Housing Retrofit: Developing a strategy

various existing houses



The overall guiding principle in design for energy efficient new build and refurbishment is to first conserve energy before having to generate it:

1 Insulate the building to the highest possible standard, make it airtight and ensure suitable levels of ventilation.

2 Install an efficient heating system that is sized correctly to match the anticipated space and domestic hot water demands.

3 Where possible, install solar collectors to provide a proportion of the hot water required.

4 Consider other renewable water and space heating systems

5 Consider renewable electrical power systems.


But first ..... understanding the existing structure

Fully comprehending how an existing building ‘works’ is key to a successful energy efficient refurbishment. Understanding how it has been constructed, how it has measured up to the elements and how it has used energy are the foundations to designing a strategy for its improvement.

As part of the overall initial structural and fabric survey, the designer should identify aspects of the building that will determine the eventual design strategy, including:


Thermal performance:

It is essential to understand how the building performs thermally. Understanding and quantifying energy useage, heat loss and air permeability will be critical in identifying performance targets and eventually measuring the degree of success realised through refurbishment. Assessing existing performance will include:

- thermal modelling
- air tightness testing
- thermal imaging (useful if the budget allows)
- meter readings will show exactly how much energy is currently consumed
- understanding occupant behaviour will also identify how the energy is being consumed (eg space heating frequency, air temperature, bathing habits, use of appliances etc)



How does the building deal with rising damp and water penetration – is the house one that relies on an absorption and evaporation cycle where the walls become wet and then dry out, or is the house of a more modern construction that forms a barrier against moisture through using impermeable materials?



is the building in a particularly exposed location – is it subject to wind and driving rain? How has the fabric coped with any unusual conditions?



If the house is old, it will probably come with alterations and works that have addressed particular defects eg damp, draughts, leaks etc. These defects are a useful guide to how the building has performed in the past and will guide the designer to address the particular issues that are unique to the house as part of the refurbishment process.



The refurbishment is likely to alter the appearance of the building to some degree. It is important, particularly when involved with historically important buildings, to establish some ground rules in collaboration with Local Authority planners (and conservation officers if need be). What alterations that will be permitted by Planning will likely influence the degree of performance that can ultimately be achieved through the refurbishment.


insulation locations

Solid wall insulation

30% of the existing housing stock is of solid wall construction. Walls can be insulated internally or externally.

In designing an insulation strategy for external walls, it will be realised that not all buildings will present an ‘either/or’ scenario. Often the practicalities of adding insulation will force a compromise upon the designer. In these situations, it is likely that a combination of external and internal insulation will evolve to meet the needs of the brief.

An example of combined system might be where the appearance of the front façade is to be maintained to comply with Planning requirements, or, as in many cases, the façade borders a pavement. In these instances, it is common to apply internal insulation to the front wall and external insulation to other walls.

Note that where a terraced house is refurbished and the adjoining properties remain un-refurbished, the party walls will need insulating.

Each method of external wall insulation, whether insulation applied externally or internally has its advantages and disadvantages:





Internal insulation (Insulation fixed to the inside of the wall)

Pro Maintains the external appearance of the building
Pro The living spaces are quick to warm-up
Con The adding of insulation reduces internal space, and, in historical buildings, will likely compromise decorative features
Con The necessity to minimise encroachment on space will restrict the designer’s choice of materials and possibly restrict achievable u-values
Con The occupants will probably have to re-locate during the period of the works

Internal insulation options

Options for insulating walls internally


External insulation (insulation to the outside of the wall)

Pro Applying insulation externally will change the appearance of the building. This might be an intended benefit, or it might be considered detrimental to valued historical building
Pro External insulation usually provides the designer with a greater flexibility in the choice of insulation materials and insulation thicknesses to obtain optimum u-values
Pro The majority of thermal bridges can be eliminated
Pro External insulation will preserve the existing internal thermal mass. The thermal mass might be considered important in regulating the internal room temperatures
Pro The works will not unduly inconvenience the occupants.
Con It might need planning permission - check with LA
Con Living spaces will continue to be relatively slow to warm-up
Con Junctions between the added insulation and other elements (eaves, verges, openings etc) will need re-designing
Con Replacing windows at a later date is difficult


External insulation options

Insulated render

Insulated render is the most cost-effective form of external insulation, though the system might be prone to damage and the selection of the render must be carefully considered in respect of a potential need for the existing wall to ‘breath’.

More information about insulated render

Timber, tile and slate cladding

Cladding offers the best performance characteristics by separating the thermal insulation from the weatherproof layers. It also offers the designer a wide flexibility in the specification of cladding material and façade design. On the other hand, it is likely to be expensive, require a higher degree of design input and present the greater depth of construction.

More information about timber, tile and slate cladding

Rainscreen cladding

Rainscreen cladding is a common choice for buildings of over 3 stories. It is usually applied to high-rise post-war apartments.

More information about rainscreen cladding



Pitched roof insulation

Additional insulation is likely to be required if the space within the pitched roof is used as living space.
The vast majority of existing roofs will be of the ‘ventilated’ type. Though perfectly acceptable, the designer will have the option of replacing the roof with the better-performing ‘unventilated’ type.

'Ventilated roof'

The insulation strategy chosen for a pitched roof will be based upon a number of factors:
The existing roof can be maintained if:
- The roof covering materials are sound (tiles/slates, battens and sarking)
- If insulation is to be applied, then there is sufficient depth, or that depth can be added, suffice to provide the required thermal performance combined with ventilation.
- It is undesirable to raise the roof height.

More information about 'ventilated roof' insulation


'Unventilated roof'

If the roof covering is to be renovated, including stripping off of existing tiles / slates, a number of options are available including: Adding insulation between and over the rafters (if adding height is possible); Between the rafters with added battens to the u/s; Or between and below the rafters. The type of ventilation strategy will be between the traditional ‘ventilated’ type (see above), or the more modern ‘unventilated’ type.

More information about 'unventilated roof' insulation


Loft insulation

If the loft space is to remain uninhabited, then loft insulation should be installed. Adding loft insulation is the easiest, most efficient and cost effective method of contributing to the overall thermal performance of the building.

More information about loft insulation



Flat roof insulation

lat roofs are the most prone to incurring defects. If the option exists to replace the flat roof with a pitched type, then this course should be seriously considered.

The majority of existing flat roofs will be of the ‘cold roof’ type. Though common and workable in many instances, problems of maintaining cross ventilation within the roof often lead to condensation issues and thermal bridging which inherently reduces the thermal performance. Designers should opt for the ‘warm roof’ type where insulation is located over the waterproofing layer.

More information about timber flat roof insulation


More information about concrete flat roof insulation



Ground floor insulation

Some designers will consider adding insulation to existing concrete slabs to have little relative impact on the overall thermal performance of the building. If replacing the slab, adding below or above slab insulation might be considered.

Where the ground floor is of the suspended timber type, insulation should be installed.

More information about ground floor insulation




making the building air-tight

Once regarded either as of little or no importance or its lack a means of ‘passive’ ventilation, air tightness is now regarded as critical to the thermal performance of a building.

In a high-performance building where insulation is optimised, the proportion of overall energy then lost through warm air leaking to the outside becomes relatively high. The implementation of a strategy to enforce air-tightness using materials, careful detailing and construction, is key to realising maximum thermal performance.

More information about air-tightness




house ventilation

Perhaps of all the issues associated with highly energy efficient buildings, mechanical ventilation is probably the most emotive in its potential to influence the way people live in their homes.

A well-insulated, airtight house performs at its best when the ventilation to its spaces is controlled to minimise heat loss. Heat loss occurs when warm air is drained from the building. The potential for energy wastage becomes particularly acute when the heated air loss is un-controlled – usually through the opening of windows and doors. Modern ventilation systems are designed to provide the residents with a degree of control that allows fresh air to replace stale air at an optimum rate sufficient to maintain a comfortable and healthy internal atmosphere. In addition ‘Mechanical Ventilation with Heat Recovery’ or MVHR systems enable the heat from the expelled air to warm-up the incoming air.

Despite the undoubted advantages of providing mechanical ventilation, the main obstacle to its widespread adoption will be a cultural issue of occupants learning control systems, changing filters and resisting the urge to throw open the windows in hot weather.

More information about ventilation




house windows

The appearance of windows in an old house often contributes to its character. This is particularly true of historic buildings and buildings within conservation areas. Consultation with a Local Authority planning / conservation officer is recommended at an early stage in order to determine a window treatment strategy.

Existing windows, particularly if they are old, single glazed and air-leaky around their casements and frames, will probably need replacing.

Dealing with more recently installed windows that might be double-glazed, well-made and reasonably air-tight (check pre-refurbishment air-tightness testing), is slightly less straight-forward. The designer will have to plan a treatment of the windows based upon budget, window size (affect on overall heat loss) and ability to carry-out any remedial works such as draught-stripping, replacement of single glazed panes with double glazed, or the addition of secondary glazing.

It will likely prove difficult to balance the appearance of new/renovated windows with the need for energy efficiency – expect compromise!

More information about windows




Space heating and hot water

house heating


Space heating

Once the best possible standards of insulation and air-tightness have been achieved, the proposed refurbishment should be thermally modelled to determine what will probably be a much reduced space heating load. A new heating system should be planned to meet this new load.

A decision will be taken on whether to use the existing boiler or replace it. Likewise, the existing heat distribution system will need reviewing. What will probably be the case, will be that both the boiler and the radiator (or other) system will be over-sized for the revised heat load.

If the gas boiler is of the old non-condensing type, operating efficiency requirements will require its replacement with a new correctly-sized gas condensing boiler (though beware that durable boilers that maintain their condensing efficiency in the cheaper end of the market, are difficult to find). A slightly less conventional alternative worth considering might be a bio-fuelled boiler using, for example, wood pellets or chips. If choosing the latter, the designer should be aware that the installation needs to be matched with an assurance of fuel supply.

If the building is off-mains, the first consideration might be again for a wood-fuelled boiler – these types of boiler offer a greater carbon reduction possibility than the closest efficient alternative, LPG, and are a much better choice than oil.

More information about gas heating systems
More information about wood fuel heating systems
More information about radiators and underfloor heating

Another strategy for space heating might include a heat pump, usually a ground source heat pump. Heat pumps in some circumstances can be used on their own, but they will usually be installed in conjunction with a boiler heating system. Use of heat pumps should be considered judiciously – any benefit is likely to be achieved only within certain conditions.

More information about heat pumps

In any circumstance, any form of space heating using electricity should be avoided. Any water heating with electricity should also be minimised to that of back-up in times of greater-than-usual water usage.


Hot water

Though the specified primary heating system will provide for the base load for domestic hot water (DHW), the designer should consider the contribution that can be made from solar hot water heating.

Of all the technologies available under the popular banner ‘Renewables’, solar water heating is the most efficient, cost effective and simplest to install. Installed correctly in a location optimised for solar gain a ‘solar thermal’ system will typically provide between 50 – 60% of a home’s annual hot water needs.

More information about solar hot water collectors



Electrical power

house power

Electrical power represents high-grade energy. Conventional electrical power is drawn from the national grid, where, in most part, it is generated inefficiently using fossil fuels. Though the proportion of energy generated from fossil fuels is likely to be reduced through the introduction of large-scale renewable energy along with a re-vitalised nuclear industry, renewable systems at a domestic or community scale can offer a significant contribution in both reducing the dependence on the grid and to the proportion of electricity used in the home.

The choice of a reliable domestic-scale system is currently realistically limited, particularly in urban areas, to photovoltaic (PV) panels; Though with the prospect of workable micro CHP (combined heating and power) technologies in the offing, the range of opportunities for micro generation is set to expand. Either way, specifying domestic renewable generators involves relatively large capital costs and it is important, therefore, to ensure the overall budget is directed primarily towards more efficient and effective energy reduction before considering renewable electricity systems.

Domestic wind generation, despite a promising start, has widely failed to match expectations – though this was a result of inadvisable marketing to adopters living in urban areas where conditions are largely unfavourable. Small-scale generation in rural areas is still, given the correct conditions, very feasible.

More information about electrical power systems






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