Why you need two layers of attic insulation

Loft conversion using Kingspan Kooltherm K118 Insulated Plasterboard

Have you ever noticed that our online U-Value calculator only offers options for two layers of pitched roof insulation?

We also only ever advise two layers in our Kooltherm Quick Guide.

So, why isn’t one layer of attic insulation enough?

For one, we want your home to be as well insulated possible… Of course! Secondly, and perhaps more importantly, it is so as to comply with Building Regulations.

Recommended U-Values – Domestic Pitched Roofs

Attic Building Regulations* Column A is for extensions where the existing dwelling’s U-values are worse than 0.70 W/m.K in the walls and worse than 0.25 W/m.K in the ceiling. Column B is for other extensions, upgraded existing thermal elements, non-exempt conservatories and conversion of unheated buildings.

As per Part L of the Building Regulations (England and Wales), or Section 6 in Scotland, we recommend that the U-value of a pitched roof insulated at rafter level should be 0.18 W/m2.K for a domestic refurbishment. If insulating a domestic refurbishment at ceiling level, the U-value should reach 0.16 W/m2.K in England and Wales and 0.15 W/m2.K in Scotland.

For a new build the U-value should reach 0.11 W/m2.K in England and Wales and 0.10 W/m2.K in Scotland, whether insulating at ceiling level or at rafter level.

Put simply, one layer of insulation fixed either between rafters or between joists would not achieve these required U-values, and therefore your home would not pass Building Regulations.

So – if you have to have two layers of the stuff – what are your options for pitched roof insulation?

Well, this would depend as to whether or not you want your attic area to be a habitable space.

If you’ll indeed be living in the loft, we would advise the following attic insulation installation methods:

You also have the option to put up dwarf walls and insulate these accordingly.

If you are only using your attic for light storage (think luggage for that holiday you never get around to having, or a bin bag full of Christmas decorations for 11 months of the year), then ceiling level insulation is fine. This will prevent heat rising into the unused attic space, keeping the rooms below warmer instead.

You can install insulation at ceiling level via the following methods:

Please note however that you can’t walk, or store heavy goods, on top of ceiling level insulation; for this, we advise installing insulation at the rafters as detailed above.

The specifics on how to install attic insulation at both rafter and ceiling level are available within the Kooltherm K107 Pitched Roof Board and Kooltherm K118 Insulated Plasterboard product literature.

If you want to learn more about how to insulate a pitched roof at the rafters, do take a look at our dedicated blog post on the subject.

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

0 comments on “Why you need two layers of attic insulation
  1. There seems to be no mention of ventilation In the case of a loft, do we need to keep the space ventilated if insulating at rafter level ?

    • Hi David

      Thanks for your comment. There is generally a choice between unventilated and ventilated constructions, except in the case of refurbishment / attic conversions. In these instances, unless the whole roof is to be stripped or there is a breathable sarking membrane already in place, it is impossible to use an unventilated roof. If you’re in this position and you’re creating a ventilated roof, Building Regulations / Standards require at least a 50mm air gap between the insulation and the sarking felt so as to avoid condensation.

      Take a look at our Kooltherm K7 Pitched Roof Board product brochure. This gives advice and guidance on how to insulate at the rafters of a pitched roof, including ventilated (recommended for loft conversions where re-roofing is not intended) and unventilated (recommended for new build or re-roofing) options. You can also read a little more on ventilation considerations on pages 10-11.

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