Retrofitting thermal mass

Off form finish on a tamped sand wall.
Retrofitting thermal mass to timber-framed buildings is possible and can make your home that much more comfortable and thermally efficient. Dick Clarke looks at how it’s done.

The benefits of thermal mass in passive design are well known; warmth (and coolth) is stored in the core of a house. However many houses lack any thermal mass where it is most beneficial, such as brick veneer where the thermal mass is on the outside and is subject to the vagaries of the weather. But there are ways to retrofit thermal mass into existing houses, even brand new ones.

Thermal mass is useful in most climates, especially those with cool winters and some sunny days, or those with hot days and some cooler hours overnight. Here, we’ll focus on two of the most difficult retrofit situations – walls and floors. Other (easier) options should be more

Thermal mass comes in a number of physical forms and many conventional building materials – such as rammed earth, bricks and blocks of various kinds, concrete and Hebel AAC – offer useful amounts. Some of these materials can be incorporated into existing buildings quite easily, such as when building new walls, but there are also ways to retrofit thermal mass into existing walls. Another option is phase-change materials, usually waxes or salts that absorb large amounts of energy when changing from solid to liquid, and release that energy again when reverting to solid form. These can be incorporated into the structure, or can stand alone in ‘heat banks’. Water, too, can provide a neat substitute for high-mass materials, most often manifest in hydronic heating.

Internal walls are a very good place to put thermal mass, assuming the building has reasonable solar access, good glazing and is well insulated. Walls are extremely effective in regulating thermal comfort as the vertical thermal mass is exposed near to people inside a room. Retrofitting thermal mass to walls works well both in passive heating and cooling
modes. Many project home builders in southern Australia offer floor plans which, with some
tweaking, can provide good orientation of living areas to the north, and have internal walls that will benefit from added thermal mass. A few more progressive project home builders provide the option of making these walls brick instead of empty timber frames. Case one looks at options for retrofitting thermal mass into a timberframed wall.

Floors are a more conventional location for thermal mass and concrete slab-on-ground
construction is de rigueur in the project home sector. This is good, provided solar access is available to help heat in winter, and that overnight purging of warm air occurs in summer to cool it; otherwise thermal mass will not provide maximum benefit. If it’s not already part of the building structure, there are opportunities for renovators to add thermal mass floors when re-orienting their homes to take advantage of solar access. Case two explains one method of substituting a suspended concrete floor for a raised timber floor when chasing direct solar access.


This relatively non-disruptive, non-structural procedure to turn a timber-framed wall into a high mass wall can be achieved with the relevant building permit and once engineering checks have been done. In essence, stabilised sand (selected for colour and texture) is used to fill the space between wall studs. One side of the selected wall’s plasterboard is removed and the other left in place but screwed more securely to the studs at 300 millimetre centres. Bracing is retained and all services are enclosed in backing boxes, clip conduits or insulation as appropriate. Then a piece of formwork (the length of the wall by about 300 millimetres in height) is temporarily screwed to the studs on the open side and the space between the studs is filled with selected sand and five per cent cement in a stiff and slightly moist mix – not sloppy like mortar. This is tamped (packed) into place lightly by hand, using a short piece of ‘4×2’. Ensure the plasterboard is not pushed off the other side – the tamping is just to remove large voids so high pressure is not needed, the cement holds it all together.

The process is repeated up the wall, with the formwork moved progressively up until just shy of the ceiling. The finish can be exposed ‘off form’ as a visual feature and the surface sealed with a coat of 10:1 Bondcrete (mason’s lime), and the stud faces covered with a suitable dressed timber batten. Or the whole wall can be covered in a fine expanded mesh and rendered. The original plasterboard can even be replaced, although this will reduce the immediacy of the thermal mass contact slightly.

If solar access is not available to the space adjacent to this wall, it can be introduced by solar hydronic heating pipes installed into the wall before the rammed sand is installed. [Note: Solar hydronic heating can use direct heat collected in evacuated tubes like a normal solar hot water system, or it can use a heat pump powered by renewable electricity, either by onsite photovoltaics, or off-site from an accredited supplier.] The heated pipes, in the centre of the
thermal mass, provide very effective distribution of heat, and in the hot months the vertical mass still absorbs heat very nicely.

In most timber-framed buildings, especially those with truss roofs, it is also possible to prop and brace the surrounding structure and remove the existing timber-framed wall altogether, replacing it with selected brick or blockwork. Retaining existing services is a bit trickier if this method is pursued, and adding hydronic heating requires additional thick render on one side. The additional loads are usually higher than the ‘tamped sand’ method too. In slab-on-ground buildings the existing slab’s structural capacity must be assessed by a qualified engineer and there may be limits to how much thermal mass can be added. In houses with raised timber floors, it may be necessary to add piers (or stumps) between the existing piers, to halve the additional load.


Replacing a timber structure with an insulated concrete slab can add very useful thermal mass if it is installed in the right location, but this is a more disruptive measure than infilling walls. It requires a building permit and is best done when there is more than marginal benefit and when renovating the house. Needless to say, all spaces being changed become uninhabitable during the works.

The suspended timber floor structure is simply replaced by a suspended concrete slab with insulation on the underside. The technicalities are reasonably straightforward for a competent builder.

The engineering checks having been done and any additional piers identified, and temporary bracing installed if required, it is time to surgically remove the old structure. Salvage the flooring if you can – it is worth something to someone. If the building has cavity brick walls, the floor structure will simply come away from the walls. If it is a timber-framed building, the wall frames sit on the bearers and joists and surgical incisions are required: cut the bearers and joists around the perimeter in line with wall frames above. Any new piers or foundational work is installed now.

The slab can be designed to distribute its load uniformly in all directions or, using permanent structural formwork, can distribute the loads in one direction only. This is the simplest method and is described here, but the former method is very similar. Using the existing internal piers grid is usually best, with new steel bearer(s) installed. The formwork is laid over
this, fitted into the gaps left by the old bearers or the old joists. Using the line of the old bearers will usually allow a drop in finished floor level, which has the added benefit of increasing ceiling height. However, it’s important to check that the new step in floor level is compliant with the Building Code of Australia.

The original foundation perimeter wall or piers with new steel bearers added become the supporting structure for the formwork, and sufficient bearing must be provided – the structural engineer will make this clear. This also allows existing termite physical barriers to remain in place, or new ones to be added in the same plane. Additional temporary propping
may be required until the concrete has cured. Perimeter slab edge insulation should be installed wherever the slab might touch the cladding.

Once the top reinforcement is installed, and the concrete placed by pump, the slab can be finished. All the usual options are available – ie polished, burnished, or finished with marmoleum or tiles. It is important not to cover the slab with a soft, insulative material like carpet as it defeats the purpose of installing thermal mass where it is needed. You can also install solar hydronic heating to the slab if needed, but this must be designed in. Finally, and very importantly, the insulation can be installed underneath the slab, and proprietary fixing systems are available. Subfloor height is a consideration, so don’t leave that as an afterthought.

Replacing a timber structure with an insulated concrete slab may require the attention of a builder, but the actual materials and procedures are fairly mainstream – albeit applied in a slightly unusual way. Therefore, each step in the process (demolition, formwork and reinforcement, concrete placement, and finishing trades) can be priced easily, and the
overall cost should contain no surprises.

Dick Clarke is the principal of Sydney-based Envirotecture. It provides design and consulting
services for people and organisations who want buildings that work well, feel good, are culturally appropriate – and reduce their ecological cost toward zero. Dick is a contributing author to Your Home building manual.

[Ed note: Any work affecting the structure of a building (such as altering or removing/replacing a wall frame, or providing support for additional floor loads) is regarded to be building work and requires that appropriate permits be issued. This carries with it obligations relating to registration, home warranty insurance and domestic building contracts.]

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