BUILDING BRIEFING:

Energy Efficiency

- part 2; insulation and renewables

 

The introduction of Energy Performance Certificates coupled with the significant rise in fuel costs provides landlords with a timely opportunity to review the ways in which energy consumption can be reduced.  In the first article we explained how a range of strategies can be adopted with some indication of priority order, approximate costs and payback times. The second article looks at fabric, services and lifestyle strategies in greater detail, and touches on renewables.  By Adrian Birch.

Pitched roof insulation (cold roofs)

Where the thickness of insulation is not an issue then the most economical approach is to lay 100mm thick insulation quilt between the ceiling joists and a further 200mm quilt above the joists at right angles to the joists. This is regarded as a ‘cold roof’, as the structure above the insulation will remain cold in winter.

However, this can lead to condensation forming on the underside of rafters and other roof timbers and can lead to wet rot decay. This can be overcome by increasing air flow in the void above the insulation and by creating air flow paths at eaves and ridge by retro-fitting plastic ventilation systems.

It is important to ensure the insulation does not block the air flow so plastic vent spacers may need to be fitted between rafters at the eaves to permit this.

All roof spaces should be accessible for inspection and so a roof access hatch should be provided for the purpose. A considerable amount of heat can escape from hatches so they must be a tight fit in their frame and fitted with neoprene sealants and insulation board bonded to the top of the trap.

Pitched roof insulation (warm roofs)

Where accommodation is to be provided in the roof space the insulation must be laid in the slope of the roof for part or all of its height.

If the roof is fitted with a microporous sarking membrane above the rafters then insulation can be laid to the full depth of the rafters and below them, if necessary. If the roof covering is to be renewed, insulation can also be laid above the rafters, but this can create detailing problems at the junction with adjacent roofs of terraced properties.

If the roof is not lined with a microporous membrane but the more traditional reinforced bituminous felt, then an air gap of 50mm must remain between the top of the insulation and the top of the rafter to allow the space to be ventilated to eaves and ridge vents.

In order to comply with the current Building Regulations it is necessary to insulate with 50mm PIR board between the rafters and a further 80mm layer below the rafters to which plasterboard must be bonded. This can present a fixing problem, as well as potentially overloading the structure and reducing usable space. A space and weight-saving alternative is to provide 50mm PIR board between the rafters and a 30mm multifoil insulation such as Thinsulex below. However, for the foil to perform satisfactorily a 25mm air gap is required on the inside face of the foil insulation.                                                                              

Insulating existing flat roofs

If the roof is to be stripped and re-covered then 120mm PIR rigid board insulation can be fitted above the timber deck but below the new membrane (warm deck).

A sealed vapour control layer is required on the warm side of the insulation, above the deck. Care must be taken to ensure that the increased height of the roof does not compromise existing outlet and junction details, especially if the roof abuts a cavity wall, as cavity trays will require re-positioning.

Alternatively, if insulation is to be laid between and under flat roof joists (cold deck) then a 50mm ventilation gap must be maintained between the insulation and underside of the deck as described above. Each joist space must be adequately vented, which may involve adapting the roof structure. If the deck is found to be of chipboard then it should be renewed with 22mm plywood which is more resistant to interstitial condensation which could form in the air gap.

Insulating timber and solid floors

This is quite disruptive and so is best done when the property is vacant.

For timber floors the boards are lifted and a garden mesh stapled under the joists. The insulation quilt is then laid between the joists and the boards replaced ensuring there are no gaps around the edges. Any gaps should be filled with a silicone or other sealant.

The floor void should be properly ventilated and any gaps in the external walls reduced to prevent the entry of rodents who use the quilt as bedding material.

Concrete floors can be insulated by removal of the screed and replaced with a rigid PIR board. This is then laid on a polythene membrane over the floor slab with a floating floor covering of 22mm tongued and grooved chipboard, loose laid, but with glued joints. This can only be justified if the screed requires renewal, otherwise insulation can be laid on a screeded floor but doors and skirting will require adjustment.

Windows and doors

In older properties the windows and doors are often poorly fitted. Draught stripping can be a relatively inexpensive way of improving thermal comfort by the control of air leakage.

Double hung sash windows can be difficult to weatherstrip and maintain but are probably the most efficient form of ventilation and should be retained where possible.

If the windows are sound and the character of the building would not be compromised, then secondary double glazing may well be preferable to window renewal, particularly as it can improve sound insulation as well. Some systems can be removed in the summer when ventilation is most needed.

Whilst Polyvinyl Chloride (PVC) replacement double glazed windows undoubtedly have their merits, they are often installed by poorly trained personnel who rely heavily on the mastic gun to seal around the frames. PVC windows are also environmentally unsustainable, being derived from oil.

Softwood windows from Forest Stewardship Council approved sources are more environmentally friendly and may have better thermal characteristics. If repainting every few years is a chore then there are timber windows available from the USA with a vinyl cladding, but durability is as yet unproven in the UK climate.

To meet current Building Regulations replacement windows have to have a U value of 1.8W/m2C. This can be achieved by the use of low emissivity glass coupled with Argon fill to the glass cavity. Windows should ideally meet The British Fenestration Rating Council rating in band C. The window installer should be a FENSA approved contractor.

Replacement external doors should ideally be an insulated type and the frames fitted with good quality draught stripping that will withstand wear and tear.

Building Regulations now require the use of a thermal damp proof course to be fitted around the window or door to prevent heat loss at this vulnerable point.

Letter box flaps should be insulated, draught stripped and spring loaded to ensure they shut properly.

Renewables

Given the high initial costs and long payback periods renewable energy technologies should generally only be considered after carrying out all basic energy efficiency improvements. They are also very wind and solar-dependent and, therefore, inconsistent sources of power that should only be used as a back up for more conventional systems, which cannot normally be eliminated.

Existing dwellings may not be oriented to receive solar heat gains as a south- facing roof slope is needed. Roof-mounted wind generators may not perform adequately in sheltered or urban environments and result in producing energy inconsistently.

A major issue is the ‘carbon payback period’as well as the ‘financial payback period’. Most renewable technologies consume a considerable amount of energy in the manufacturing process so it can take many years before the energy used in manufacture is paid back. In some cases the carbon payback period may be close to or exceed the design life of the product concerned. This is particularly the case with photovoltaic panels to generate electricity.

Around half the heat lost in a typical home escapes through the walls and the roof. In the UK there are estimated to be around 8 million homes without adequate insulation. Source: Act on CO₂

 Solar water heating systems

This is probably the most affordable and tried and tested technology. Provided the system is correctly sized and managed it should provide up to 50% of hot water needs over a year.

Evacuated tube systems are more efficient than flat plate collectors and can provide up to 65% of hot water needs as the heating season can be extended. They are off course more expensive and, despite the hike in oil and gas prices, payback periods can be more than 10 years.

Normally, the storage cylinder is replaced with a larger cylinder with at least two heating coils; one for the solar water supply and one for the condensing or conventional boiler.

More sophisticated controls are necessary. The primary flow to the solar collector will require a pump, which adds to the electrical demand. They are not yet suitable for use where there is an existing combination boiler. The various manufacturers are looking into the provision of a storage cylinder served by a combination boiler, which rather defeats the major space saving benefit of fitting a combination boiler.

Building mounted wind generators

These generators do not make sense in most urban locations due to the low average wind speeds as a consequence of the dispersal effect of a mass of buildings. There are also planning issues to be considered.

Roof mounted photovoltaic cells

Providing electricity from photovoltaic cells is also very expensive at present and, until such time as production costs and carbon payback periods have been reduced, there can be little justification at present for fitting them in the UK. However, payback periods are likely to reduce as manufacturing costs in China are reduced and fuel prices continue to increase.

Heat pumps

These are mechanical devices that operate in the same way as a refrigerator, but in reverse, and supply more energy than they consume by extracting heat from their surroundings.

Sources include air to water, ground to water, air to air. They have a coefficient of performance of 3-4, which means that 3-4 kW of energy is delivered for every 1kW of electricity used to power the pump.

However, electrical energy is inefficiently produced in the UK - only 25% efficiency is the norm - so by using a heat pump the energy inefficiency is neutralised.

Heat pumps are expensive to install but cost effective where mains gas or oil is not available. Most cost effective where there is low heat demand so buildings need to be well insulated first. They work best with under floor heating systems so are often inappropriate for retrofit situations.

Summary

In these two articles we have advocated a strategic approach to energy efficiency which should be tailored to suit the timing of other works to the building.

The Energy Performance Certificate will indicate where the shortcomings in the existing building are to be found and act as a trigger for improvements. Improvements to the fabric will help contain heat losses and make the building more comfortable. Improvements to ventilation and services will help reduce demand for energy only if occupiers are encouraged to adopt a more energy conscious lifestyle. The cost benefit table on page 22 in the October issue of The Letting Update Journal should be consulted to determine the most cost effective solutions that generate the most energy savings.

More Information:

Energy Savings Trust: www.energysavingtrust.org.uk

British Fenestration Rating Council: www.bfrc.org

Directgov: www.campaigns.direct.gov.uk/actonco2