Is it better to insulate between rafters or joists?

Author: Dr. Chasity Ziemann  |  Last update: Wednesday, July 9, 2025

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.

Should I insulate between rafters or joists?

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).

Is it worth insulating between roof rafters?

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.

Where should you not put insulation in the attic?

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).

Is it worth insulating between joists?

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.

How to Insulate Timber Rafters - ISOVER G3 Metac Touch

How thick should insulation be between joists?

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.

Is it better to insulate attic ceiling or floor?

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.

Should you insulate the underside of your roof?

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.

Can you put too much insulation in attic?

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.

How much insulation do I need between rafters?

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.

What is the best insulation for attic rafters?

Fiberglass batt is considered to be the best insulation for attics. Compared to other insulation materials, it is the cheapest and easiest to install.

What is the best insulation between ceiling joists?

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.

When would attic insulation be placed between the roof rafters?

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.

What is the best way to insulate an attic roof?

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.

Do I need blocking between rafters?

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.

Where should you not put insulation?

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.

Should I use r30 or r38 for attic?

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.

Can your R-value be too high?

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.

Should I insulate between roof joists?

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.

Why don't you insulate the attic ceiling?

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 there be an air gap between insulation and roof?

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.

Should there be vapor barrier between ceiling and attic?

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.

What part of the attic should be insulated?

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.

What is the best flooring for a finished attic?

Resilient flooring, such as laminate or vinyl, is one of the best choices you can make for attic floors.

Previous article
Which is cheaper laminate or Formica?
Next article
What happens if you run your air conditioner 24/7?