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Figure 5-15: (C) J Wiley, S Bliss Install Wood or Tile Floors over Radiant Heat

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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.



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Detailed Guide to Installing Wood Floors or Tile over Radiant Heating

Figure 5-15: (C) J Wiley, S Bliss

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 [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:

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.

Figure 5-15: (C) J Wiley, S Bliss

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, by Steven Bliss, courtesy of Wiley & Sons.

-- Adapted with permission from Best Practices Guide to Residential Construction.

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:

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.

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