams in the floor. Though they may appear tight visually, each of these junctions affords additional room for the material to expand or grow into with increases in moisture. It’s important to note that wider width planks can do well over radiant heat in areas where the relative humidity averages 50% or lower or where humidity controls insure stable levels of moisture at all times within the structure. Nevertheless, I would strongly encourage the use of a “full spread” elastomeric urethane adhesive in conjunction with nailing, for any wooden plank 6” or wider installed over a radiant heating system.
For a better overall “feel”, appearance, and longevity, I recommend both gluing and nailing any wood floor over radiant heat – unless its installed utilizing a full-floating method. Gluing and nailing is a considerably more expensive and time consuming method of installation (particularly if repairs are required once the flooring is in place). However, it is proving itself more and more every year as the most foolproof method for potentially problematic installations and one that provides the most “acceptable” “feel” down line with the inhabitants of the residence or structure. Although the various wood flooring associations are still lagging in their full acceptance of such procedures, each passing year finds more and more of the top wood flooring installers throughout North America and Europe adopting it as “their standard” for solid wood flooring (and in particular solid plank flooring) over radiant heating systems.
Whenever possible, we try to recommend the use of an antique reclaimed product, such as flooring re-milled from old timbers, decking, or structural lumber. Not only is this one of the “greenest” flooring choices you can make, but from our experience, one of the best performing types of wooden flooring available for use over radiant in-floor heating systems. It has proven itself repeatedly with our most
demanding clients as the overall top choice for appearance,
longevity and out-and-out effectiveness over radiant heat.
Excellent sources for information about wood flooring over radiant heating:
If you would like to learn more about installing wood flooring over in-floor radiant heating systems, I would suggest you review all the information on our two web sites
http://www.theoakfloors.com and http://www.woodfloorco.com
For a continuing topical dialogue on wood floors and radiant heat as well as many other subjects of our trade, check out
http://www.hardwoodfloorsmag.com. For a somewhat more homogenous treatment of the topic, you might also check out
http://www.nwfa.org and http://www.nofma.org. These are the web sites for the Radiant Panel Association, the National Wood Flooring Association and the Wood Floor Manufacturers Association (formerly the National Oak Flooring Manufacturers Association). I agree in principle with both associations stand on installing wood flooring over in-floor radiant heat but not to the exact letter. I have helped both organizations write, edit, develop and revise their technical manuals and other data on this and loads of other subjects over the years. I have also helped organize and develop most of their schools and training programs going back to the 1970s. Wood flooring (particularly “solid wood” flooring) over radiant heat issues are still very much “leading edge” and being associations, they are only as strong as their weakest links
Some differences in in-floor heating systems:
I have written a good deal about installing wood flooring over radiant heat, but very little specifically relating to systems that require the installation of wood flooring immediately over and in direct contact with the heating tubes, metal fins or panels themselves. Some utilize sections of plywood that support the finished flooring, others are attached to subflooring or underlayment, while still others are simply panels laid atop a structural surface with the flooring laid directly over the panels.
There are products that allow wood flooring to be nailed into or through the fins or panels. A few allow wood flooring to be nailed and glued to their sections. Still others allow wood flooring to only be glued or free floated over the tubes, fins, panels, cementitious slab or other
There are so many different manufacturers and systems out there and new ones coming on to the market almost daily. It’s hard to really know what works effectively and what doesn’t. From my own personal viewpoint and experience, I can tell you that the simpler the system the more fool proof and effectual it will likely make obvious over time. It will do that by expressing less of itself
in maintenance, repairs and malfunctions over the long haul.
Engineered & floating wood flooring systems:
Many wood flooring manufacturers “recommend” only engineered products (if they make such a critter). Those that don’t, don’t. There are some manufacturers that say their products must be installed “free floating” over radiant in-floor heating systems. The individual strips, pieces or planks of “free floating” wood flooring systems are held together through the use of dry snap-lock attachments, metal clips or the planks or pieces are glued to each other with flexible glues inserted inside the tongued and grooved seams down their lengths and/or at end butts. The edges of these floating floor systems are held down by base molding at wall lines or cabinets and by “lip-over” landing nosings at the tops of stairs and “lip-over square edges” or “lip-over reducers” adjacent to carpet, tile or other floor coverings and in other open stretches.
Systems that fail:
Whether engineered, solid, free floating, dry locked, clipped, glued, nailed or nailed and glued, I’ve personally seen all of them work and fail. Most “failures” though, are due (at least in part) to moisture issues. Excessive moisture (or lack of it) in liquid or vapor form is almost always at the root of wood flooring malfunctions, perceived “problems” or out-and-out failures.
Since radiant heat exacerbates the ingress and egress of moisture into and out of wood, one can easily see why installations of wood flooring engineered or solid, free floating or glued and/or nailed over radiant heating systems can be a bit more problematic than those with forced air heating. Still, in nearly every scenario, it’s the lack of information and education that create the most significant roadblocks to successful wooden flooring installations over in-floor radiant heating systems.
Systems that do well:
From my experience, the more efficient the radiant heat installation, the kinder it is to any wood flooring system installed above it. This should provide ample reason (if there wasn’t already enough) to build the most efficient in-floor (wall or ceiling) system possible. Extremes of heat (or lack there of) expressed in highs and lows cause a great deal of fluctuating in the tubing, fins, boilers or whatever, and should be strongly discouraged or made impossible if the wood flooring is to perform well above it (or below it). When wood flooring is made to come into localized direct contact with the primary heat transfer devices (tubes, fins, pans, etc.) rather than with a more indirect generalized contact from a larger mass (slab, plywood or other larger structure), the effects from these fluctuations can and usually are more pronounced. As it turns out, wood, stone and other natural substances, prefer coaxing over
Systems that do the best:
For out and out efficiency, simplicity and total comfort, I prefer systems that work with greater amounts of mass. Usually this means a cementitious slab with the tubes set inside. My all time favorite, though presently condemned by most building codes is one I helped design and install many years ago. Most of those systems utilized a solar heated liquid, that was then transferred to aggregate stored beneath the house in an enclosed crawl space. This warmed the entire structure much in the same way as many geothermal systems transfer heat to a slab or aggregate. A ground loop system of tubing (taking advantage of the constant ground temperature) was utilized in combination with these devices as heat sinks (on warm days), but more importantly as preheating and precooling systems for the homes domestic water
Comparing some newer systems:
I generally avoid comparing styles or types of systems by brand name. Unless or until a manufacturer exemplifies themselves with an exceptionally good or bad product, I will almost always leave specific products or manufacturers out of my discussions and speak more to specific “types” of products or procedures. Radiant systems that require that the flooring installer lay the floor directly on top of their tubing with the surrounding plywood supporting the flooring’s weight have come into vogue in recent years. The increased popularity of these styles is not only due to the reduced weight loads on the homes substructure but also for their reduced installation cost. Probably the biggest advantage of these systems is that they can be reasonably added to homes during fairly minor remodels. Those considering such an arrangement should know that although quite “effective”, the efficiencies are significantly reduced utilizing this technology. A little more down payment will seriously increase your dividends down
My personal preference:
Given the choice, I would almost always opt for a radiant tubing system set inside a cementitious pour overlaid with two layers of plywood glued and screwed together floating over the slab with some type of membrane separating them from the cementitious material. The wood flooring is then nailed and glued to the top layer of plywood. This system, or some variation of this system, is one I’ve utilized or specified most often when working as a consultant for architects, engineers or homeowners. It is the most successful prevalent system on all but just a few of the most discerning wood over radiant installations around North America. This does not mean the direct over systems are bad – far from it. I have specified it when weight or other restrictions eliminated the use of a cementitious pour. As a wood flooring contractor, it probably doesn’t make much difference, although there is an increased potential for hitting a tube with the “direct over” procedure. The scientist in me favors the efficiency of the added mass to accentuate the positive effects of the radiant system. In all fairness I should say that as a consultant and inspector, I’ve observed an increased propensity with “direct over” projects for reiterative gaping among flooring boards (engineered or solid) and/or detectable color striations within the finished wood flooring some time later. This color striation appears to be more prevalent with some wood species and stone types than with others.
The best case scenario:
That said, if the general contractor, flooring contractor, radiant heat designer/installer, plumbing and electrical contractor and owner are all well advised experts on the proper procedures, sharing information and working together as a team, there is really no reason for a pessimistic approach with either system. I’ve described installing solid wood flooring over radiant in-floor heating systems in a few articles I’ve written akin to surgery. If everyone is experienced, knows their job and does it correctly (assuming the patient is in otherwise good heath), there is little cause for doubt or worry. On the other hand, if there’s a loose cannon on board, almost anything can happen – and it has on more than one
It’s important to note a this juncture that regardless of which style or type of radiant system you choose to install, the odds are strongly in your favor that you’ll be rewarded for your decision with a substantially more efficient and effective heating system than possible with almost any forced air system available.
Personally, I view a solid wood floor, or a mixture of stone and wood installed over an in-floor radiant heating system, as the best of all worlds (when properly designed, constructed and maintained by all involved).
If you have any questions after you’ve completed your research, please feel free to give us a call. We have many excellent references from all kinds of wooden flooring installations over radiant in-floor heating systems, but the best ones are always those we don't hand out to potential clients.
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