In:Carpentry DIY, Insulation DIY, Window DIY

Comments Off on How to insulate a bay window | Warton Woodworks

How to insulate a bay window

By Warton Woodworks

Wondering how to insulate a bay window? this guide will help.  Most older properties lack insulation around bay windows, some have none! Insulation must be installed correctly for it to be effective.  This how to insulate a bay window guide, covers windows with single skin walls only.

Why insulate a bay window?

  • Reduces heat loss and heating bills.
  • Reduces condensation due to warmer interior surfaces.
  • Reduces drafts.
  • Eliminate mold around bay window, due to warmer internal walls.

Things to consider when insulating a bay window

  • What type of insulation you will use?  We prefer to use celotex insulated plaster board.
  • What insulation thickness?  Use this insulation calculator as a guide.
  • What walls needs insulation.  Most people only insulate under the bay window.  Side walls also need insulating if no cavity exists.

 How to insulate a bay window – Step by step guide:

Whats needed?

  • PIR Insulated plaster board.  50mm insulation for wall under bay window.  25mm insulation for window side walls.
  • Treated softwood timber battens.  About 25mm depth and 47mm wide.  Length depends on bay window height.
  • Drywall screws.
  • Wood saw.
  • Tape measure
  • Pencil.
  • Spirit level.
  • Work bench.
  • Electric screw driver
  • Hammer.
  • Rawl plugs.

Safety wear

  • Dust mask.
  • Googles.

Step 1 – Preparation:

  • Planning on replacing your bay window? if so then change it before you start insulating.  We replaced an old bay window for a new uPVC one.  Cladding above the window had no insulation behind it, just a cavity.  Will filled this cavity with 100mm of celetox.  Walls surrounding the window are single skinned.
Bay window before

Bay window before

Bay window after

Bay window after

  • Next prepare the room inside, so that the bay window can be insulated.

Bay window room before


  • The image below shows the room completely bare.  It is important to remove all wall paper from around the bay window.  Any lumps protruding from the window wall should be removed.  Also remove your internal bay window boards.  All loose plaster should be chiseled off.

Preparing bay window surfaces


  • You are now ready for step 2.

Step 2 – Fix timber battens to the wall:

  • Ensure your bay window single skin wall is permeable.  This means the wall must allow moisture to pass through it.
  • Fix timber battens below your bay window (interior).  Use 4 screw for each batten.  Fix to the wall vertically at 600mm centers using rawl plugs and screws.  Battens support edges of the insulated plaster boards.  The gap between wall and plaster board should be 25mm minimum.  This is to allow moisture to escape through the porous bricks. Ensure each batten has a 5mm gap at the bottom to allow for expansion.
  • Now fix some battens to the wall horizontally, so they will support the top and bottom of each plaster board.
  • Read this guide from Celotex for more information regarding insulation applications.

Step 3 – Fix insulated plaster boards to timber battens:

  • Cut insulated plaster boards to the width of your battens.  The bay window below needed 8 panels, cut 600mm wide, then 2 smaller width ones at the ends.  This is a great example of how to insulate a bay window.

Bay window insulation

  •  Fix the insulated plaster board sheets to the timber battens.  Use 8 screws per sheet.


  •  We also recommend insulating the window side walls.  However, you will need  thinner insulated plaster board, around 30mm.  This bay window was also insulated above using rock wool.  It used to be a recessed ceiling.

Bay window ceiling insulation

  •  Below are images of the whole room finished.  The room is much warmer since the bay window has been insulated properly.  Did you manage to follow this how to insulate a bay window guide? please let us know.

Finished room with insulated bay window




We hope you have enjoyed this how to insulate a bay window guide.  Please feel free to comment below.

Tags: , , , ,