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How Much Insulation Do You Need for Your Loft or Roof?
- Part L of the UK Building Regulations requires at least 270mm of mineral wool or similar blanket-style insulation in a loft or roof to achieve effective thermal efficiency.
- UK Building Regulations also set a u-value expectation of 0.16 W/m²K for roofs and lofts of existing homes.
- Some insulation materials - such as PIR boards - are more thermally efficient than others, achieving the desired u-value with less insulation.
Roof and loft insulation are the most cost-effective measures to improve your home. By cutting up to 26% of heat loss, loft insulation can bring the detached UK home up to £380 in annual savings and up to 1 ton in carbon cuts, benefitting you and the planet.
But you might be wondering; how much insulation do I need in my loft? Simply put, you need as much roof insulation as it takes to achieve a desirable thermal efficiency. That way, you can enjoy a home environment that is temperature-regulated, condensation-free, and safe from mould growth and structural rot.
This ultimate guide by GreenMatch UK will walk you through all you need to know about insulating roof and loft structures, and material amounts, including tips on how to determine what you need, the ideal insulation thickness, and much more.
Ready to insulate your roof? Let GreenMatch UK be your one-stop solution for all your needs. Instead of spending endless hours researching and vetting installers, spend just 30 seconds filling out our online form to receive up to 3 free home-tailored quotes. Free of charges and obligations. Click below to begin!
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How much roof insulation do you need?
Asking yourself; how much roof insulation do I need? You’re in the right place. The amount of roof insulation you need is always parallel to the amount of thermal efficiency your insulation needs to achieve. The UK Building Regulations outline a series of requirements expected from roof and loft insulation. Here are some factors that will shape your choice:
- U-value requirements: Part L of the UK Building Regulations set out that existing roof and loft structures should achieve a u-value of around 0.16 W/m²K to be considered thermally efficient. A u-value is a numerical measurement of the rate of heat transfer of a material. It tells us the rate at which heat can pass through a material. This means that the lower the u-value, the better a material is at resisting heat transfer.
- Insulation thickness: Normally, it's required to use at least 270mm of mineral wool or similar blanket insulation for roofs and lofts, however, some insulation materials have a higher thermal function. For example, if you use PIR boards, you only need a thickness of 125mm to achieve the same u-value as 270mm of mineral wool.
Whether you have a flat or pitched roof or want cold or warm loft insulation, these general rules still apply. Naturally, larger jobs (like warm-pitched roof insulation) will require more insulation material than smaller jobs (like cold flat roof insulation).
Hiring a professional insulation expert to assess your current roof and loft is the best call. That way, an installer can tailor the best approach for your home.
How to determine the amount of insulation needed for a loft
To determine the amount of insulation needed for a loft, it’s best to consult a professional who can assess your space and tailor the exact amount you need to insulate your loft. As a rule of thumb, how much insulation in attic you need is parallel to the u-value you are trying to achieve.
There are some rough calculations you can do to have an idea of what you need:
- Measure the loft area: By knowing the total m2 area of your loft space, you can already have a rough idea of the minimum amount of insulation material you’ll need. Simply multiply the length of your loft by its width to know the area.
- Measure gap between joists: The width and depth of your loft floor joists will determine the amount of insulation they can contain. This is important especially if you plan to do a cold roof insulation.
- Measure gap between rafters: For a warm loft insulation job, you’ll need to secure insulation material on the inside of the roof structure. Measure the width and depth of the gaps between your roof rafters to know how much insulation material it can contain.
- Begin calculating: Once you have all your measurements, you can roughly figure out what you need. For example, the total area of your loft can tell you how many rolls of insulation you may need, and the depth of your rafters and joists can tell you how much thickness of insulation it can contain.
If the depth of your loft floor joists and not deep enough to host up to 270mm of insulation material, then you can get some loft legs to raise the depth of your loft floor.
This is important because you also need to account for at least 50mm of air space between your insulation material and the loft floor for healthy ventilation.
Alternatively, some online loft insulation calculators can give you a rough estimate of what you need, but for the most accurate understanding of what you need, it’s important to work with an accredited insulation expert who can tailor your material needs to your property.
How much roof insulation do you need according to its thickness?
The amount of insulation material you need is also dependent on the thickness of the insulation you are purchasing. For example, if you buy thinner mineral wool blankets for pitched roof insulation, you’ll need a larger quantity to create the intended 270mm thickness than if you got thicker blankets.
Here’s a breakdown of the estimated thickness you’ll likely need per three of the most popular loft insulation materials, as well as the m2 u-value range each material can achieve:
| Insulation | U-value (W/m²K) | Thickness (mm) |
|---|---|---|
| Mineral wool | 0.030 - 0.040 | 270mm |
| Cellulose | 0.036 - 0.042 | 270mm |
| PIR Board | 0.022 - 0.028 | 120mm |
For the most budget-friendly and efficient insulation purchase, consult a professional insulation expert who can assess your roof and tell you how thick should roof insulation be and how much of it you need to achieve thermal efficiency.
However, many prospective customers get stuck in an endless loop of websites and phone calls in an attempt to land the best bargain. GreenMatch UK can free you from this burden.
Simply spend 30 seconds filling out our online form to receive up to 3 free home-tailored quotes from our nationwide network of pre-vetted installers. The best part? Our services are completely free of charges and obligations. Click below to begin!
- Describe your needs
- Get free quotes
- Choose the best offer
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