Post-works assessment

Owner’s feedback

The works were completed in autumn 2012, following which the energy benefits and comfort value of the works appeared to vary throughout the house.

The ground floor return room (kitchen/dining room) which features the deepest level of intervention (underfloor heating & insulation, calcium silicate lining to walls, double glazing, removal of flue) has become the most comfortable room in the house.

The first floor return bedrooms similarly benefitted significantly from the calcium silicate lining and this measure was seen to have the biggest overall impact. Although the calcium silicate does not perform to the same standard as other modern insulants (such as PIR board) and a relatively thin lining (50mm) was applied, this nevertheless has had a significant impact reducing the wall U-values (rate of heat loss) from 2.10 to 0.81 W/m2K.

Improved thermal comfort was reported to the rear ground floor room of the main house which was fitted with double-glazing and floor insulation only (note that the exposed wall area of this room is quite low).

The above rooms all benefit from some level of solar gains as the back of the house faces south. The (north-facing) first-floor front bedroom only received double-glazing to two windows as original cornices mitigated against using a thermal lining. A small, less dramatic improvement in comfort levels was noted here.

Although the slim double-glazing has reduced condensation build-up and staining on sashes in winter and underfloor draughts have been eliminated, little or no improvement to the front living room was reported which is still very cold. This room (which is exposed on two sides) did not receive any thermal lining, has a northerly aspect and has a large bay window with an uninsulated flat roof and weights voids. Being located on the ground floor, the room would also tend to lose heat to the bedroom above which may at times be unheated.

Issues were reported in relation to heat delivery of the radiator-based element of the reorganized heating system. This may be due to circulation pressures as the pump size was decreased to reduce noise generated within the house by moving the boiler from an outhouse into the house.

Impact on BER rating/fuel bills

The BER of the building was improved from an E1 to a C3 following the works. The BER would indicate that the heating energy load of the house has theoretically reduced by 36%. The house was historically partially heated with a low occupancy by comparison with BER temperature and occupancy assumptions, and as such, the BER calculations are unlikely to be an accurate reflection of actual energy consumption.

The post-retrofit energy value of the house per m2 is some 30% higher than the Upper Leeson Street case study house which is a somewhat similar two-storey return house. This difference may be attributed to the fact that Albany Road is an end-of-terrace house with an exposed gable and resulting higher heat loss are/floor area ratio, as well as having a higher window area/floor area ratio.

At the time of the study (circa 2013), the building had only experienced one heating season since the works were completed during which the heating system was experiencing teething problems. As such, the effects of the measures on fuel bills, many of which were estimated, are inconclusive. While kWh consumption and unit costs were not available, overall fuel bills appear to be marginally less than an average of the previous three years. Assuming a 10% increase in gas prices before and after the works similar to the other case studies, it could be concluded that there has been a reasonable increase in comfort levels in association with a reduction in energy consumption on foot of the works.

Impact of works on architectural heritage

The external appearance and setting of the house were unchanged by the works. Internally, the plan form was unchanged apart from the removal of a screen in the return which was introduced in 1990.  Apart from the introduction of double-glazing to replace the original single glazing, the principal rooms to the main house were unaffected by the works. Historic window glass should be respected where possible and its removal has had an impact on the heritage value of the house (such an intervention on a protected structure would require planning consent). In this instance, its removal was predicated by the severe conditions experienced by the owners, including ice on the inner face of the glazing during the winter of 2010/2011. Since the time of the project some eight years ago, the performance and certification of reslim double-glazed units has been called into question, with secondary glazing gaining ground as a preferable retrofit route for period windows.

The introduction of the thermal lining to the return rooms has had a visual impact on the window reveals (see image) but without any loss of building fabric (original skirting boards were re-instated).

The rationalising of the heating system has not had an impact on the fabric of the building (apart from a new balanced flue outlet to the side wall of the return) and similar to many other projects is seen as the first thing to be considered in an historic house.

The removal of the surviving section of flue at first floor level in the return resulted in the loss of original fabric. However, given that the fireplace had been removed to the floor below and the chimney had been removed above roof level, it was deemed reasonable to remove this redundant section of masonry.

The insulation to the suspended timber floor was removed in late 2014 following an outbreak of dry rot and cellar rot in the underfloor void. Dry rot had been discovered prior to the original retrofit works and its treatment was therefore included in the original programme of works. Nevertheless, the dry rot returned following the renovation works. The problem was subsequently linked to high moisture levels in the subfloor and rising walls caused by a long term, concealed leak within below-ground drainage. The insulation had also effectively reduced airflow through the floorboards and subfloor. The leak was repaired, and floor insulation removed to fully ventilate the sub floor area. Following this drying-out period, the humidity levels in the underfloor void (without insulation) were measured at a healthier 80% RH. The client has been advised that this remedial period should be a year long so that further measurement and drying can occur, and so it can be determined with certainty that the sole cause of the moisture has been identified and removed. Although woodfibre board had been installed (see image), woodwool strand board (which has a cement or magnesium wash) is far more resilient to high humidity and more suitable to subfloor conditions. For more information, see section on Insulating a suspended timber floor [link to follow].

The lesson learned was that the "precautionary principle" should be applied when considering energy-efficiency focused renovation works – conditions within the building should be thoroughly understood and that knowledge included in the planning. Works to historic buildings (which have found their own state of equilibrium in relation to moisture and are therefore sensitive to changes) are better informed by more detailed preparatory investigation.