TECHNICAL
Q&A Rafter-level insulationinpitchedroofs
Warmer yes, but do itwisely –Douglas Kent, the SPABʼs Technical Secretary, focuses on the installation of thermal insulation at the level of rafters – the inclinedwoodenmembers that formthe top of the frame supporting the roof
How advantageous is it to fit insulation at rafter level? Insulating roof slopes on old buildings can be trickier than laying insulation conventionally over horizontal ceilings in lofts, butmay frequently be necessary where living accommodation is located directly under pitched roofs. It will combat excessive heat loss and solar gain. Caremust be taken, however, not to harmthe character and significance of a building or increase the likelihood of condensation and associated timber decay.Where thermal upgrading is necessary under UK Building Regulations, important exemptions and special considerations exist for historic buildings that enable lower levels of insulation where full compliance would cause unreasonable harm.
How is roof insulation installed at rafter level? Pitched roofs can be insulated between, below or above the rafters. It can be advantageous to combinemore than one of these approaches – for example, by insulating between and below rafters. Permeable insulation often works well with old buildings, though non-breathable forms should not be discounted where adequate ventilation and detailing can be achieved. The use of sprayed foamor loose-fill insulation is inadvisable. Non-combustible insulation will be required for any fire separation. Avoid insulating above recessed light fittings (unless they have a heat shield)
and route any electric cables outside the insulation to reduce the risk of over-heating and fire
What are the benefits of insulating below rafters? The advantage of insulating beneath rafters where there is sufficient access is that the thermal efficiency of a roof can be improved frominside without stripping and re-covering it. The drawback is a loss of headroominternally. This option will also be unsuitable where sound, historically important plaster ceilings or linings exist or when rafters are to stay exposed internally. Insulation boards, including thosemade of wood-fibre, can be applied and covered with a “breathable” plaster. It is possible to fit the insulation to a ceiling, but future removalmay result in extensive damage.
How might insulating above rafters be advantageous? Insulating above rafters can allow old ceilings to be retained, avoid the loss of headroomand enable beams to be exposed internally, but is generally not an option unless a roof is being stripped and re-covered anyway.
Rafters in the recently refurbished loft at the Merchant’s House, Shepton Mallet, Somerset
Another disadvantage can sometimes be an unacceptable increase in the height of the roof, necessitating alterations to details (at verges, etc). Rigid insulation boarding is commonly used, but a thin, flexiblematerial (for example, a “breathable”multi-foilmembrane) or aerogel productmay be desirable to maintain the character of an uneven roof, avoid significantly increasing its thickness and obtain air-tights joints between boards.
What are the pros and cons of insulating between rafters? Positioning insulation between rafters where access is availablemay avoid having to increase the height of a roof by insulating above or, alternatively, reducing headroomor disturbing a historic ceiling internally by thermally upgrading below. The disadvantage, however, is that insulation can not always be inserted to the required thickness because of insufficient rafter depth, though this can sometimes be overcome by screwing timber battens below. Theremay also be a risk of “thermal bridging” (cold spots) along the uninsulated rafters themselves but other approaches are sometimes limited. Soft, pliable insulation, including hemp, can be appropriate
Providing sufficient ventilation above or below a breathermembrane or impervious underlay respectively. Special attention is needed where the roofline
How do I cut the risk of condensation if rafter-level insulation is installed? Minimise possible condensation problems by:
is interrupted by rooflights, etc. Ensuring that insulation is fitted tightly. Lagging pipes and ducts where condensation could cause damage.
Bear inmind that underlay is not necessarily needed. Additionally, the batten cavitymay not require a ventilation path when using permeable, air-open roof coverings (such as clay tiles or rough natural slates) so long as there is no “torching” (mortar to the underside) or a likelihood of airflow through joints becoming restricted bymoss or lichens.
Further reading Garrand, C (2008) “Roofing, Technical Review Series”, London: RIBA Publishing Stirling, C (2002) “Thermal Insulation: Avoiding the Risks: A Good Practice Guide” “Supporting Building Regulation Requirements”, 3rd edition, London: CRC Ltd
The SPAB runs a technical advice line for technical enquiries, between 9.30am and 12.30pm, Mondays to Fridays, on 020 7456 0916. This number is sometimes subject to change.
Cornerstone, Vol 32, No 3 2011 77
ADRIAN SHERRATT/CONSTRUCTION PHOTOGRAPHY
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