Have we ever mentioned how thermal mass keeps our homes warm in winter and cool in winter?
No doubt if you have visited us you’ve heard the stats and felt the benefit, but a day like today makes the benefits all the more evident.
The thermal comfort of our homes is met through the application of three key design principles:
- Thermal mass to store heat in the summer months to keep the home cool in summer and warm in winter
- Passive solar gain to reduce the need for space heating and artificial lighting
- Super-insulation and buffer zones to provide a reduced temperature gradient between the inside and outside of homes.
So on a day like today we shut the triple-glazing between the living space and buffer zones, along with curtains and shutters if we are out and about, to keep out warm air and solar gain; and let the thermal mass soak up the heat when we are around the house. We have built passive ventilation into our buffer zones – otherwise known as skylights in the conservatory and porch area – which are vital to keeping the temperatures in those spaces comfortable.
Given the warnings yesterday from the Committee on Climate Change, we think cooling, or overheating, is an issue that needs addressing as part of the government’s home energy strategy. It should not be an add-on as there can be a conflict between approaches that keep heat in during winter and keep it out over summer.
The positioning of insulation in a construction element was completely disregarded by SAP up to and including SAP2005, and continues to be ignored by RdSAP. But it is essential if you want to get the heat storage benefits of thermal mass throughout the year for cooling, heat storage & release. Our walls, floor and roof could have the insulation placed on the inside, which would give exactly the same U-Values and hence RdSAP result, but completely different and appalling thermal performance of the house as a whole in warmer weather. Instead of being absorbed into the thermal mass, the passive solar gain would continue to raise the temperature until vented in some way.
Even though this is beginning to be recorded by SAP, RdSAP does still not differentiate between internal and external solid wall insulation. Neither assessment reflects the benefits in their overall assessment of thermal comfort. If the Government wants to prepare homes for the 21st century, and beyond, these tools will need to both recognise and reward the way external insulation can“lock in” the mass of the walls to deliver summer cooling and winter heating.
Yesterday the Government launched its Energy Efficiency Strategy, and we were particularly interested to see if any mention was given to the role of SAP, the Government’s energy performance assessment tool.
Hockerton Housing Project takes pride in its low-tech, low-cost approach, and the homes consume 15-25% of the energy used by homes built today. But the Government’s energy efficiency measurement assessment tools (SAP and RdSAP) cannot compute the benefit of homes like ours:
- SAP cannot cater for our use of passive solar gain as our main heating system
- Use of thermal mass to reduce heat demand through the year is not recognised (the thermal mass in our homes essentially stores the summer heat and keeps our homes warm in winter)
- SAP assumes that an element of mass thicker than 100mm has no additional thermal capacity is flawed (SAP2009 Table 1e: Heat capacities for some common constructions), contrary to evidence at HHP. As long as the mass is well insulated (externally) the full thickness of the mass will be effective as a heat sink.
- RdSAP does not differentiate between internal and external solid wall insulation, so the benefits of external insulation to “lock in” the mass of the walls, which can then aid summer cooling and winter heating, are not recognised for existing dwellings.
- SAP assumes that thermally separate conservatories are not present, ignoring two benefits:
- The sunspace provides sheltering of the dwelling from the external environment, therefore reducing heat losses.
- The sunspace can be used to harvest passive solar energy which can then be brought into the main dwelling to top-up the heat stored in the thermal mass as required.
All this matters because the Government tells us in the Strategy that it intends to make more policies conditional on energy efficiency. Access to feed-in tariffs and the renewable heat incentive are already affected, and RdSAP or EPC ratings could also be used to introduce differential council tax or stamp duty. All this will mean that energy efficiency improvements will be made to meet whatever measure of energy efficiency is applied. Whilst a policy to drive up the value of energy efficiency in the property market would be very welcome, as this is potentially the simplest way to drive investment in existing homes, this must not be so broad-brush as to drive out innovative approaches and a process for ‘exceptions-handling’ must be incorporated into future policies.
Seeing is believing
On the upside, whatever documents come out of Westminster, here at Hockerton we’re enjoying ‘zero’ energy bills as our investment in additional solar PV starts to pay off and the summer heat stored in our thermal mass continues to keep our homes warm.
If you are interested in homes that are comfortable yet consume only 15-25% of the energy used by homes built today, this time of year is the best time to visit to truly feel the difference. There are some spaces left on the tour this coming Saturday 17 November so book your place on a tour of Hockerton Housing Project here.
Its an Aero chocolate bar in reverse of course!
(not that we’re endorsing a Nestle product)