ThermalSheet Insulating Underlayment Frequently Asked Questions

When installing electric floor heating over a concrete slab, an insulating underlayment like ThermalSheet is highly recommended to ensure efficient operation, faster heating times and warmer floor temperatures.

When WarmlyYours electric floor heating systems are being installed on a concrete slab, we strongly recommend adding a layer of insulation over the slab prior to installing the system. While our electric floor heating provides up to 25% more heating power per square foot than many of our competitors, the slab will always act as a "heat sink." Some of the heat that would otherwise be transferred to the flooring surface will remain in the slab, causing the floor’s surface temperature to be considerably lower. This is true with any electric radiant floor heating system.

When installed on top of a concrete slab without insulation, it is generally accepted that electric floor heating will take the chill away from the floor and provide a small amount of warmth. Adding insulation on top of the slab and beneath any electric radiant floor heating system will allow a greater percentage of the heat generated to transfer to the flooring surface. This results in greater efficiency and therefore faster warm up times, higher expected surface temperatures and lower energy usage. The floor will have the capacity to warm to a comfortable temperature, and in some cases can be employed as the primary heat source for that room.

The recommended types of insulation are natural cork, insulated tile backer boards and insulated underlayment, such as ThermalSheet™ synthetic cork.

Insulating underlayments and uncoupling membranes share some key characteristics like providing crack isolation benefits, but they are separated by their primary functions.

Insulating underlayments, like WarmlyYours ThermalSheet, provide a thermal break between a subfloor and the heating elements, which can help prevent the concrete from acting as a “heat sink” and increase energy efficiency. ThermalSheet has a 1.5 R-value, and it offers crack isolation of up to ¼”. This can be particularly beneficial if you are trying to heat a room above a cement slab or above an unheated garage. 

Uncoupling membranes, like the Prodeso membrane, are great for protecting relatively fragile floor coverings like tile from foundational movement. They also allow for faster installation times than many other types of floor heating.

No, for the installation of an Environ heating system with a floating floor like laminate, it is not necessary to adhere the ThermalSheet underlayment to the subfloor. The underlayment should instead be allowed to float freely on the subfloor. Make sure to stagger adjacent sheets of underlayment and tape them together to prevent any overlaps, ensuring a flat and even layer beneath your flooring. This method helps accommodate the natural expansion and contraction of the floating floor.

Any type of underlayment/padding may be used as long as it has a density of 6-8 pounds per cubic foot. Certain underlayment, such as ¼” (6mm) thick cork, or 6mm ThermalSheet are recommended for Environ™ Flex Rolls. It is also acceptable to place a vapor barrier, such as a plastic sheet, under the underlayment. Underlayment papers are not compatible with Environ™ Flex Rolls. Underlayment is necessary to prevent Environ™ Flex Rolls from damage from the weight of the floor, furniture, and people. Unavoidable wooden floor movements will not harm Environ™ Flex Rolls. Wooden floors expand and contract with temperature and moisture change. The wooden floor “moves” on top of Environ™ Flex Rolls. This movement will not damage Environ™ Flex Rolls when the installation is done correctly and the underside of the final floor covering is installed evenly. Some underlayment materials also have a noise reducing and insulating effect. When installing over concrete, a ¼” underlayment is mandatory. Any underlayment available in ¼” size is acceptable in most cases. ThermalSheet, cork, or ¼” to 3/8” of expanded polystyrene (EPS) with a density of 2 pounds per cubic foot or more, are recommended for installation under Environ™ Flex Rolls. When installing over a concrete slab, choose the underlayment with the highest R-Value.

ThermalSheet Insulating Underlayment is a synthetic cork, a lightweight thermal barrier made of hemi-crystalline thermoplastic.

WarmlyYours recommends ThermalSheet underlayment when installing a floor heating system over a concrete slab to prevent heat loss and protect against moisture. It will not rot, support mold and mildew, or absorb moisture.

- ThermalSheet synthetic cork underlayment (6mm) has a 1.5 R-value, crack isolation up to 1/4“, reduces sound by up to 22 dB, and is Robinson tested (TCNA) and rated for residential loads.

- Cork (6mm) has a .72 R-value, crack isolation up to 1/32“, reduces sound by up to 23 dB, and is Robinson tested (TCNA) and rated for residential loads.

No. Put a skim coat of SLC (Self-Leveling Cement), or even plywood over it first. Then, the vinyl flooring product can be installed.

Yes. Without insulation, a concrete slab draws heat down from the floor heating, so the floor warms slowly and uses more energy. Over an existing slab, lay ThermalSheet, WarmlyYours’ insulating underlayment, under the heating system to send the heat up into the floor. ThermalSheet replaces CeraZorb, which is no longer sold.

Yes, ThermalSheet should be bonded to the subfloor with any high quality modified thin-set, or with a bond coat adhesive. Further installation information can be found in the installation manual. 

It depends on your flooring type and heating system. For electric radiant floor heating, underlayment is often beneficial but not always required.When underlayment is recommended:

• Floating floors (laminate, engineered hardwood, LVP): Use ThermalSheet, WarmlyYours' insulating underlayment, to direct heat upward and improve efficiency.
• Cold subfloors (concrete, basements): Insulating underlayment prevents heat loss downward.

When underlayment is not needed:

• Tile and stone installations: Heating cables/mats are embedded directly in thinset or self-leveling compound—no underlayment required.
• Glue-down LVP over concrete: System can be installed in self-leveling compound without additional underlayment.

Important note: WarmlyYours currently offers ThermalSheet as our recommended underlayment solution. While CeraZorb and cork underlayment were previously available and may still be found in older installations, ThermalSheet is now our preferred product for optimal performance and compatibility.Our free SmartPlan service includes underlayment recommendations tailored to your specific flooring and subfloor. With over 600,000 custom floor plans created, we ensure you get the right system configuration.