Insulating a Flat Roof: Everything You Need to Know

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The benefits of insulation for your home are well known – reduced energy bills, a lower carbon footprint, and in some cases, even better soundproofing. Since we lose significant heat through our roofs (25% according to the Energy Saving Trust), installing roof insulation is a popular way to reap rewards. 

But how do you insulate a flat roof vs a pitched roof? What’s the difference between warm roofs and cold roofs? And how much can you expect to pay to have this insulation installed? 

Insulating a Flat Roof vs a Pitched Roof

When insulating a pitched roof, you’ll have two options – whether to create a cold loft space or a warm loft space. A warm loft is mainly necessary when you use your loft space as a bedroom, office, or other space.

Most flat roofs, on the other hand, don’t leave enough space in the loft to use it as a full room, so creating a warm loft isn’t really an option. 

Flat roofs can also be more energy efficient than their pitched counterparts because of the way they’re insulated. A membrane system is installed on top of rigid flat roof insulation board. 

Pitched roofs, on the other hand, tend to use a cavity insulation method, which involves installing insulating material between ceiling joists. Rigid insulation doesn’t have as many gaps as a cavity system, making it more efficient. 

If you’re building a home and interested in making it as thermally efficient as possible, consider opting for a flat roof.

How Do I Know If I Have a Flat Roof?

This might sound like a silly question, but it can sometimes be difficult to distinguish just how flat your roof must be to be considered a flat roof. In general, a flat roof is defined as a roof with a pitch of less than 10 degrees.

Flat roofs appear horizontal and tend to lack the space you’d expect to see in a loft with a pitched roof. Houses with flat roofs, for example, and less likely to have a bedroom in the loft.

Warm Roof vs Cold Roof Insulation

When insulating your roof you can either create a warm roof deck or a cold roof deck. The main difference between these two options is where the insulation material is placed. In a warm roof, the insulation layer is applied on top, so the roof side of the structure is warm.

In a cold roof, you’ll insert the insulation between the rafters and the roof deck, leaving a cold space under your roof’s boards. Warm flat roofs offer better thermal performance and suffer from fewer of the problems commonly associated with cold roofs.

Warm roof decks are more popular for plenty of reasons, one of which is the need to install ventilation into cold flat roofs. This need for ventilation is to avoid damp and mould, which can easily occur if improperly installed insulation is affected by condensation. Since in warm roofs the insulation fits above the structure, no ventilation is needed.

While insulating a flat roof from inside used to be more popular, more and more people are starting to opt for a warm roof, which can avoid some of the problems that come with the former.

How Should a Flat Roof be Insulated?

Where possible, opt for a warm roof, i.e. a flat roof that was insulated from above with a layer of rigid insulation board. You can add a layer of rigid insulation board either to the top of the roof’s weatherproof layer or on top of the timber surface with a new weatherproof layer applied over the top.

Insulating a flat roof from underneath is another option, but this type of insulation does run the risk of condensation and damp problems. 

In a cold roof, the insulation is fitted inside the roof below the weatherproofing materials, such as beams and brickwork. With this type of insulation, you absolutely must leave a ventilated air gap, and it’s also worth installing a vapour barrier membrane.

Advantages and Disadvantages of Flat Roof Insulation

While the pros of installing insulation generally outweigh the cons, every type of insulation has disadvantages to consider before you go ahead with the installation.

Advantages of flat roof insulation

When your home is energy efficient, it takes less energy to cool down and heat up. This not only means less money spent on energy bills, but it’s also a simple way to reduce your carbon footprint by reducing how many fossil fuels you burn to keep your home at a stable temperature.

Insulating a flat roof costs less than insulating other parts of your home, such as your walls, so if you’re on a budget it’s the perfect way to make your home more thermally efficient without breaking the bank. 

The best thing about flat roof insulation is it pays for itself within a few years. Plus, it can increase the value of your home.

You can expect your flat roof insulation to last at least 40 years before you’ll need to replace or renew it.

Disadvantages of flat roof insulation

Cold roof insulation has its advantages – it’s easier to install and tends to be cheaper than warm roof insulation. The problem with this way of insulating your home is that it leaves your roof at risk of condensation, which leads to long-term issues with damp and mould.

While leaving adequate space for ventilation and installing a vapour barrier can help, there’s still a risk involved with this type of insulation. If you’re not sure whether it’s suitable for your property, consult a professional. If your loft area is already prone to damp, discount this option.

Note that this disadvantage is more prevalent in a cold flat roof installation than the warm alternative.

If you were thinking of saving costs by insulating a warm roof yourself, think again. Because of the complicated process of creating a warm roof with external roof insulation, this method of insulation is best left to professional insulation installers.

Installing flat roof insulation inside your roof can be done as a DIY project, but in general, it’s less effective and riskier than the alternative.

As well as being labour-intensive, warm flat roof insulation can only be installed in good weather conditions – easier said than done in the UK. This can make installation more difficult to organise.

How Much Will I Save by Insulating My Flat Roof?

The Energy Saving Trust estimates that the amount saved by insulating a flat roof is similar to the amount saved by loft insulation. With loft insulation, you can expect to save up to £590 on a detached house, which means the insulation will pay for itself within a few years. 

If you assume flat roof insulation offers similar results, it proves to be one of the most cost-effective types of insulation. 

One of the biggest advantages of insulation is that installing it not only knocks valuable pounds off your energy bill, but it also reduces your carbon footprint quite significantly. 

The Energy Saving Trust‘s own figures indicate up to 1000 kg of carbon saved per year as a result of roof insulation. 

How Much Does Flat Roof Insulation Cost?

The average cost of flat roof insulation is between £30 and £60 per square metre, including labour costs. If you decide to install the insulation yourself, you can expect to pay less per square metre as you’ll only be covering the cost of the materials themselves. Supply only costs range from around £5 to £30 per square metre.

The rate you pay for flat roof insulation can vary depending on which part of the country you’re in. The size of your home will, of course, determine how much the insulation costs overall, but you can expect it to cost between £400 and £700 for the entire project.

Typically, roof insulation installation takes approximately 1-2 days depending on the scale of the insulation, but if it takes longer, you may pay more for labour costs. Luckily, roof insulation isn’t as disruptive as other methods of insulation such as internal wall insulation or underfloor insulation.

Thanks to the effectiveness of roof insulation, you’re only looking at a few years of use until it pays for itself because of the reduction in energy bills.

How to Save Money on Insulating a Flat Roof

The best time to insulate your roof is when the flat roof needs to be replaced anyway – that way you can save money by getting everything, new flat roof with all associated insulation, done at the same time. Do bear in mind if you replace your flat roof, you’ll have to insulate it anyway to comply with building regulations.

If you’re having your roof insulation installed by a professional – which is advised with this type of insulation – you want the confidence that your tradesmen are reliable installers. This is where we come in, by putting you in touch with trusted installer who can provide you with a highly competitive quote.

One way to save money is to install the insulation yourself, and while retrofitting flat roof insulation can be done in some instances, this can throw up complications if you lack experience with similar projects.

Can I Install Flat Roof Insulation as a DIY Project?

In general, flat roof insulation tends to be more effective when it’s carried out by a professional. If you have a lot of experience with similar projects or you’ve insulated parts of your home alone, you’ll have a better chance of effectively installing insulation yourself.

If you’re installing a cold flat roof, it’s easier to do it yourself than warm roof insulation, which involves accessing your roof externally from your property.

How Long Does Flat Roof Insulation Last?

As with most insulation materials, flat roof insulation can last for years, in fact usually much longer than the external flat roof itself which might last around 20 years when properly maintained. In general, however, the insulation material will tend to need replacing or updating after around 40 years.

Warm roof insulation can require replacing more quickly than the cold roof alternative because of its exposure to the elements, but you can expect to have it in place for at least a couple of decades.

During this time, it will pay for itself multiple times over, so it’s certainly a worthwhile investment in the long term. 

Flat Roofs and Building Regulations

In the UK, there are building regulations you must adhere to for installing flat roof insulation. These regulations relate to the insulation’s U-value. A U-value is a UK-wide unit used to tell engineers, insulation installers, and builders how effective a material is at insulating. The lower the U-value, the better the material is at insulating properties.

Building regulations in the UK stipulate what your insulation’s U-value should be, with the aim of making homes more thermally efficient. Achieving better energy efficiency across the board will help the UK meet its net-zero targets.

According to building regulations, new or replacement flat roofs must have a U-value of 0.18W/m2K. There are plenty of U-value calculators you can find online, or alternatively, you can discuss your insulation’s U-value with your installer.

How to Find a Flat Roof Insulation Installer

Finding the right flat roof insulation installer is key for the success of your project. You’ll need an installer that’s both experienced and reasonably priced, so seek out professionals with positive reviews and testimonials as well as affordable pricing. 

The best way to find good value for money is by using a comparison site like Insulation Advisor, who will match you with local insulation installers. Don’t be afraid to ask to see testimonials, reviews or qualifications from your installer. After all, if they successfully install flat roof insulation, it will be part of your home for decades to come. 

Is Flat Roof Insulation Worth It?

It’s easy to underestimate the difference insulation makes to your home’s energy efficiency, carbon footprint, and bills, but with such a significant amount of heat lost through your roof, insulation should be a top priority. To limit heat loss through a flat roof is in most cases well worth the investment.

Although it involves some financial outlay at the outset, you will over time more than make your money back rapidly from the savings on heating bills. Plus, you might even be able to add some value to your property when you come to sell it.

Flat roofs are notorious for having problems when they have not been properly maintained, so it may be prudent to look into the current level of insulation the next time your flat roof requires attention or needs a replacement.

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