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How to Insulate a Bay Window

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?

  • Save money on heating bills by reducing heat loss.
  • Reduce condensation on your bay window single skin wall.
  • Reduce drafts coming from your bay window.
  • Eliminate mould on your bay window single skin wall.

Things to Consider when Insulating a Bay Window

What Type of Insulation to Use?

When insulating your bay window wall you need to take into account the window board depths, and also the overall space you have in your room.  We prefer to use Polyisocyanurate (PIR) board that has been bonded to plasterboard (such as ones manufactured by Celotex or Kingspan).  This is ideal becuase PIR board is an excellent thermal insulator, and having it bonded to your plasterboard helps to save time and space.

What Insulation Thickness?

For a solid brick wall under your bay window, any PIR insulation that is 50mm of more would be good.  In general, PIR insulation is twice as effective as rockwool or glass fibre insulation.  This means PIR insulation does not have to be as thick to achieve the same insulating properties.  For example, 50mm thickness of Kingspan or Celotex is equivalent to 100mm of rockwool or glass fibre wool.

What Walls require Insulation?

Most people only insulate under the bay window.  Side walls also need insulating if no cavity exists.  Also consider the space above your bay window, such as inside the cladding.

List of Materials

50 mm Celotex bonded to plaster board

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To be used on a single brick wall under a bay window, on the inside.  Celotex PL4050 is a thermal insulated plasterboard.  This high performance thermal insulation board has 50mm of PIR insulation bonded to 12.5mm tapered edge plasterboard.  Insulated plasterboards are not usually sold in local shops in the UK (such as B & Q or Wickes), which means you will probably need to source them online.  Just Google search for the code “PL4050” if they are not available at the Amazon store.

25 mm Celotex bonded to plaster board

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This is similar to the thermal insulated plasterboard above, except it has 25mm of PIR insulation bonded to 12.5mm tapered edge plasterboard.  These types of board are ideal if you cannot use thicker boards like the one above.  For example, you may require thinner insulated plasterboard for the interior side walls surrounding your bay window.

Treated Softwood Timber Battens 

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These treated timber battens will be attached to the interior wall under your bay window.  Once fixed to the wall, your insulated plasterboard will then fix to the battens allowing a 25mm gap between your wall and insulated plasterboard.  The gap helps to stop damp being transferred through your wall.  We recommend battens about 25mm in depth and 50mm wide.  Length depends on bay window height.  They must be resistant to moisture.

75mm Deck Screws

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The screws are 75mm long, which should be long enough to penetrate thorough the insulated plaster board (50 mm insulation + 12.5 mm plasterboard) and into the 25mm of batten.

Water Repellant Liquid

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A water seal should be applied to the external brick surface to help prevent moisture seeping through the brick.  You must ensure the water seal is also breathable, like the one below.

List of Tools

Electric Drill with Pozi Drive

Buy this Compact Drill/Driver Kit at the Amazon store

An excellent cordless drill popular with building contractors.  This powerful Milwaukee drill comes with charger, battery and case.  Ideal for screwing your timber battens to the wall, and the insulated plasterboard to the battens.

Red or Brown Rawl Plugs

Rawl plugs are a well known brand of wall plugs.  Do you need advice on how to select and install wall plugs for solid brick walls? If so, then please check out our wall plug article

Hand Wood Saw

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You will need a handsaw to cut your timbers and insulated plasterboards to the correct lengths.

Tape Measure

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Need to measure your timbers and insulated plasterboards.

Work Bench

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Cutting timbers or anyting else is always easier with a workbench.

Carpenters Pencil

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You will be surprised how much easier things are to mark with a larger pencil.

Spirit Level

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A spirit level is needed to make sure your new insulated plasterboard wall is completely vertical.


Buy this hammer from the Amazon store

Safety Wear

It is recommended a dust mask is worn during cutting.

How to Insulate a Bay Window.  Step by Step Guide

Step 1.  Understand the Construction

The diagram below shows how your bay window single skin wall will be insulated.  Let me explain how and why this insulating system prevents mold, and improves the thermal insulating properties of your wall.

Why it Prevents Mold:

Bricks are a porous material (like a sponge), which means rain water can easily seep through the bricks from the outside.  This means moisture can easily build up on the internal surface of the single skin wall, and eventually leading to mould build-up.  The single skin wall is also cold on the inside, which means warm moist air inside your home will codensate on the wall, creating more moisture and mould build-up.

The system shown in the diagram below shows a water repellent layer (Item F) on the external brick surface, helping to prevent this moisture transfer in the first place.  A layer of insulated plasterboard (Item C) has also been fixed to the internal wall surface with a 25mm gap in between.  This creates a cavity wall system which prevents further transfer of moisture from the single skin brick wall, to your internal plasterboard wall.  It is also important to note there is a 10mm gap at the bottom of the insulated plasterboard to allow airflow, and any moisture leakage.  Another benefit of this insulated plasterboard system is that your internal wall will be warmer, which helps prevent mosit air condensating on it.

Why it will make your room warmer:

Household bricks are not good thermal insulators.  This means heat can easily escape from your room via a single skin wall, especially if that wall is damp.  The insulated plasterboard and the 25mm air gap helps to slow down this transfer of heat, therefore keeping more heat in your room for longer.

How To Insulate A Bay Window Single Skin Wall

Step 2.  Preparation

Planning on replacing your bay window? if so then change it before you start insulating.  We replaced an old aluminium framed bay window for a new uPVC one.  Also we replaced the old timber cladding above the window for new uPVC cladding.  When changing the cladding above the bay window we noticed it was just a viod behind it, so decided to fill it with 100mm of celetox.  Walls surrounding the window are single skinned.

Bay window beforeFig 1.  Bay Window Before.

Bay window afterFig 2.  Bay Window After.

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


Fig 3.  Bay window inside.  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.

how to insulate a bay window

Fig 4.  Preparing the bay window surfaces.  Inside view.

Step 3.  Apply a Water Repellant to the Outside Brick Surface

It is essential you select a water repellent liquid that will also allow vapour to pass through.  This will help prevent the transfer of moisture whilst also allowing your brickwork to breath.  Ensure all single skin walls surrounding your bay window is coated with this liquid on the external brickwork.

Step 4.  Fix Timber Battens to the Wall

  • Fix timber battens to your single skin brickwork, below your bay window (interior).  Using 4 screws for each batten, fix to the single skin brick wall vertically at 600mm centers using wall plugs and screws.  Battens provide a 25mm air gap between your wall and the insulated plasterboard.  This is to allow moisture to escape through the porous bricks.  The battens also help support the side edges of the insulated plasterboards.  Ensure each batten has a 10mm gap at the bottom to allow for expansion and airflow.
  • 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 5.  Fix Insulated Plasterboards to Timber Battens

Cut insulated plasterboards 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.

how to insulate a bay window

Fig 5.  Bay window insulated plasterboards attached to the battens.

Fix the insulated plasterboard sheets to the timber battens.  Use 8 screws per sheet.

how to insulate a bay window close upFig 6.  Bay window insulated plasterboard (celotex PL4050).  

We also recommend insulating the window side walls.  However, you will need thinner insulated plasterboard, such as 25mm of PIR bonded to 12.5mm of plasterboard.  This bay window was also insulated above using rock wool.  It used to have a recessed ceiling.

how to insulate a bay window topFig 7.  Bay window ceiling insulation (100mm rockwool)

Step 6.  Decorate your Room

Next you will need to plaster over your insulated plasterboards and attach the window boards.  You will then need to decorate.

Below are images of the finished room, turned from a bedroom into a modern dining room.  You will probably agree that it is quite a transformation!  You may be wondering how we fixed the wooden skirting to a curved wall.  Well, we simply bridged the skirting across two chairs, then placed a brick in the centre.  Then left it for a few days.  This made the skirting naturally want to bend, and therefore easier to glue or screw in place.  MDF skirting board that is thinner is easier to bend.  You can also make shallow cuts in the rear of your skirting to help encourage it to bend further.

dining-room-finished-3Fig 8.  Finished room with insulated bay window.

dining-room-finished-2Fig 9.  Finished room with insulated bay window.

We hope you have enjoyed this how to insulate a bay window guide.  Please feel free to share this post on your social media.


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