How to Insulate a Dormer Roof: Your In-Depth Guide

insulating a dormer roof

Dormer roofs are an excellent architectural choice to add an extractive looking extra space to your home. Unfortunately, these areas in a property are often overlooked when it comes to properly insulating it.

Keep reading our guide to learn how to insulate a dormer roof and understand the different insulation options. 

How to Insulate a Dormer Roof

There are many methods you can use to insulate a roof. These depend on your choice of insulation material. Typically, the following materials are best for insulating a dormer roof:

Rigid Insulation

If you want a well-insulated roof, your first option would have to be rigid insulation. In this type of roof insulation, the material is a rigid foam board.

Accordingly, rigid insulation is the go-to system for flat roofs. However, it still works for dormer roofs, particularly as it fits flush between the rafters.

The rigid foam is usually 150 mm thick, providing optimal insulation. The boards fit between the rafters, with a gap about 50 mm wide from the roof. This air gap helps efficiently trap heat and prevent cold bridging.

If you want energy-efficient insulation that’ll last a lifetime, rigid insulation is for you. This product has a thermal conductivity of 0.02 to 0.023 W/m K. This low value means the heat will remain trapped, keeping you warm in the winter and cool in the summer!

Having said that, rigid insulation can be pretty hard to install on a dormer roof, especially if you’re not hiring a professional. That’s because you’ll have to trim the large sheets to the exact size of the roof rafters.

Still, if you have experience with DIY home projects, you should be able to manage these handy polyurethane boards.

Spray Foam Insulation

While spray foam insulation is a relatively new solution, it has quickly become one of the most popular insulation materials for good reason as it can get into hard-to-reach areas, moulding into the shape of the roof.

You might believe spray foam insulation offers subpar insulation compared to rigid foam boards. However, the opposite is true.

Spray foam insulation provides a breathable, airtight seal, meaning there’s no cold bridging or potential for leaks. It fills any gaps in the dormer roof, keeping rodents and insects at bay. Since it’s impermeable but breathable, there’s no chance of mould growth.

The lightweight foam has higher thermal conduction than rigid insulation, at 0.035 W/m K. Yet, the difference is negligible.

Mineral Wool Insulation

Another perfect dormer roof insulation option is mineral wool. This insulation process includes layering 150 mm layers of mineral wool until reaching a thickness of 350 to 400 mm.

Naturally, this can be a lengthy process. Yet, mineral wool is inexpensive. It’s also not as tough as rigid insulation. Instead, it consists of malleable sheets, making it the perfect fit for dormer roofs.

Mineral wool also doesn’t require an air gap. However, in the case of cold bridging, it’s simple to create a gap in the already installed insulation.

As for the thermal conductivity of this insulation material, while it’s somewhat higher than foam boards and spray foam, it’s still incredibly low. Mineral wool has a thermal conductivity of 0.042 W/m K.

Which Dormer Roof Insulation Should You Use?

Now that you know all the different materials you can use to insulate a dormer roof, you might still be wondering which is best for your needs.

The first thing you should consider when picking insulation material is the shape of the cavity you’re insulating. Well, we’ve established that with dormer roofs, your top choices are rigid foam, spray foam, and mineral wool.

That’s because these materials are lightweight and malleable enough to fit the structure of a dormer roof.

Then, the next thing to reflect upon would be your budget. Spray foam is straightforward for a professional to install, but you’re paying extra for the material itself. In contrast, mineral wool and rigid insulation are on the cheaper side but can be tricker to install, especially if you are aiming to do it yourself.

Benefits of Insulating Dormer Roof

Insulating your dormer roof can seem exhausting, especially after factoring in the cost, labour, and safety precautions. That said, choosing to insulate your roof can be beneficial for the following reasons:

Getting a Warmer Home

The most obvious reason why you’d want to insulate your roof is to regulate your home’s temperature. A slight draft from the former roof is enough to freeze your home during winter.

Fortunately, even a thin layer of insulation can make a world of a difference. After all, the materials used for insulating a dormer roof have thermal conductivity lower than 0.05 W/m K.

Saving on Energy Bills

Many people decide against insulating their homes due to the costly process. Yet, home insulation can actually save you thousands in the long run.

The reason is that dormer roof insulation can efficiently maintain the internal temperature of your home. There’s no need to turn up the heat or have to rely on AC or cooling fans. In turn, your energy bill will be much lower.

Helping the Environment

Home insulations make a huge impact on the environment. Since insulating the dormer roof means you’ll be reducing your energy consumption, it also leads to less carbon emission.

You can be even more environmentally friendly by using eco-conscious recycled insulation material. Unfortunately, spray foam is chemically produced and contains very little recycled material. Instead, you can opt for mineral wool.

Insulating a Dormer Roof On Your Own

If you know what to expect, insulating your roof can be a fulfilling project to take on. So, here are a couple of tips to help you:

  • The insulation material should be cut into smaller pieces, as they’re put in between the joists.
  • It’s best to install the dormer insulation internally, to avoid increasing the height of your dormer roof.
  • If you want to insulate the roof from the outside, check with your local planning commission beforehand.
  • If you’re working with spray foam or mineral wool, make sure you’re wearing an appropriate face mask and gloves to avoid the material getting into your body.

How to Insulate a Dormer Roof: Wrap Up

Dormer roofs are often neglected areas in your home. Learning how to insulate a dormer roof can be the first step to making full use of the area in your home, as well as save on energy bills.

The job depends on the type of insulation you’re using. Spray foam is more expensive and is probably best installed by professionals.

On the other hand, foam boards and mineral wool are cheaper alternatives, but you’ll need to trim them to the size of your roof rafter, further lengthening the process.

So, you’re ready to transform your roof into an integral room in your house? Get your insulation quote with us, to assess the best solution based on your needs!

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