So I've done a decent amount of research on radiant heat. In particular radiant heat with hardwood floors. However, I still have a lot of question; questions that are kind of hard to find answers to.

House Info

- New Contstruction
- 2800 sq/ft
- Multi level
- Hardwood floor: Quarter Sawn Virgin Pine

Questions

1. There are a ton of subfloor systems out there. There's Engineered Subfloor, Subfloor Board, Staple up Subfloor, Hanging Joist Space subfloor, etc. What is the best subfloor to use with hardwood floor?

2. What are the pros and cons of some of the more popular subfloor systems?

3. People mention that you have to be very careful with humidty when it comes to having a radiant systems with hardwood floors. What is the best way to deal with humidity when it comes to radiant heat? A whole house humidifier system?

4. What are some general tips when it comes to installing a radiant system with hardwood floors? Words of advice?

Thanks so much in advance!
Ryan

What is the best way to deal with humidity when it comes to radiant heat?

Build it tight. Ventilate it right.

I would read this.

The link Jonr provided contains good advice. Just be aware that you lose significant heating efficiency when you use hardwood flooring as compared to concrete or tile. You can enter the hardwood flooring R-value into the HR floor heating design software to determine the higher supply temp that will be needed.

The information in the link is marginally accurate, rehashing some old information. As with most such, the quartersawn wood planking, and old standards make for more stable floors regardless of the heat source or application. Same goes for "floating" floors. By the way, the industry standard of 85F maximum surface temperature is often exceeded with solar gain by an adjoining window. I have recorded idle radiant wood floors well over 140F surface temperature, a design condition that any self respecting wood installer or know-it-all radiant "expert" would deny or or least condemn with visceral contempt...just wait.

The first step in designing any radiant floor heating system is a proper heat load to discover if a heated floor will satisfy the load presented. If so, what is the required output. At what design water temperature will the radiant floor need to operate? Will the chosen design temperature produce a surface temperature detrimental to the wood species chosen or the comfort of those walking on it?

Efficiency is a matter of economy and comfort. If you have confirmed that the radiant floor system chosen will work as designed and the working system operates at design water temperatures conducive to your heat source e.g. heat pumps and condensing boilers more efficiently at design water temperatures below 140 degrees F.

My own 3/4" pre-finished plainsawn oak flooring has been in for 5 seasons now without an issue, but the engineered floating floor in the "most stable" material available (bamboo) was a huge dissappointment, as it cupped irregularly after the first season, indicating an inconsistency in production and reflecting the "bargain' price I paid. In contrast, one of the first retrofit radiant sub-floor systems I installed for one of my brothers back in 1991 was stapled directly to the bottom of the 3/4" sub-floor planking and heated the seamless 5" southern yellow pine to a maximum 80F in the coldest weather (WI) without cupping, curling or cracking! A miracle I know. And, the system has been running 180F since I installed it. Dumb luck I guess.

Of course, the lower the design temperature, the lower the operating cost and the less likely the radiant floor will be blamed for poor quality flooring...

Use of hardwood flooring over HR floor heating makes increasingly less sense as the required heat load increases. The larger the required heat load, the more likely you will experience hardwood flooring failure issues like you experienced Badger and the greater will be the efficiency performance hit. Once your HR floor heating supply temp exceeds 100F, you are better off just increasing it further and supplying a radiator instead of supplying a HR floor.

If you are willing to spring for the expense of a heat pump, you would likely be better off abandoning HR floor heating altogether and just use a mini split that will operate at high efficiency while also fully addressing humidity to better mitigate hardwood flooring failure issues.

Over the years we have installed many radiant systems with hardwood floor cover.
Systems,
2x4 sleepers flat parallel to pipe infilled with concert. Nail hard wood to sleepers.
2x2 same as 2x4 system This works well as there is heat mass and where there is a cross of pipe no wood so floor contractor will know to glue not nail.
RHT floor panel system nail direct to the 3/4 plywood. (same with any sandwich type system)
Floating floors, Got to love the no nail systems
Engineered hardwood floor glued to concrete substrate
Many systems decades old, no problems.
In early radiant days use to be 3/4 ply over 3/4 ply stitched together, then the flooring. Never did that system, seemed a waste.
When installing wood floors think building acclimation, moisture content of wood at time of install, is the platform wet, is you're flooring still high moisture (stored out in the shed for the last 5 years waiting for the day).
Seasoning the wood to the building (storing the wood sticker for air-flow in the rooms to be used for a month) or using moisture meeter will help any wood install be it radiant or not.
Last if you are concerned about moisture in slab, or platform dry it out, turn on the boiler.
Dan

Some will miss the point, though I tried to type slowly. The quality of the flooring and installation outweighs any radiant heating factor (real or imagined) that may effect the long term performance of any finished floor.

Arbitrary design water or surface temperatures are of little use to the qualified hydronic radiant panel designer, given the diversity of available prefabricated panels and those made from scratch, such as Dan suggests.

Having the proper room-by-room heat load performed and matched up to an appropriate system with manufacturers data to compare is the right solution in every radiant heating equation.

One should never be discouraged from radiant floors by anecdotal evidence regardless of its source. I will replace my sub-standard floating floor with a higher quality, probably bamboo like the old one, and consider it another lesson learned. My experience was in fact one of the few I have personally witnessed in 25 years of radiating floors, but oh the things I've heard....

@BadgerBoilerMN

"The first step in designing any radiant floor heating system is a proper heat load to discover if a heated floor will satisfy the load presented. If so, what is the required output. At what design water temperature will the radiant floor need to operate? Will the chosen design temperature produce a surface temperature detrimental to the wood species chosen or the comfort of those walking on it?"

Forgive me for being naive to this, but what does "first step in designing any radiant floor heating system is a proper heat load to discover if a heated floor will satisfy the load presented", mean? I'm building a new house that will have radiant heat on the first and 2nd floors.

Does the above quote mean figuring out how much energy it's going to take to heat the house? If so, what kind of tests do you need to run?

Thanks so much!
Ryan

Sorry, working with Architects and Engineers all day will get you talking technical.

In most of the new house designs the "loads" (how much heating or cooling will be needed) are determined by construction of the "building envelope" (the walls, windows, ceilings of the house exposed to the outside).

New homes meeting the new energy code make radiating floors with relatively mild (body temperature) water generated by a boiler, heat pump or domestic water heater quite easy to do. Such temperatures make flooring choices much easier than renovation work where the walls may be entirely uninsulated and higher water temperatures are required to heat the room to the temperature set at the thermostat (inside design temperature). All of this is base on the weather in your particular location and plugged into an ACCA "Manual J * program, the current industry standard.

The Wrightsoft program I use generates the room-by-room heat loads and allows me to choose the best form of delivery, be it sub-floor plates, sandwich systems or over-pours with gypcrete or concrete. I chose to use aluminum plates in my own retrofit and found the design water temperatures quite reasonable and the comfort perfect as expected.

Or just use our free DIY heat loss analysis and HR floor heating design software to confirm what we previously advised:

http://www.borstengineeringconstruction.com/Heat_Loss_Analysis_Calculator.html

http://www.borstengineeringconstruction.com/Hydronic_Radiant_Floor_Heating_Design_Calculator.html

Please be sure to read the associated instructions for the software. All is very clearly explained.

You are NOT naive and you should question anything that you perceive as being gibberish.

Thank you all for the responses. They are much appreciated.

I'm in the initial stages of researching radiant floor designs, and I'm at a crossroads. That crossroad is choosing between a concrete subfloor or to go with Warmboard. Warmboard has very nice reaction times, which is something that I'd really like to have. However, is god awful expensive. My contractor has relatively cheap labor costs, so the Warmboard sales pitch, on that end, falls a little flat.

So my question is this: Is there anyway to get the reaction time of a concrete system to come close to that of Warmboard? And two, if not, are there any cheaper alternatives to Warmboard, that come close to matching its efficiency.

Reportedly, Warmboard is a good product. However, as you discovered, it can be rather expensive. We always avoid these products and plate systems in general because the ROI is often longer than the lifetime of the homeowner, especially if the homeowner hires out the installation labor. Our company only designs/installs maximum efficiency and affordable concrete slab HR floor heating, which is often integrated with aggressive passive solar heating in well-insulated, new building construction. The increased thermal mass lag time associated with concrete slabs is not an issue (it actually provides a more stable and comfortable indoor environment) if the control system methodology is accomplished properly. For folks doing remodels in poorly-insulated, old building construction who feel they must have HR floor heating at any cost, we refer them to another company and wish them well with their project.

With regard to your specific question...Yes, there are ways to further reduce the R-value of concrete slabs and thereby reduce the associated thermal mass lag time my introducing additives having very low R-value into the concrete mix. This is typically only needed and accomplished by companies who specialize in integrated HR and passive solar heating systems. Of course, if you going to place a high R-value hardwood covering on top of your HR emitter, it makes little sense to reduce the thermal mass lag time of the HR emitter... BTW, these days one can affordably finish concrete floors to look like whatever you want, including old-style hardwood floors.

"Reaction" or rather, response time is only relevant to the load e.g. if you live in Aspen, CO and have a lot of south facing glass, you may need a radiant panel that responds quickly with an appropriate output when the sun goes down.

ROI is not always a financial equation, perhaps the reference was to payback. Though we work on many low-energy designs, all of our customers are interested in comfort first. We are currently designing mechanical systems for a new house here in Minneapolis. It will include fire protection, plumbing and radiant floors on two of three levels using Warmboard as a structural radiant panel replacing the deck on the upper to levels. I have personally installed nearly every type of radiant panel and (experience precluding prejudice) find Warmboard a bargain in certain applications. Since the south wall facing the lake is 100% window and the floors will be 3/4" hardwood, a fast response time and relatively high output at low design water temperatures, will be ideal. On the walk-out basement level we will staple PEX to rigid insulation and pour the slab with hardwood over all. I designed it all with Warmboards help with the panel layout and will integrate it with the other HVAC systems in the house.

We do a great deal of renovation work in historically significant homes including several on the Historic Register. Since the architecture is important to the owners of such houses and they can afford to be comfortable we use radiant floors, walls and ceilings to heat their homes and with the addition of high efficiency boilers often cut their fuel bills in half.

Naturally no concrete floor, we polish, inlay, color and stamp quite a few each year, will match the tactile qualities of real wood. When you heat a wood floor (your feet won't care how it was heated) you may have the ultimate in comfort, economy and style. As for special concrete mix, unless you are interested in Passive House performance you can likely get by with best practice for your area and application.

Well, this company and several others we love know how to accomplish concrete floor finishing and can make it look like a real wood floor:

http://www.harmonconcrete.com/concrete/diagonal-concrete-floor-made-to-look-like-wood/

I suppose if you like real wood floors that squeak and cup, it is true that you won’t experience this with concrete. Quality concrete floor finishing is a lot like quality HVAC design/installation, the people doing the work actually need to know what they are doing.

petrified wood, wont cup, or squeak.

We have wood in multi million dollar homes in Seattle over RHT floor panel system. I have systems over platforms with sleepers ( to nail flooring to) & concrete infill no cup, no squeak. Some decades old.
Real wood is sometimes what is required by the client, get over it radiant works fine.

We have installed RHT floor panel system. Part of the floor will be tiles and the rest we want wood. We are in silicon valley,CA.
I am looking for specific recommendations for hardwood floor to be installed over RHT floor panel system. Thanks.

@sailawayrb

Thank you for the info on reaction times. It's nice to know you can lower the R-value of concrete floors. That's good news!

On another note, the more I research radiant floors, the more questions that come up.

Questions

1.) What would you recommend to control whole home humidity? From researching, controlling humidity is something extremely important when installing radiant heat under Hardwood Floors.

2.) Since the radiant heat is going to provide the hardwood floor with a constant amount of heat, don't you have to worry about the hardwood floor drying out over time? From a layman's perspective, you're just cooking the wood at a very low temperature for years on end, right?

However, I did read, the quote below, from a comment in this thread:http://www.houzz.com/discussions/786884/Radiant-heated-wood-floor

The moisture content of the floor is going to come to equilibrium with the moisture content of the air and has very little to do with the floor temperature.

If that is true, my slow cooking hypothesis, seems to fall apart. Can anyone shed any light on this?

3.) Why do you even have to worry about reaction times? Can't you just install an outside reset control, and have the radiant heat adjust to the outside air temp. For the most part, the outside air temp won't fluctuate drastically in a short period of time.

Thank you SO much for helping. I can't tell you how much I appreciate it.

All of our HR floor heating design/installations are for energy efficient buildings so we never have any wood between the PEX and the living space. I would suggest consulting with companies that do remodels and such that don’t worry about energy efficiency and customer ROI, etc. Perhaps get their customer references and directly contact their customers to gain a better understanding of how well their hardwood floors have performed over time and how much they spend on heating their buildings.

Right, as I indicated previously, only companies who specialize in integrated HR and passive solar heating systems get involved with reaction times. Passive solar heating can quickly over heat the living space of an energy efficient building if you don’t have the means for quickly moving the excess heat. Yes indeed, even simple outside reset control can typically keep up with typical outside air temp fluctuations, even in a poorly insulated and leaky building.

Energy efficiency is relative. How much you spend to heat your home will depend on more than "special" construction techniques.

The heat load for each room will determine the required output of any radiant panel designed to heat the space. We recently designed and installed a radiant ceiling in a "energy efficient" sun room with an design water temperature of 110°F, this is with an outdoor design temperature of -13°F mind you. We used the ceiling because the floor was wood covered with rugs and furniture.

In another sun room we used a new slab and integrated an old with triple-pane windows from Germany and a design water temperature of 87°F.

Wood flooring has no practical effect on energy efficiency in most construction, new or old. Try not to fall into parochial views of radiant floor heating. In experienced hands radiant floor heating infinitely flexible, responsive and efficient. We are involved with all kinds of radiant heating systems and do consider reaction times, most especially at the two extremes of super-insulated and houses with no insulation.

When designing heating systems for new houses one can design around the floor coverings to keep people and wood floors looking good.