Our Conclusion. We recommend
Popular Insulation Materials for Roof Underside
The choice often comes down to fiberglass batts, spray foam, rigid foam, and mineral wool insulation. Each has its own pluses and minuses. Fiberglass batts are praised for being budget-friendly and simple to install. They're cut into panels perfect for home projects.
Insulation helps to regulate the temperature inside the house by blocking out heat and cold during the summer and winter, respectively. It is placed against the underside of the roof deck where it can prevent heat or cold from entering the attic.
When the choice is made to insulate along the underside of the roof deck, spray foams have advantages over other insulation types because of the ability of spray foams to effectively air seal complex assemblies.
Spray foam can provide the thermal, air, and vapor control layers in both new and retrofit construction. Unvented roof strategies with open cell and closed cell SPF insulation sprayed to the underside of roof sheathing have been used since the mid-1990s to provide durable and efficient building enclosures.
You can put all the insulation on top of the roof deck, all the insulation on the underside of the roof deck, or some on top and some below. You can put it all below the roof deck but use two different kinds. Your insulated roof can be a cathedral ceiling or it can be above a conditioned attic.
The general advice we give is that you should always insulate between and above the rafters (warm roof) or between and under the rafters (cold roof). On our U-value calculator, we don't give an option for putting insulation between the rafters.
This kind of underlayment is commonly called tar or felt paper and can either be made with natural materials, such as wood cellulose, or synthetic materials, like polyester or fiberglass. The base materials are then soaked in a protective coating made of bitumen (asphalt) or something similar.
Best roof insulation for roof rafters
Rigid foam boards: These boards, including polyisocyanurate (PIR) and extruded polystyrene (XPS), offer high R-values, providing excellent thermal resistance. They can range between £10 to £21.50 per m2 depending on the type of rigid foam boards you choose.
Foil insulation
Single or multi-layer rafter insulation foil is an aluminium-based membrane that is both affordable and easy to install onto roof rafters.
Some common types of insulation used for roof rafters include fiberglass batts or blanket insulation, cellulose, and spray foam. Fiberglass batts are one of the most affordable options and offer decent resistance to heat flow with an R-value between 3.2 and 4.3 per inch, depending on where they are being installed.
Soffit. A soffit is the underside of an exterior or interior architectural feature such as a beam, arch, staircase or cornice. In roofing, it generally refers to the horizontal protective board on the underside of eaves. Stack Pipe.
Compared to fiberglass batts, mineral wool batts offers superior insulation and has about a 22-37% higher R-value per inch. For commercial settings, the higher cost of soundproofing with rockwool insulation may be offset by the thermal regulation it provides, helping to reduce heating and air conditioning bills.
Yes, too much loft insulation can cause dampness in some situations. Loft insulation works by trapping warm air inside a building, preventing it from escaping through the roof. This helps to keep the interior of the building warmer in cold weather, reducing the need for heating and saving energy.
The most common thickness of insulation between rafters is 170mm, and there is a general rule that thicker insulation is installed first, followed by a thinner layer of 100mm that can be installed perpendicular to the rafters to fill in any remaining gaps.
Should you leave a gap between the insulation and the roof? Yes, you should leave an air gap between the insulation and the roof to allow proper ventilation and prevent moisture buildup.
For optimal energy efficiency, your home should be properly insulated from the roof down to its foundation.
Fiberglass batt is considered to be the best insulation for attics. Compared to other insulation materials, it is the cheapest and easiest to install.
Directly against electrical wiring or fixtures: Spray foam insulation should not be applied directly against electrical wiring, fixtures, or recessed lighting unless they are rated for contact with insulation. Contact with insulation can cause overheating and pose a fire hazard.
The Cons
The main things you need to worry about with spray foam roofs are small installation windows, overspray, maintenance, and repairs.
Properly installing foam board insulation under a metal roof is crucial for optimizing energy efficiency, preventing condensation, and enhancing the overall performance of a building.