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Attic Insulation Thickness: The Ultimate Guide

Attic Insulation Thickness

Attic Insulation Thickness - The Ultimate Guide

If your house is located in a cold climate, keeping it warm in Winter is expensive. Many homes belong in the “under-insulated” category. And with the prices for natural gas and heating oil continuing to rise, ensuring that adequate insulation is installed is crucial for enjoying your home.

Yes, you can always dial down the thermostat, or you can get used to wearing bulky sweaters indoors. That will save money and cut your costs. But if you have an unfinished attic, adding insulation is one of the simplest, most effective ways to keep heating bills in a normal range, and enjoy spending time in your house.

With that in mind, here’s our ultimate guide to attic insulation thickness…

Increase Your Insulation Efforts to Decrease Your Bills

A properly insulated attic can save between 10 and 50 percent on your heating bill. It all depends on your attic insulation thickness, and what material you use for insulating.

A simple insulation inspection will help you determine whether you need attic insulation or not. Go into your attic, inspect the condition of your insulation, and calculate the current level.

The recommended level of R-value in your home should be between 38 and 45 for a properly insulated home. It depends whether you are facing the South side (sunny side), or North (not so sunny side).

loft insulation Attic Insulation Thickness

How Much Insulation Do You Need To Install?

Two factors that contribute to the amount of attic insulation thickness currently in your home are age and location. Location as in where is your home located. If you’re located in the Southern part of the UK, you probably need less insulation.

If you are located in the North, you need to add an inch or two more to your insulation. Different climates require different insulation R-values.

The age of your home is also a factor. New buildings need less insulation. If your home is 10 years or older, you need to get more insulation.

The best way to determine whether you have enough insulation is to look across the span of your attic. Is your insulation just level with your floor joists? Or is it below your floor joists? Can you easily see the joists? If you answered yes to any of these questions, you need to add more insulation.

If you cannot see floor joists because insulation is well above them, chances are you have enough insulation. One thing to note. Make sure that insulation is evenly distributed, meaning there are no low spots.

Sometimes, people put enough insulation in the middle of the attic, but very little along the eaves.

As a general rule of thumb, you need to reach an insulation R-value of R-38, and add between 10 and 14 inches of insulation. But the attic insulation thickness depends on the material you use. Some offer higher R-value, meaning you need less insulation to achieve the desired effect.

Types of Attic Insulation

When you add additional insulation to your existing insulation, some experts recommend that you use the same type of insulation that is currently in your attic. That means loose fill on top of fiberglass batts, and vice versa.

However, you can also add a different type of insulation, like spray foam, for example.

An important factor in deciding which type of insulation you add is the type of roof you have. There two types of roofs. Pitched, or sloping roofs offer more options. They are straightforward, and as such, they can be insulated with any type of material.

Flat roofs, and dormer roofs, on the other hand, are more challenging because of their form and shape.

What is a warm loft, and what is a cold loft? This is another typical question in terms of attic insulation. A warm loft is where you insulate immediately under the roof, which means that your attic space is warm as well.

A cold loft, on the other hand, refers to when you insulate above the ceiling of the top storey. This means no heat from your home gets into the loft.

In terms of cost, some options are more expensive, some are cheaper. But it is always better to look for the best option overall long-term, one which might be more expensive at start, but offers better return in terms of value.

Some of the attic insulation options can be installed as a do-it-yourself project, while others require a professional contractor.

Here is a breakdown of attic insulation thickness, in terms of material, R-value, and what you can ultimately achieve.

Material R-value/in 3 ½ “ 5 ¼” 10”
Fiberglass blown 2.2 – 4.3 7.7 – 15 11.5 – 22.6 22.0 – 43.0
Mineral wool blown 3.1 – 4.0 10.8 – 14.0 16.3 – 21.0 31.0 – 40.0
Cellulose blown 3.2 – 3.7 11.2 – 12.9 16.8 – 19.4 32.0 – 37.0
Open cell spray foam 3.5 – 3.6 12.2 – 12.6 18.4 – 18.9 35.0 – 36.0
Closed cell spray foam 6.0 – 6.5 21.0 – 22.7 31.5 – 34.1 60.0 – 65.0

Loft Insulation Thickness

What Is The Best Option For Attic Insulation?

In order for you to determine which is the best option, let’s break down the traditional choices. To start with, let’s focus on fibreglass, cellulose, and spray foam insulation.

Cellulose Attic Insulation

The oldest material used for insulation, cellulose can be installed as a loose-fill material, but also as a blow-in material.


– Due to the boric acid within it, cellulose provides resistance to mould, pests, and fire

– When applied as a blown-in insulation, cellulose can get to the areas that are hard to reach

– Cellulose is inexpensive

– Can be done as a D.I.Y project


– Cellulose weighs more than fibreglass. That means you cannot use it for anything more than the attic slope

– Due to airflow, cellulose can shift and settle to one side of the attic. This makes some areas less well insulated than others

– Cellulose does not provide a sufficiently airtight seal

– Cellulose must be kept dry in the attic, as it can absorb water

– After absorbing water, cellulose dries slowly, which results in deterioration

Fibreglass Attic Insulation

Fibreglass has become one of the most commonly used materials, due to its price. You can find it in two forms, blankets and loose fill.


– You can use it for a D.I.Y project

– Inexpensive compared to other materials

– Good for standard stud and joist spacing


– Fibreglass can trap allergens in the attic

– Fibreglass also traps moisture, which leads to mould growth

– Small particles that come to contact with the skin can cause itchiness

recommended loft insulation depth

Spray Foam Attic Insulation

By any measure, spray foam is the best option for insulation. Be it the attic, walls, or any other part of your home, spray foam always gets the job done.


– Unlike any other material, spray foam fills in cavities, and provides air-tight seal

– In addition to air barrier, spray foam also provides noise barrier for your home

– Spray foam does not retain water, which means there is no risk for mould growth

– Lowers your energy bills as it makes your home a thermal envelope

– Spray foam does prevent pests and allergens from entering your home


– The price is higher compared to traditional forms of insulation

– Cannot be used for a DIY project, you must hire professionals

What About Other Areas In The Attic?

In addition to installing insulation in the roof, you need to take care of some other areas of your attic. That way, you can make the space liveable and improve the energy efficiency. Here are some aspects you need to consider:

– Insulation should go up the side and over any tanks. You should never insulate under any tanks

– Cover the pipes in the attic with pipe insulation. Without it, you might get burst pipes in freezing weather

– If there are electric cables in the attic, leave these exposed so they remain cool

– Insulate the loft hatch, which usually involves attaching a block of polystyrene insulation to its upper side. Ensure all the gaps around are sealed

– If you have recessed halogen lights in the room below the loft, make sure to protect them before you cover them with insulation.

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