Radiant heat design specifications for wood floors:

This article explains the recommended details for installing wood flooring over a radiant heated floor system and properties of the radiant heat tubing installation under the floor.

Detailed Guide to Installing Wood Floors or Tile over Radiant Heating

What's the best design for installing radiant heat below wood flooring or below tile flooring built on slabs, subfloors, etc., as described in the book, Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) [reference] Chapter 5, INTERIOR FINISH [web page].

This article includes discussion of installing radiant heat below both wood flooring and ceramic tile.

How to Install a Wood Floor Over Radiant Heating Systems

Radiant heating is a challenging application for wood floors due to the high temperatures, excessive drying, and greater temperature cycling to which the wood and finish are subjected.

Careful monitoring and control of the moisture levels of the flooring and structure at the time of installation are critical for success. Also, because a 3/4-inch wood floor over 3/4-inch plywood has an R-value of almost 2, similar to plush carpeting, wood systems generally must run at higher water temperatures than tile or vinyl floors.

Large area rugs make it more difficult for the heating engineer to design a system that will heat the room without overheating the flooring.

Wood flooring can be installed over radiant slabs, or over dry systems where the hydronic tubes are stapled directly to the subflooring (staple-up) or laid on top in grooved panels.

Dry systems are more common in retrofits and generally require water 10°F to 20°F higher than thin slabs, leading to reduced efficiency, and often ruling out low-temperature heat sources like heat pumps or solar. Also, with less thermal mass than slab-based systems, dry systems are more prone to temperature fluctuations.

All radiant floor heating systems are designed to heat floors to about 80°F. Floors heated above 85°F are uncomfortable for occupants and may be damaged from the heat.

With any approach, the radiant slab or subfloor must be dry prior to installation. With slab systems, run the heating system for at least a week, up to three weeks if necessary, to dry the slab to a moisture content of 8 to 12% before installing the subfloor.

The subfloor and hardwood floor should be acclimated to the average annual moisture levels for the region and be within 2 percentage points of each other (see “Acclimatization,” page 167). Flooring that is installed too wet can lead to shrinkage cracks; flooring installed too dry can lead to expansion problems or cupping in humid summer weather.

To steer clear of problems, also follow these recommendations:

• If possible, choose an engineered wood product rather than solid wood. Floating floors are best, since they are designed to accommodate movement.
• If solid wood is used, avoid flooring wider than 3 inches—the narrower the better. If possible, use quartersawn wood, which swells and shrinks 30%–50% less than flat-sawn.
• Choose a prefinished flooring coated on all faces. Prefinished flooring with chamfered edges (microbevels) will help conceal shrinkage cracks.
• Increase nailing of strip flooring to 4 to 5 inches on-center.
• If finishing on site, use a recommended sealer to reduce the chance of edge-bonding when the flooring shrinks, leaving large gaps every several courses.
• Always use a vapor barrier under the flooring, but do not use asphalt-impregnated felt, which will give off fumes.
• Avoid glue-down flooring. If used, make sure both the flooring and adhesive manufacturers approve the application.

Specifications for Installing Radiant Heated Wood Floors over Slab-on Grade

Over traditional concrete radiant slabs at least 4 inches deep, use either a floating hardwood floor or install subflooring and nail on 3/4-inch strip flooring.

The subflooring can consist of two layers of 1/2-inch plywood floating over the slab (see “Installing Over Concrete,” page 168), or a single-layer 3/4-inch subfloor nailed to the slab with powder-actuated fasteners. Because it is thicker, the floating subfloor (Figure 5-15) will take slightly longer to heat up, but it does not risk puncturing the hydronic tubing.

Specifications for Installing Radiant Heated Wood Floors over Thin Slabs

In wood-frame construction, use a minimum 1 1/2-inch-thick slab of Gyp-Crete® or lightweight concrete, which provides thermal mass for the radiant floor. Above the slab use a floating hardwood floor, or nail strip flooring to 3/4-inch sheathing installed over the lightweight concrete.

Fasten the sheathing to 2x4 sleepers placed 12 inches on-center, with the lightweight concrete and tubing in between (see Figure 5-15 shown above). A two-layer floating subfloor, as described above, is also an option for larger rooms where the subfloor will be heavy enough to stay solidly in place without nails.

Specifications for Installing Radiant Heated Wood Floors over Staple-up and Panel Radiant Floor Heat Systems

There are a variety of dry radiant systems that install just under or over the subflooring, making them ideal for retrofits. The tubing is either stapled to the underside of the subflooring, laid over the joists (with spacers to fur up the sheathing), or placed over the subflooring in grooved plywood panels.

Engineered wood floating floors are best with these systems, but nail-on hardwood flooring can work if installed with care.

This article series discusses and provides a best construction practices guide to the selection and installation of building interior surface materials, carpeting, doors, drywall, trim, flooring, lighting, plaster, materials, finishes, and sound control materials.

This article includes excerpts or adaptations from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , by Steven Bliss, courtesy of Wiley & Sons.

- - Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) .

How to Install a Tile Floor Over Radiant Heating Systems

Ceramic tile should not suffer the same problems of curling, warping, gaps that may occur with wood flooring that we discussed earlier. But there are still some concerns to attend, including at least:

• Effect of radiant floor heat on tile mastic or adhesive
• Effects of radiant floor heat on some tile grout or on caulk used instead of grout at expansion joints and intersections ( do this or you'll be sorry later)
• Protection of the slab against cracking that will telegraph up through the tile
• Proper insulation below the slab and proper placement of tubing high in or over the slab.
  
  See RADIANT HEAT MISTAKES

 

Question: Install Tile Floor over Slab Using 3mm Polystyrene Sheets Under a Radiant Floor over Concrete

At last I found a website I can make sense of.

I need to replace my heating boiler and am looking at underfloor heating.

Your website gives me the information that I am as well to use particle board water pipe retainer than the expensive pre-constructed boards, I hope to find a 3mm polystyrene sheet (coffee cup type material) which can be placed between the existing concrete floor and the particle board before tiling on top of that.

Thank You for providing me with such useful information. - Anonymous by private email 2017/01/14

Reply:

Thanks for the very nice note, Robert. We work hard to provide researched, unbiased and useful information so I'm really happy when a reader finds it so.

Now about that floor plan, let's research as much as possible before you do anything that's expensive, since as I document at RADIANT HEAT MISTAKES https://InspectAPedia.com/heat/Radiant-Slab-Heat-Mistakes.php
it can be really ugly trying to un-do a radiant heat floor that was improperly installed.

at INSULATION R-VALUES & PROPERTIES we note that polystyrene board has an R-value of about 3.5 to 5 per inch. 3 mm is about 0.11 inches, giving an R-value of 0.3 to 0.5 - really not much in insulation. The material might also serve as a vapor barrier, since it's waterproof, and there's no reason not to also opt for a teensy bit of insulation too.

The thin material you describe is indeed readily available if you shop for it in packaging and wrapping materials. For example, companies selling packing and mailing supplies like Uline sell "UPSABLE Foam Rolls" that is flat, in thicknesses from 1/32" to 1/4" and in 12" and 24" widths. A 1/8" thick UPSABLE polystyrene foam sheet is 3.2 mm - about what you described.

Wider sheets of thin polystyrene sheeting is available from ULINE as well as other manufacturers. Wider rolls of this same material are sold in 48" rolls (about $165. U.S.D.) but that material isn't dead flat - which may not matter for your application.

I'm unclear on just how you plan to install, secure in place, and cover your radiant floor tubing.

I GUESS you are saying you're installing radiant tubing over a slab by using these layers:

1. Ceramic tile
2. Tile mastic - check with the manufacturer about the effects of radiant floor heat when using the mastic you choose
3. 3/4" OSB or particleboard subflooring or 2 layers of 1/2" as per Steve's article
4. Radiant heat tubing - secured in place somehow
5. Thin polystyrene sheeting vapor barrier with modest insulating properties
6. Concrete with no insulation below the slab
7. Dirt or gravel - mother earth

Particularly, how do you plan to hold the tubing in place between the subflooring and the insulated slab?

at WOOD FLOOR RADIANT HEAT (above on this page) Steve Bliss describes such an installation with no insulation shown, though in new construction I would insist on insulating below the slab as well. Key is keeping the tubing as high as possible in the design. Steve used a full inch of plywood over the tubing set in the concrete. Your tubing, atop the concrete, will lose less heat into the concrete than Steve's design.

But I'm not sure about what the radiant heat tubing manufacturer will say about how the plywood bears directly on the tubing - is compression a concern?

In this design I'd give a call to the manufacturer of the tubing you plan to use. Ask for technical help from one of their engineers, and ask about your design and get her recommendations. Be sure to review the proper operating temperature range for the tubing too.

Please do keep me posted - what you learn will definitely help other readers.

• Uline UPSABLE Industrial Rolls, ULINE, Uline 12575 Uline Drive Pleasant Prairie, WI 53158 USA, Tel: 1-800-295-5510, Website: https://www.uline.com/BL_857/Uline-UPSable-Industrial-Rolls

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-03-13 by Riki

thanks so much for your help

On 2021-03-12 - by (mod) -

@Anonymous, imo,

Yes,
Generally no
But check for product reviews

On 2021-03-12 by Anonymous

thanks for your reply:) Will a thinner engineered wood reduce output to a lower degree? Would the grout gaps create the biggest problem in that regard due to insulation?
If a manufacturer deems the wood safe over radiant heat, could it still pose a problem over the tile?

On 2021-03-12 - by (mod) -

@Riki, I would have at least two concerns with what you proposed. One is that you will reduce the heat output from the floor and two you may find that the heat from the radiant system actually damages the flooring causing gaps or warping or it may colors off-gassing. So you should not install engineered wood flooring or other flooring products over a tile floor that contains radiant heat without first discussing your plan with the manufacturer of the new flooring system

On 2021-03-12 by Riki

Can you install engineered wood flooring over tile with radiant heat underneath? If so, do you need to add any barrier between the layers? The radiant heat system is estimated to be about 20 years old (from previous owner)
Thanks

On 2020-12-31 - by (mod) -

SC

It may be quite reasonable to add a layer of thin-set tile on top of existing tile radiant heat floor.

However I would want to diagnose the cause of the tile cracking first.

It also may be helpful to include crack isolation membrane material under the the new thindet tile floor.

On 2020-12-31 by SC

Our relative has a house with radiant floor. Do not the type of radiant floor. There are uneven/cracked tiles. He thinks he can add another floor over it. Is this possible or advisable?

On 2020-08-24 by Lynne

What kind of grout should be used with 18' porcelain tiles over hydronic radient heat floor? The former grout has dried out. Should silicone be added?

On 2018-01-01 - by (mod) -

Stu

If you build a wood framed, insulated floor atop an existing radiant heated slab, you will not get heat up through the new floor unless you also abandon the existing radiant heat system and install a new radiant heat tubing system under your raised floor.

On 2018-01-01 by Stu Lamb

We currently have radiant heat in our home and wish to raise the floor in the sunken (step down 18") living room. The room is 17 x 22' with 12' ceiling. The living room floor is concrete and we have had no problems with the heating the room in the cooler southwestern months. I could use some advice on the joist and type wood to use, if "any" insulation is necessary, subfloor or decking and options for floor covering, (wood, ceramic tile or carpet. My concern is making sure the radiant heating system is still able to heat the raised living floor even if the system is not as efficient.

...

Continue reading at GAPS in RADIANT HEATED WOOD FLOORS - caused by running the radiant heat at too-high a temperature, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• FLOOR, WOOD RADIANT HEAT - guide to installing wood floors over radiant heat systems - this article is provided just below
• GAPS in RADIANT HEATED WOOD FLOORS - caused by running the radiant heat at too-high a temperature
• RADIANT HEAT - usage guide, strategies for using radiant heat
• RADIANT HEAT TEMPERATURES - at the boiler, in tubing, under floors, floor surface, thermostat & control settings
• RADIANT SLAB FLOORING CHOICES - wood vs ceramic tile over a radiant heated floor
• RADIANT SLAB TUBING & FLUID CHOICES - tubing and fluid choices for radiant heating
• RADIANT HEAT FLOOR MISTAKES - proper and improper radiant tubing depth in slabs & slab insulation needs - what can go very wrong when the building contractor substitutes brawn for brains
• SLAB INSULATION, RADIANT / PASSIVE SOLAR - slab insulation & vapor barrier placement in heated floor slabs
• WOOD FLOOR DAMAGE

Suggested citation for this web page

WOOD FLOOR RADIANT HEAT at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to RADIANT HEAT

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