Saturday 8 September 2012

High Thermal Mass

At the expense of boring any readers who have already had enough evangelising from me about the necessity of thermal mass (see earlier post "Passive Solar Design and Thermal Mass") I am yet again revisiting the topic since I cannot emphasise too strongly the importance of this design concept.
High Thermal Mass!

High Thermal Mass (HTM) mortared concrete block or poured in place concrete wall construction is recognised as a much better sustainable passive solar answer than super insulated air light construction for any climate. Conventional stick framing, log homes, straw bale, autoclaved aerated concrete and especially insulated concrete forms (ICF's) are simply not feasible passive solar, sustainable design materials because they don't effectively store and release energy. For true sustainable passive solar construction the building material of choice is HTM. Although HTM is a more commercial approach than residential in its construction details I decided, at an early stage in the design process, that it was the way I wanted to go for my earth sheltered solar passive house.

Passive solar gain "re-charges" the home's huge thermal mass automatically - even on cloudy winter days there is still some solar gain removing the need for mechanical devices  to operate climate control systems which is a priority in my design.
Part of our high thermal mass

Thermal mass may be regarded as a giant heat/coolth storage battery - the larger it is the easier it is to keep the temperature at a stable/constant level. Passive solar heat gain or cooling breezes must be quickly absorbed and released, as needed, to be effective.

Conventional passive solar design relies upon airtight high insulation construction to produce "energy efficient" homes and businesses. When you ventilate in a high insulation home you lose all your comfort unless some of the recently developed heat exchanging stack vents and associated ducting are employed. More simply High Thermal Mass building materials allow the heating and cooling effect to be stored within the homes walls and floor. This allows for the HTM home to be vented without "losing" all heating and air-conditioned comfort. Fresh air is vital to the health of any building. That is why with HTM one can never have too much solar gain since can always vent more in winter and shade more in summer.

House to be "buried" here to increase thermal mass

HTM Example - The Hockerton Project

This excellent example of HTM construction has 200mm concrete block intermal walls on a 300mm concrete slab, a concrete beam and block roof and 500mm thick external walls of two skins of concrete blockwork used to contain poured concrete mass infil. A polythene waterproof geo-membrane waterproofs the building from the surrounding soil.

Walls, slab and roof are super-insulated with 300mm expanded polystyrene (CFC-free!) with the mass on the inside of the insulation. The roof is covered with 400mm of topsoil and the north side and ends of the terrace have soil heaped up and over them.

The building envelope is clay brick for the exposed exterior walls and all interior walls are wet plastered. It is interesting to note that there are no holes through the slab or soil pipes or services so the insulation and waterproof membranes are not perforated.

Main door and windows opening into conservatory are triple glazed with low-e glass and argon filling whilst the conservatory has double low-e glazing.

For more construction details see Hockerton Project website.