Last edited 28 Jun 2016

Roof insulation

Contents

[edit] Introduction

Because of its buoyancy, hot air tends to rise, and as a result buildings that are inadequately insulated can lose a quarter of their heat through their roof. There may also be significant heat gains through roofs during the summer. This means that insulating a loft, attic or roof is a simple and effective way of reducing heat loss, the size of heating and cooling systems, energy usage and so carbon emissions.

Selection of the type of roof insulation depends on considerations such as; the roof design, the use of the building, climate, space availability, access, whether the roof is pitched (sloping) or flat, and so on.

[edit] Pitched (sloping) roof

This type of roof is generally straightforward to insulate because of space availability and access, and so there are range of insulation options to choose from:

  • Warm roof: This is where insulation is installed immediately under the roof, within the plane of the roof pitch, meaning that the loft space beneath is also kept warm.
  • Cold roof: This is where insulation is installed immediately above the ceiling of the top storey, meaning the loft space is not heated. This generally involves insulating between and over joists immediately above the ceiling of the top floor. Cold roof solutions are generally less expensive to install because there is a lot of available space, and so more economic, deeper insulation materials can be used.

[edit] Flat roofs

These types of roof can be more of a challenge to insulate, and generally there are three types of solution.

  • Warm deck (or warm roof): The ‘deck’ of the roof is below the insulation.
  • Cold deck (or cold roof): The insulation is installed below the roof deck. For ventilation purposes, and to avoid condensation forming, a ventilation slot is left usually around the perimeter of the roof.
  • Inverted roof: The insulation is installed above the uppermost weatherproof membrane, of the roof structure, effectively protecting it from heat and cold which can cause damage in the long term.

For more information see Flat roof defects.

[edit] Condensation

As warm, moist internal air permeates through the external envelope of a building towards the outside, it will tend to cool. When it reaches its ‘dew pointtemperature, the moisture it holds will begin to condense as water. If this happens within the structure of the building itself, this is known as interstitial condensation. If interstitial condensation occurs within the inner skin of the building envelope, it can cause problems. To prevent this, vapour control layers (VCL) or vapour barriers are positioned on the warm side of the structure, preventing the warm moist air from penetrating to a point where it might reach its dew point temperature.

Great care must be taken when retrofitting insulation to an existing roof. If the insulation changes a warm roof into a cold roof, it may be necessary to install additional ventilation so that cold damp air can escape from the roof space and does not cause timber to decay.

[edit] Types of insulation

There are a range of materials available for roof insulation, some of which are relatively inexpensive and can be installed without specialist expertise:

[edit] Blanket insulation (also called matting insulation)

This is the most common and easiest to install. It generally comes in foil-backed rolls which can be placed between joists or rafters, and may be held in place by timber battens attached to and across the joists or rafters. It is usually made of mineral or glass wool but can also be made from plastic fibres, and natural fibres such as cotton and sheep’s wool. The depth of blanket-style insulation may be in the range of 250-270mm.

[edit] Foam boards

These are rigid panels of insulation which are cut and fitted in place. Most commonly they are made from polystyrene, polyisocyanurate, and polyurethane, and may fitted to a depth of around 175mm.

[edit] Radiant barriers

These inhibit heat transfer by thermal radiation. They are capable of being stapled to the underside of rafters. They are more effective in hot climates, and studies have shown that the reduced heat gain may allow for smaller air conditioning systems. In cool climates, installing more thermal insulation it is usually more cost effective. The thickness of a radiant barrier is usually between 3-5mm.

[edit] Blown-in insulation

This involves cellulose mineral fibres being blown into a void in the roof space. Although requiring specialist equipment, it is quick to install and can be effective for spaces with limited access, such as gaps between roof joists.

[edit] Spray foam insulation

Typically, spray foam is formed of polyurethane and is sprayed as a liquid which gradually expands to up to 100 times its original volume. Once set, it creates an effective thermal and noise insulating layer. As spray foam can produce dangerous fumes and damage the structural integrity of the building if applied incorrectly, professional installation is recommended.

[edit] U-values

The building regulations require that reasonable provision be made to limit heat gains and losses through the fabric of new buildings and works to existing buildings. The approved documents to the buildings regulations set out the limiting standards for the properties of the fabric elements of the building, including roofs.

Approved Document L specifies minimum U-values for domestic and non-domestic buildings in England. Calculations by Kingspan have produced best starting point values that are thought to be suitable for common situations.

U-values for roofs in domestic buildings

Pitched roof – ceiling level

  • New Build: Best starting point (fabric only) – 0.11
  • Existing buildings: Extension – 0.16
  • Existing buildings: Refurbishment – 0.16

Pitched roof – rafter level / Flat roof

  • New Build: Best starting point (fabric only) – 0.11
  • Existing buildings: Extension – 0.18
  • Existing buildings: Refurbishment – 0.18

[edit] U-values for roofs in non-domestic buildings

Pitched roof – ceiling level

  • New Build: Best starting point (fabric only) – 0.19
  • Existing buildings: Extension – 0.16
  • Existing buildings: Refurbishment – 0.16

Pitched roof – rafter level / Flat roof

  • New Build: Best starting point (fabric only) – 0.13
  • Existing buildings: Extension – 0.18
  • Existing buildings: Refurbishment – 0.18

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

External references