By Tim Martel

A third of our greenhouse gas emissions are from heating. We need solutions that save more than 50% of the CO2 given that our country’s target is to reduce CO2 by 80% by 2050.  The UK has a very old building stock, Victorian houses are common and about 80% of the housing stock that will be around in 2050 is already built today so solutions are needed for our current buildings.  Over the last 4 years I’ve been working with the AECB, the Association for Environmentally Conscious Builders and have been finding low energy retrofits are surprisingly cost effective when accurate calculations are done which include co-benefits. I’m a Chartered Architectural Technologist, Passivhaus Designer and a tutor for the AECB’s Carbonlite Retrofit course.
So firstly what co-benefits are we talking about? When an eco-renovation or retrofit is done, the owner gets much more than energy savings. They’ll have a smarter, more comfortable and possibly more stylish house which they are likely to heat warmer than before and the comfort is likely to be a main reason. Heating savings would pay for a large part of the work, but you wouldn’t expect heating savings to pay for improvements in style or an increase in living temperature. The typical average temperature in UK houses during the heating season is 17⁰C so we assumed they’d want 20⁰C after the retrofit and they would be willing to pay a little bit extra for that.
Not all energy calculations are the same. There is usually quite a large difference between the energy performance of the design of a building and the actual performance in use, called the ‘performance gap’. The Green Construction board say that typical performance can be double the EPC, the energy figure produced by SAP calculations.  The AECB’s REALcosting software was used for this study eliminates this performance gap by using the much more accurate Passivhaus Planning Package for energy calculations and it also gives indicative costs for the work.  Energy costs were compared over the next 30 years in today’s money.
The graph below left shows that all the retrofit options are cheaper than the existing house. What has become clear is that money to pay for the project is not a problem; over the life of the building there are more than enough energy savings to pay for a high standard of deep retrofit, including heat recovery and triple glazing.

Heating, maintenance and capital costs over 30 years for a semi-detached house using External Wall Insulation (EWI)

The same data plotted over time. Owners will change but the building life after retrofit is expected to be in the range of 60-100 years based on replacement rates of current buildings.

The issue is more how to spread this cost out so that the occupants who pay for it get the benefit.  Even houses 100 or more years old are likely to have at least another 60 -100 years of life because existing houses that are structurally sound are not usually replaced that often. That timescale is usually much longer than the interest of one owner, who is unlikely to be concerned beyond 30 years.  The Passivhaus Institute have a solution for this, they call it ‘Incremental Retrofit’ where the house is retrofitted in stages over many years according to a plan. At the end of the planned time the owner (not necessarily the original one) will have a Passivhaus, and this spreads the cost over a long timescale while starting to add benefits immediately. Owners would start to have a building easier and less costly to heat; and the same insulation that keeps houses warmer in the winter can also protect them from overheating in the summer.