What’s the best way to effectively insulate my bungalow roof?

I have a 30-year-old dormer bungalow and I now need to replace the roof slates. I want to increase the insulation on the roof when I do this. I have some internal insulation but it’s not adequate (all I have under the slates is felt).

I have two questions.

Is there any grant or financial assistance available?

What would be the best approach to insulating the roof?

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You are replacing your roof covering and this gives you a rare opportunity to access the concealed space below the slates. Dormer roofs can be subject to extreme heat loss and heat gain and it is vitally important that good levels of insulation are achieved so that the attic rooms are comfortable in both winter and summer.

It is generally well understood that heat rises and up to 30 per cent of heat loss occurs through the roof of our homes. The introduction of insulation to combat this helps not only in saving heat but also saves money and reduces our carbon footprint.

In your case you will clearly be limited by the space available or the depth of the common rafters used to construct the roof of your house. Insulation placed in the raking sections of the roof needs to have a minimum of 50mm ventilated air gap above. This is required to prevent condensation forming in the roof structure. Condensation can lead to mould and high levels of moisture which could trigger decay or a woodworm outbreak. Increased moisture can also reduce the effectiveness of insulation. Therefore, the depth of the common rafter less 50 mm is all that is likely to be available to you to accommodate new insulation.

In terms of insulation type, best results are likely to be achieved with a dense, foil-backed polyurethane insulation. This type of material can achieve good insulation qualities while using lower thicknesses. This insulation must be cut and slotted snugly between the common rafters without any gaps as these will lead to heat loss, cold spots and mould on the ceiling below. As always with insulation a high standard of workmanship is imperative in achieving the desired results.

The Sustainable Energy Authority of Ireland (SEAI) has grants available for improvement of attic insulation up to a value of €400. To qualify you need to achieve a target U-value (the rate at which heat will transfer through insulation) and your contractor must also be on an approved list of contractors maintained by SEAI. Good information on grants and attic insulation is available at seai.ie.

Apart from the areas below the slates you should also look at increasing insulation to the enclosing walls to the dormer accommodation and the ceilings within any eaves voids. Elimination of draughts is also paramount in improving comfort so if possible draft proofing should be provided in tandem with insulation.

Spend some time with your chosen contractor and ideally you should seek input from a chartered building surveyor or other professional with expertise in this area. They should assess your particular project so that any obstacles are predicted and all available solutions are researched. Items like ventilation, insulation type and thickness, treatment of downlighters if present, potential U-value achievable, eligibility for grant, draught-proofing options, treatment of remaining attic areas and so on all need consideration. Time spent planning your project will be rewarded once the best fit for your case is chosen.

Noel Larkin is a chartered building surveyor and member of the Society of Chartered Surveyors Ireland, scsi.ie