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Rules of thumb for surface thermal insulation

by Darren
Rules of thumb for surface thermal ISOBOARD

Mark Russell from IsoBoard shares his rules of thumb for insulating elevated surfaces, floors and perimeters.

Where slabs are elevated and the occupied space is above an open void, such as accommodation or office space above parking, or rooms created above an awning, it is essential to insulate within or below the floor slab, whether there is in-slab heating or not.

Insulating floor slabs is a common approach when performing a rational design to satisfy the requirements of 10400XA, as installing some floor insulation, even if only around the perimeter or on balconies, helps greatly to regulate the temperature and save on energy usage.

When planning a suspended floor, particularly a wooden one, insulating between the floor joists is also a good idea to prevent heat leakage.

Perimeter insulation
Insulating the building perimeter usually means insulation fixed against the outside of foundations, generally to a depth of 600mm below the surface, though it can go deeper if moisture exclusion from foundations might be an issue. Insulating the perimeter edge of large buildings requires having insulation laid horizontally below slab for a distance of 2m to 3m in from the outer edge. This is the area that is likely to be affected by daily temperature fluctuations.

Standards
There is no mandatory requirement for in-slab thermal insulation in terms of the National Building Regulations SANS10400XA, unless the floor will be heated, in which case the slab must be insulated from underneath with a thermal resistance (R-value) of minimum 1,0m2K/W.

The voluntary SANS 204 standard, guiding energy efficiency in buildings, requires that floors of buildings less than 500m2 are insulated under slab or along their perimeters, particularly in cooler climatic areas, while larger building floor areas need only to be insulated along their perimeters. In both cases, a minimum R-value of 1,0m2K/W is required, with an insulating material that resists moisture transfer and absorption. 30mm IsoBoard extruded polystyrene is ideal for under- and in-slab insulation, as well as all forms of perimeter insulation.

In the Cape and central interiors of South Africa, insulated floor slabs make a difference to the internal temperatures of homes and small office buildings, being frequently warmer by between 4°C and 6°C, before any warming intervention.

Buildings likely to require more thermal resistance in their floor insulation are usually specialised and the subject of heat flow design, such as cold rooms and ice skating rinks.

Incorporating heating systems
There are two philosophies when installing insulation below a floor slab for a heating system:
•    If you are heating the screed electrically, insulate above the slab, under the screed, to reduce the volume of slab to be heated and to speed up the availability of heat to the room above.
•    If you are using solar heating systems, insulate under the slab, so that when the slab is warmed during the day, heat energy is stored by the mass and released into the rooms above overnight.

Perhaps these are rules of toes, rather than rules of thumbs. Stay comfortable!

Isofoam SA (Pty) Ltd
Tel: 021 983 1140
Website: www.isoboard.com

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