Use the approved carpet-and-cushion assembly for US Environ Flex installations.
For the US carpet installation covered by the Environ Flex manual, place the heating roll over the carpet cushion and directly under the carpet. Prepare a structurally sound, clean, dry slab and remove anything that could damage the cushion or heater.
The manual recommends an insulating underlayment with a high R-value over concrete and specifies 6–8 lb/ft³ density for stretch-in carpet padding in its Compatible Underlayments section. Obtain the actual pad’s density and thermal data; thickness alone does not establish either.
The layer above the heater has different limits: the carpet must be ¼–1 inch thick and have an R-value no greater than 1. Rubber- or foam-backed carpets are excluded. Confirm the exact carpet’s compatibility with its manufacturer.
Use the installation plan to preserve the unheated perimeter for the carpet stretcher and the required cable/sensor routing. Do not install permanent fixtures over the heating rolls. The manual labels installation under carpet USA only; this is not an approval for the same assembly in Canada.
Source: Environ Flex installation manual (Compatible Underlayments and Installation Under Carpet).