Cold flat roof insulation involves installing insulation between the rafters of an existing timber joisted roof structure. This method can be used for refurbishment and boasts great fire and acoustic performance while remaining cost-effective to install. However, it must comply with British Standard 5250 for adequate ventilation to work effectively.
Wood rafters conduct heat more effectively than insulation and cause thermal bridging – visible via thermal imaging as purple marks on the ceiling – which leads to thermal bridging.
Cost
Cost of cold roof insulation depends on your requirements and preferences, but typically costs less than warm roof insulation and is often preferred for outbuildings. Cold insulation also has more fire resistance properties than other forms, making it eligible for government grants; however, its use should not be in primary living spaces where condensation issues could arise.
Insulating a flat roof with this method requires installing insulation between the joists of the roof. A vapour control barrier and plasterboard ceiling must also be installed underneath this layer of insulation for complete coverage. This method works well in either new builds or renovation projects and insulation options include rigid PIR boards like Celotex XR or more flexible glass mineral wool such as Rockwool Hardrock. However, proper ventilation must also take place as otherwise moisture-laden air can enter and cause condensation problems on the roof structure itself.
Installation
If you opt for this insulation option, make sure that you select a high-quality product with at least an R-value of 5. It should also be fire-rated; PIR (Polyisocyanurate) insulation would be an ideal choice as it can withstand being walked upon and is moisture resistant, helping prevent condensation and dampness issues.
Cold flat roof insulation is best suited to new builds as it can be fitted during construction. Retro fitting it into an existing property could prove more costly and time consuming than installing warm roof insulation.
Flat roof ventilation requirements call for both cold and warm roof constructions to use a vapour barrier, in order to protect their insulation from becoming warm and creating interstitial condensation. AIRTRAK roof void ventilation systems are designed to meet these criteria, helping you meet building regulations compliance while prolonging roof service life.
Ventilation
Cold flat roof insulation is an effective and energy-saving way to lower energy bills while increasing environmental performance in your home. To meet Building Regulation requirements, however, a 50mm gap must remain above the insulation for ventilation purposes.
Ventilation in your home is necessary to prevent condensation and humidity issues from developing, as the presence of moisture can damage both roof and ceiling plasterboard and alter its temperature in an adverse fashion. Without sufficient ventilation, moisture accumulation could potentially damage these elements as well as disrupt internal temperatures within your house.
Cold flat roofs should consider installing a fully waterproof breather membrane, as this will help control condensation and minimise thermal bridging. The membrane should be secured to its substrate with adhesive tape as per manufacturer recommendations and should not puncture or tear as this could reduce insulation performance; additionally, ensure it’s highly vapour permeable.
Height
Cold flat roof insulation works on the principle of insulate between or under timber rafters to increase U-value of an existing structure. While cost effective, good ventilation must also be ensured to avoid condensation forming under the deck and protect it from potential damage.
Building regulations stipulate a gap must be left between insulation and decking to allow air flow. Mineral wool provides superior performance in terms of fire, acoustics and thermal qualities compared to alternative options such as foam.
Polyurethane spray foam insulation provides another high R-value option for flat roof insulation, available either as 2-component spray or 1-component options depending on its application. It can be used to insulate cold storage buildings with freezers or coolers as well as commercial applications like food factories; additionally it prevents the formation of ice dams in cold climates.