Installing or upgrading loft insulation is probably the quickest and most efficient method of improving the thermal performance of an existing building. But though adding insulation is straightforward enough, an understanding of the moisture and ventilation dynamics within the roof space is important to avoid the risk of condensation.
Ventilation: prevention of condensation
• One of the major threats to the integrity of a timber structure is through the build-up of condensation
• Condensation occurs where warm moist air comes into contact with a cold surface. Within building fabric, without the prevention of condensation, the presence of damp is likely to lead to the decay of vulnerable, particularly timber, components.
• The way moist air is handled within a roof helps define the type of construction.
• BS 5250: 2002 provides guidance to the control of condensation in pitched roofs
Ventilated roof space (traditional loft construction)
• The traditional ‘cold roof’ or ‘loft’, found in the vast majority of existing buildings, handles moist air from inside the building by allowing it to diffuse through the ceiling into the roof void from where it is removed through cross ventilation.
• A ventilation path of at least 25mm in width is required between the insulation and the u/s of the sarking.
• Holes and gaps in the ceiling should be sealed to restrict the amount of moist air entering the roof space (see also ‘air tightness’)
• For roofs over 35o pitch, or over 10m span, ridge ventilation is required equivalent to a continuous 5mm gap.
Tried and tested
Risks of condensation
Roof space can be draughty and dirty
Un-ventilated / 'breathing' roof space
Moisture handling in the case of the more modern ‘breathing roof’ is quite different from the traditional method. Moist air attempting to diffuse through the ceiling is checked by a air tightness / vapour control layer (VCL) prevents moisture passing through it. Any small quantity of moisture that does penetrate can be diffused through the roof sarking and then dispersed by ventilation through the slates or tiles.
• Only LR sarking with third party certification should be specified (see below).
• If there is any doubt that a completely sealed ceiling can be achieved, it is recommended that a 5mm ventilation slot is included at high level.
• Insulation should be installed to butt firmly against the sarking at the eaves to close a potential ventilation path.
• Insulation should be connected to the wall insulation at the eaves to prevent thermal bridging.
Reduces the risks of condensation
Requires careful detailing to ensure a well-sealed ceiling
Ventilation: sarking technology
• The form of technology that differentiates the traditional ventilated loft from the more modern un-ventilated loft, is that of the sarking.
• Traditional sarking felt is bitumen-based and highly impermeable to air and water. Any moist air on the loft side of the felt will not be transmitted to the outside – thus necessitating it’s removal by ventilation within the roof space. Impermeable sarking is known as ‘high water vapour resistance’ (HR) sarking.
• ‘Breathable’ or ‘low water vapour resistance’ (LR) sarking allows the transmission of moist air through the membrane from the loft space to the outside, whilst inhibiting the passage of water in the opposite direction.
• BS 5250:2002 recommends that only LR sarking with third party certification (eg BBA) should be used without ventilation.
• The loft insulation method illustrated below is for general guidance only. The performance of products and methods of fixing vary. When specifying, confirm with the product manufacturer their installation methods and expected performance.
• Beware when combining different insulation materials. Check with respective manufacturers regarding possible condensation risks.
• Insulation to the required thickness is laid between and over the joists. Alternatively, if using rigid insulation, boarding with integral vapour control properties can be fixed between and to the underside of the joists.
• When insulating the loft of a 'breathing' roof, ensure that the ceiling below is tightly sealed with a combined air barrier/ vapour control layer to prevent moist air entering the roof void. If there are any doubts about achievability, then ventilate the roof void.
• When insulating a 'ventilated' roof, ensure that there is an unobstructed air path of at least 25mm at eaves level (proprietary eaves ventilators are available); and that for roof pitches of over 350 or over 10m in span, the equivalent of a continuous 5mm gap is provided at ridge level.
• Ensure that all penetrations through the ceiling and construction gaps are sealed.
• Insulate cold water tanks and pipes. The underside of cold water tanks should not be insulated (see below) to prevent freezing.
• Insulate (min. 100mm) and draught-seal the loft hatch. Avoid locating loft hatches in rooms such as kitchens and bathrooms where high levels of moisture can be expected.
• Keep electrical cables above the insulation.
• Avoid recessed lighting
• If a walkway or storage deck is required, fix using additional battens so as not to compress insulation.
EST best practice for the u-value of an insulated loft is 0.16 W/m2K, though much higher values are achievable.
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