Insulating Your Roof: Thermal Elements and Energy Efficiency

When you're dealing with your roof, it's considered a 'thermal element'. This means if you're renovating it, you'll generally need to improve its insulation to meet the standards detailed in the Approved Document, provided it makes financial sense to do so.

For comprehensive guidance on replacing these elements and minimising heat transfer, you should consult pages 60 to 64 of the Approved Document. It also explains when your renovation work will trigger the need for insulation upgrades and what other tasks might be necessary; look at Appendix C and Table C1 on pages 80 to 83 for these details.

Before you start any work, it's crucial to check the full Building Regulations and the Approved Document. If you have any questions, always get professional advice. It's important to remember that 'thermal elements' specifically exclude windows, doors, roof windows, and rooflights.

Methods for Insulating Your Roof

When insulating your roof, you generally have two primary approaches. In a Warm Deck System, you install the insulation directly on top of the rafters or joists, with the roof covering then going over the insulation layer. This method typically means your roof won't need ventilation. Conversely, a Cold Deck System involves placing insulation between the joists or rafters; for pitched roofs, you might also put it between the ceiling joists. With a cold deck setup, you'll usually need to include ventilation.

How to Insulate Flat Roofs

Warm Deck Flat Roof

When you choose a Warm Deck approach for a flat roof, you'll typically use rigid insulation. You place this insulation directly over the roof joists, then cover it with a board (usually external plywood), and finally apply the roof covering. The exact thickness of the insulation will depend on the manufacturer's specific guidelines. A key benefit of this method is that you won't need to add ventilation to your roof.

Cold Deck Flat Roof

If you're going with a Cold Deck system for your flat roof, you'll install the insulation between the joists. It's crucial to create a ventilation gap, typically 50mm, between the top of this insulation and the underside of your roof covering. This gap ensures proper airflow. You'll also need to incorporate ventilation openings either at the eaves or where the roof meets an upstand. Additionally, before you fix your plasterboard, you should attach a vapour membrane to the underside of the insulation, securing it to the joists.

How to Insulate Pitched Roofs

Warm Deck Pitched Roof

For a Warm Deck pitched roof, you lay the insulation over the rafters. After this, you add the felt, followed by the battening, and then finally the tiling. The precise thickness of the insulation you use will be determined by the manufacturer's specific recommendations.

Cold Deck Pitched Roof

With a Cold Deck system for a pitched roof, you have two main options for placing your insulation: either between the rafters or between the ceiling joists. The required thickness will vary based on the insulation material and the manufacturer's specifications.

If you decide to place the insulation between the rafters, you must install vents along the eaves (which means both the front and rear, or side to side of your roof). You also need vents along the ridge of the roof to ensure thorough ventilation. However, if you opt to place the insulation between the ceiling joists, your priority is to make sure there's sufficient ventilation within the loft space itself.