For radiant floor slabs, follow a clear thumb rule: put continuous insulation under the slab to force heat upward and prevent heat loss into the ground.

For typical residential concrete slabs used with hydronic radiant systems, aim for a minimum of R ten to R fifteen under the slab, which in practice is about two to three inches of extruded polystyrene rigid board. Place the insulation on a compacted granular base and tape seams or use compatible adhesive so you create a continuous thermal layer that resists moisture and compressive loads. Include an insulated edge at the slab perimeter extending from the top of the slab down to the footing top to create a thermal break and reduce edge heat loss that causes cold floors and higher energy use. Use a thin damping or separation membrane above the insulation when circuits are embedded to protect tubing from point loads and to allow for even heat transfer into the concrete. When zoning the system, keep tube spacing closer in perimeter bands and near heat loss areas and wider in interior zones to deliver uniform surface temperatures without excessive water temperatures. Always verify local code and frost depth requirements because in colder climates you may need to increase insulation thickness or extend insulation vertically alongside foundations.

Common mistakes include using insulation that is not rated for under-slab compressive loads which can compress and lose R value, so choose extruded polystyrene with adequate compressive strength and check manufacturer long term creep values. Another error is leaving gaps or poorly sealed joints which create thermal bypass paths, so overlap or tape joints and seal slab edges carefully to maintain continuity. A third issue is omitting edge insulation or thermal breaks which causes heat loss at the slab perimeter, so install continuous perimeter insulation down to the footing or to frost depth as appropriate to interrupt conductive pathways.

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