How To Insulate A Roof

Insulating a roof will vastly reduce the heat loss that occurs through the roof of a house, a key element of the thermal envelope. Roof insulation will give you a warm loft, allowing you to convert it or use it for storage. There are several different options for insulation materials within a traditional pitched roof, as well as options for other types of roofs like curved roofs or dormer roofs, allowing you to save energy regardless of the design of a house. 

Your Options For Roof Insulation

For traditional pitched roofs, sometimes called sloping roofs, you will be looking at three main options to insulate from the inside; installing solid insulation boards between the joists of your roof, installing fibreglass or mineral wool insulation batts within the joists, or installing spray foam insulation on your house roof. Roof insulation will be installed as an alternative to loft insulation, giving you a warm loft and allowing you to use your loft space for storage or even as a living area. You can insulate both the floors and roof of a loft if desired, but normally people will opt for one or the other. These options are suitable for both tiled roofs and slate roofs.

Make Sure Your House Roof is Suitable for Insulation

If you are looking to install insulation on the inside of an existing property, perhaps as a way to top up or replace loft insulation, you should first make sure your property is suitable to have its roof insulated. Here are some of the key points to take into account:

  • Planning Permission and Stairs: If you are looking to undertake a loft conversion, you will need to make sure your loft is suitable for conversion. There are both structural and accessibility conditions a loft conversion must meet (such as being accessible via a permanent staircase). Additionally, a loft conversion may require planning permission depending on some factors, such as if the building is listed.
  • Ventilation: Your loft roof will need to remain ventilated once you have installed insulation. This prevents the buildup of condensation, which can lead to both structural problems and health risks. However you install insulation you will need to leave gaps for adequate airflow to vents.
  • Condition of Your Roof Space: Check the structure of your roof and loft, making sure the underside of the rafters and the joists are free from dampness and rot. Any structural issues should be rectified before insulation. Also ensure that there are not any animals within the roof structure, particularly bats. If you have bats within your roof you will probably not be allowed to install insulation as their nesting areas are protected.
  • Access: You’ll need to ensure that your loft can be accessed for insulation. This means cleaning your loft, clearing everything away, vacuuming your exposed rafters, and ensuring that you will be able to access the upper areas of your roof. You may need to use a solid board within your loft for standing, potentially using a stepladder to access the full height.

How to Insulate a Roof With Boards

What You Need

  • 120mm of Rigid Foam Insulation Board
  • Flat Surface
  • Straightedge Ruler
  • Permanent Marker Pen
  • Utility Knife
  • Clamp or Vice (rubberised)
  • Measuring Tape
  • Tantalised Roofing Battens (50mm)
  • Screwdriver and Wood Screws / Nail Gun
  • Vapour Control Layer (if the board doesn’t contain one)
  • Staple Gun

How to Insulate

  1. Measure all gaps: You’ll need to precisely measure the gaps within your roof in order to cut the boards accurately. Using a tape measure mark out and make note of each board you will need to cover the entirety of the roof. These measurements will allow you to establish how much insulation board will be needed. You can choose to either insulate between the joists, over them, or both.
  2. Create an Air Gap: If looking to install insulation boards within joists, you will need to first create an air gap. This is to allow ventilation and air movement. An air gap will need to be 50mm wide, and the best way to ensure this is to attach timber battens of a 50mm width within your roof, which you can then place the insulation boards against. Use tantalised roofing battens of 50mm. These should be attached to the roof joists using a screwdriver or a nail gun.
  3. Installing Insulation Boards: Cut your boards to size using the utility knife method, marking lines for cutting with a straightedge ruler and a permanent marker pen, and cutting over a solid surface. Your insulation boards should snugly fit within your roof, though you can use some sealant on the back to fix them to the roofing battens if needed.
  4. Vapour Control Layer: Some insulation boards include a built-in vapour control layer, but in the cases where they do not you will need to install one. Vapour barriers are sheets that can be fixed to your roof with a staple gun. You can make a fold at each joist to attach with a thicker layer. If you need to overlap sheets, staple a solid join between them.
  5. More Boards or Plasterboard: At this stage, depending on the level of insulation you need you can install further insulation boards on top of the joists, attaching them with a nail gun. You can also attach plasterboards for decoration, which is affixed with plasterboard screws. There are options for insulation boards faced with plasterboard if needed.

How to Insulate a Roof With Batts

What You Need

  • 200mm of Insulated Batts
  • Solid Board
  • Tape Measure
  • Permanent Marker
  • Safety Equipment
  • N95 Respirator and Goggles
  • Bread Knife (mineral wool) or Utility Knife (fibreglass)
  • Tantalised Roofing Battens (50mm)
  • Screwdriver and Wood Screws / Nail Gun
  • Staple Gun
  • Vapour Control Later (if not included)

How to Insulate

  1. Measure and Cut: Measure all rafters, allowing you to establish how many insulation batts you will need to buy and what size you will have to cut them to. You’ll need to cut your batts depending on the insulation material type, making sure to remain fully protected throughout the cutting, handling, and installation. This means gloves, a long sleeve top, an N95 respirator mask, and safety goggles.
  2. Create an Air Gap: Install tanalised roofing battens to the rafters and joists, this is to ensure a 50mm air gap between your house roof and the insulation, allowing adequate ventilation.
  3. Install Insulation: Battens can be simply pushed into place, they should have a side with a paper facing or a vapour control layer, this side should be facing into the loft, and can be stapled to the joists to hold the insulation in place. Make sure that you do not over-compress your insulation batts, as this can lessen their ability to insulate. If your insulation does not fit properly without compression you should cut to size for a better fit.
  4. Vapour Control Layer: If your insulation batt does not contain a vapour control layer, you will need to install one over the insulation. This can be attached to joists with a staple gun, doubling up over the joists. If you need to cross over between two sheets, staple a decent join.

How to Insulate a Roof With Spray Foam

Spray foam insulation is strongly dissuaded for DIY installations, due to the technical difficulties of installation, and the health and safety issues that come with it. Before opting for spray foam insulation, you should get a survey to ensure that your home is suitable. If it is, you’ll have the option of closed-cell or open-cell spray foam. Closed-cell will make your roof space completely airtight and block moisture, so your loft will need to be ventilated to avoid a buildup of condensation. Alternatively, open-cell insulation allows moisture through, though it doesn’t offer as strong a level of thermal insulation. Spray foam is installed with specialist equipment, giving a consistent covering across your roof. Using spray foam will increase your insulation costs, and it can’t really be decorated over, so if looking to convert a loft it’s unsuitable.

Can You Insulate a Roof From the Outside?

A roof can be insulated from the outside, either during the build or as a refurbishment. If refurbishing, the tiles, slates, or roofing shingles, and the battens they are attached to will all need to be removed. Then a solid base will be made from wood, which will be topped with a damp-proof membrane. Solid insulation roof boards are then added, topped by a rain-screen membrane (designed for moisture protection). Then a new set of battens and counter-battens can be attached, to which tiles, slates, or shingles can be re-laid. This is a costly process, but can have the benefits of leaving your loft roof joists visible, and not reducing your space within the loft.

Other Roof Designs

The guides above are for traditional pitched roofs (or sloping roofs) but there are other types of roofs that will have their own considerations. Some types of roofs, like chalet roofs, bungalow roofs, and A-frame roofs are essentially the same as the roof in a traditional house, and can be insulated in a straightforward way.

How to insulate a Hip Roof

A hip roof is a pitched roof where all sides slope downwards towards the walls. Depending on the angles and height of your hip roof, there may be difficulties around ventilation, and installing insulation can contribute to these issues. You should have a full survey of your hip roof conducted before making any decisions about insulation.

How to Insulate a Log Cabin Roof

Cabins go by similar principles to normal buildings, though many opt to insulate them from the outside, to preserve the look of the exposed rafters. You’ll need to use a moisture-resistant sheet underneath the insulation boards.

How to Insulate a Curved Roof

For curved roofs you’ll be a little limited. But spray foam insulation can offer a great option, giving a full level of thermal insulation across a curved roof. Alternatively, you can opt to insulate your loft floor.

How to Insulate a Dormer Roof 

Dormers are vertical breakout windows within a pitched roof. They are normally installed within converted lofts, or houses with lived-in loft space. You can insulate the rest of the loft roof normally and attach smaller boards within the cheeks of the dormer, if possible. Upgrading to double glazing within the dormer window, and ensuring all gaps are sealed, will also increase the thermal insulation of your dormer roof.

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