Measuring is believing

Author Bob Prewett

Strategy

In fact, alongside a desire for some modest extensions, our client wanted to reduce the energy consumption in line with the Kyoto protocol ambition of 80%. While this seemed possible based on some early calculations, there was very little precedent and we were told by many that it wouldn’t work, wouldn’t get planning, wasn’t buildable and was a waste of time.

Nevertheless we came up the following strategy, as summarised in the drawing opposite:

1. Compact extensions presenting as little external envelope as possible.
2. A thick layer of internal wall insulation on the front elevation, built up as series a of layers, with a ventilation gap between old and new to manage moisture risk.
3. Multipane sashes on the front elevation, replicated using the best available thin double glazing with quadruple draft proofing.
4. The rear façade rebuilt as a highly-insulated cavity wall, reusing the original.
5. New Triple glazing to all openings at the rear.
6. The roof rebuilt as a warm roof, incorporating a new extension.
7. MVHR (mechanical ventilation with heat recovery) and a boiler located in the roof extension.
8. A small amount of heating from underfloor heating on the lower ground floor and one bathroom radiator on first floor.
9. Shading to loft windows by blinds and to the large lower ground opening by a pergola.

Following an excruciatingly slow planning process that lasted most of 2008, the project was constructed during 2009, then occupied from early 2010. It has been monitored ever since.

There are 5 key lessons from our Post Occupancy Evaluation (POE) of this project:

1. Fabric retrofit without a performance gap is possible.

The left bar in the chart represents our estimate for energy use before the works. It was modelled using SAP software and checked against real publicly available energy data for similar neighbouring houses. Typically the heating load dominated.

The left bar in the chart above represents our estimate for the house’s energy use as we found it. It was modelled using SAP software and checked against real publicly available energy data for similar neighbouring houses. Typically for such a house, the main component of energy use was the heating load. The middle bar shows the modelled predicted energy use, where the heating component has been slashed, almost to the point where it equals the heat wasted by flue losses from the boiler in the ‘before’ case. The third column summarises actual energy use measured at the meter following the work.

The similarity to the middle column prediction is uncanny. In fact it is more common to find a performance gap between modelled and measured results, with measured invariably being higher than modelled, sometimes by a factor 2 or 3. We attribute this lack of a gap to the care taken in the design and construction of the continuous thermal envelope and the attention paid to air tightness.

2. The fabric efficiency has not deteriorated.

The second chart plots the energy consumption each year following completion. While there are differences due to occupancy and weather variations, the energy consumption and generation are remarkably constant from year to year with no overall trend upwards or downwards. This degree of stability strongly indicates that the energy efficiency measures have all held up and not degraded. This evidence also indicates no ‘rebound effect’, where energy efficiency measures might be used to maintain an improved level of comfort at the expense of intended energy reduction.

3. High comfort.

During the winter of 2016/17 Indoor and outdoor temperatures were recorded, as shown on the graph above. The internal temperature, set at near 20°C by the owner, remained very stable except during vacant periods when the heating was off. Despite there being only two heat emitters, the rapid recovery upon reoccupation is noteworthy and encouraging.

Relative humidity was also recorded as a ‘proxy’ for air quality. Readings were mostly within the ‘ideal’ 40-60% bandwidth. This range is preferred because it creates unfavourable conditions for dust mites, fungal spores, and proliferation of bacteria.

CO₂ concentrations were tracked for a shorter period. The count generally varied between 400-600 ppm (parts per million) which is especially low. There is a single spike that reaches the limit of 1500ppm, beyond which air quality is by convention normally deemed poor, but this spike occurred when the ventilation system was switched off for air tightness testing, so is not representative of day to day conditions.

4. Retrofit can be done safely and responsibly.

Over 13 years, the building’s fabric and environmental conditions were periodically examined, with a final review study in March 2023. Air tightness, thermography, and mould spore detection revealed minimal defects, except for a leaking cistern valve, whicvh was rectified, and some local rot in one window frame. There were no specific issues with the energy efficiency work, underlining the house’s positive performance with minimal maintenance over 13 years.

5. Lifetime carbon very impressive too.

Shortly after completion an embodied carbon assessment was carried out which indicated an overall ‘spend’ of around 32 tonnes , or 266 kgCO2/m2, which is way below the RIBA 2030 recommendation for new build of 625 kgCO2/m2.  The graph above compares the total (operational and embodied) carbon emissions over 60 years for the pre-retrofit dwelling (in red) and the post retrofit dwelling (in green). While the ‘carbon burp’ associated with the works sets the retrofit ahead initially, the much lower resulting operational emissions mean that the embodied spend was ‘paid back’ in around 7-8 years. Since around 2017 it can be seen to be saving carbon.

Lessons Learned

There are some things we might do differently now:

1. When working on traditional buildings we now commonly specify ‘moisture open’ (hygroscopic and capillary active) insulation in order to reduce moisture risks generally. This may result in a slighter worse U-value but we find the long term benefits of material compatibility with the original fabric are very attractive.
2. Glass technology has evolved a lot. The latest commercially available evacuated glass allows convincing replica sash windows to be produced with a U-value close to 1.0, compared to around 2.3 for our design in 2008.
3. We only used two heat emitters and expected the MVHR to even out temperature across the house. That was a bit naïve really and while the client is quite happy with their ability to control temperature, we now design in a bit more controllability.
4. As summers have brought more extreme heat we have become increasing cautious about large areas of unshaded glazing, particularly on western façades. We would now almost certainly recommend external shading to the rear windows at upper ground level. We would probably slightly reduce the window area too, with glass specified to better balance heat loss and heat gains.

 

For future projects we have developed a number of precepts which are rooted in our learning on this project.

1. We see value in all built things now, in a way we did not before, and generally seek to demolish as little as  possible, focussing on optimising whole life carbon.
2. This principle extends to thinking very carefully before proposing extensions. If we can make the existing accommodation work hard enough to satisfy the brief, then generally we avoid proposals for new building, particularly where structural works involving steel and concrete are likely to be involved. We see small as normally better because smaller buildings require less heat, which means less cost and less carbon.
3. We think about heating very differently now. In 2008 gas was a relatively clean energy source, but as the grid has decarbonised, heat from electricity is now the winner, and improved heat pump efficiencies make this doubly attractive.  We now consider the heating system alongside the fabric efficiency measures at feasibility stage and seek to optimise how each can magnify the impact of the other.  Instead of ‘fabric first’ our approach is more ‘fabric and….’.
4. We have also started to experiment with cooling using heat pumps in reverse in summer. For urban heat island locations, we believe that such active cooling, at least for one ‘retreat room’ is likely to become increasingly necessary even when excellent passive measures have been designed in.

In 2008, sustainability in construction was focused on new buildings, with standards like the Code for Sustainable Homes.  Retrofit was not really acknowledged as an important endeavour and was hardly even a recognised term.  A decade or more later we have come to realise that managing our existing buildings is how we will really make the difference.

The graph below based on UKGBC data indicates just how important retrofit is for the built environment.  The purple zone, representing carbon emissions from existing buildings is far bigger than the brown zone representing that from new buildings.

Projects like the 80% House demonstrate that ambitious targets can be met and can yield very positive benefits, even for properties which have considerable constraints. The number of properties that require attention remains daunting but if every refurbishment, every maintenance programme and every estate regeneration has energy efficiency built into it, then I am more hopeful now that I was in 2007 that our built environment can each net zero in time.

The 80% House received a special Editor’s Choice ‘Contribution to shared learning’ award at the Architecture Today Awards in 2023 for projects that stand the text of time.

Awards
The 80% House received a special Editor’s Choice ‘Contribution to shared learning’ award at the Architecture Today Awards in 2023 for projects that stand the text of time..