In unfinished attic spaces, insulate between and over the floor joists to seal off living spaces below. If the air distribution is in the attic space, then consider insulating the rafters to move the distribution into the conditioned space.
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).
Insulating between roof rafters is an excellent way to improve your home's energy efficiency, create a warmer home and save you money on your energy bills – all at the same time. When deciding on the best insulation for your home, we always recommend speaking to local insulation experts for their professional advice.
Also, when laying down additional insulation, work from the perimeter toward the attic opening. Never lay insulation over recessed light fixtures or soffit vents. Keep all insulation at least 3 inches away from recessed "can" lights, unless they are rated IC (Insulated Ceiling).
Pros of insulation between floor joists
Better sound insulation and reduced noise transfer between floors. Moisture control and prevention of condensation. Space-saving compared to other insulation types, as it's hidden between joists. Potential annual energy bill savings of up to £100 for the average detached house.
If looking to use mineral wool batts between joists in a pitched roof, you will need to install around 200mm of rock wool or fibreglass to reach the desired U-value. If opting for sprayed foam insulation in your roof, a recommended depth of 120mm will help your building adhere to the recommended U-value of 0.2 W/m2k.
The floor wins every time. Insulating the floor is standard practice for an unfinished attic space. The exception to this rule takes place when families opt to convert an unfinished attic space into something usable or livable.
Do you have any plans for furnishing/fitting out your attic in the future? Then you could simply insulate the pitched roof. But the best solution is actually to insulate both the attic floor and the underside of the roof.
While loft insulation is an effective way to improve energy efficiency and reduce heating costs, over-insulating can lead to several potential issues. Firstly, over-insulation can lead to poor ventilation in the attic, which can cause moisture buildup and increase the risk of mould growth.
The thickness of insulation material between rafters is a crucial factor in achieving an optimal level of thermal performance. That's why the UK regulations state that the recommended thickness range should be between 170mm and 350mm.
Fiberglass batt is considered to be the best insulation for attics. Compared to other insulation materials, it is the cheapest and easiest to install.
If access is easy and your loft joists are regular, you can use rolls of mineral wool insulation. The first layer is laid between the joists – the horizontal beams that make up the floor of the loft – then another layer is laid at right angles to cover the joists and make the insulation up to the required depth.
Warm Roof: Insulation is placed between and above the rafters. This method keeps the entire roof structure warm and minimizes the risk of condensation. Cold Roof: Insulation is placed only between the rafters, leaving the roof space cold. This method requires excellent ventilation to prevent moisture buildup.
Fiberglass is the most common option for attic insulation and may come precut to fit typical attic trusses and rafters. Blanket insulation may also have an air and vapor barrier on one side to prevent condensation and to make fastening the insulation to the attic easier.
Where the distance from the top of the braced wall panel to the top of the rafters or roof trusses above is between 91/4 inches (235 mm) and 151/4 inches (387 mm), blocking between rafters or roof trusses shall be provided above the braced wall panel in accordance with Figure R602.
Never place insulation close to water heaters, oil burners or anything hot. The backing is especially flammable and will be a major fire hazard for your home.
If your attic has little or no existing insulation, you'll want to install an R-38 rated material. If your attic has 2-3″ of existing insulation, you'll want to install an R-30 to achieve a total of R-38. If your attic has 5-6″ of existing insulation, you'll want to install an R-19 to achieve a total of R-38.
The optimal R-value for most homes is 30, but you can go as high as 38 if you live in a tropical climate. Anything higher than 38 indicates an over-insulated space. While you might think that too much insulation can't be bad, there are some significant consequences to consider.
IS IT OK TO PUT INSULATION BETWEEN ROOF RAFTERS? Insulating between roof rafters is not only okay, but it is often recommended as a way to minimise the use of headspace while maintaining comfortable indoor temperatures.
While it may seem like the more insulation you have, the more energy-efficient your home will be, you do not want to insulate both the attic floor and ceiling. Insulating the floor and the ceiling would create a separate and conditioned environment, especially if you air-sealed your attic.
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.
Generally speaking, vapor barriers are considered an essential moisture-blocking tool for climates that experience significant levels of humidity. For that reason, building codes in more humid locations will often require them, in which case you might not have much say about whether to install one or not.
1. In unfinished attic spaces, insulate between and over the floor joists to seal off living spaces below. If the air distribution is in the attic space, then consider insulating the rafters to move the distribution into the conditioned space.
Resilient flooring, such as laminate or vinyl, is one of the best choices you can make for attic floors.