roof insulation

How to insulate a garden room, summerhouse or shed

Thinking about turning an old garden building into a place you can use no matter the season? That’s exactly what I did! I’m sharing my experiences as I insulated both a shed a couple of years ago and more recently, a log cabin that I turned into a garden office. It’s a pretty simple process and most folks can make their own cosy and functional space for enjoying all year round.

insulate a garden office

My garden room office (pictured above) was purchased from GBD. View current pricing here.

When you insulate garden buildings, you achieve two key benefits. Firstly, you enhance the building’s thermal efficiency. This means that any warmth generated inside the building stays trapped for longer, seeping through walls, floors, and ceilings at a slower rate. As a result, this insulation helps keep your heating expenses in check. Secondly, the insulation process also has a reverse effect. In the summer, it reflects the sun’s heat away from the building, thus helping to maintain a cooler interior temperature.

Within a normal house, heat is lost through the following different areas. A garden building is quite similar to this. Here are the percentages of heat loss you could expect:

  • 25% through the roof
  • 35% through the walls
  • 10% through the windows
  • 15% through the doors
  • 15% through the floor

As you can see heat is lost in all directions, so it’s important to consider insulating every aspect of your garden building.

In this article, I’ll be addressing three main topics: How to insulate a summerhouse, how to insulate a shed, and how to insulate a garden room. These techniques are versatile and can be applied to various types of garden buildings. As a starting point, I’ll provide a general guide that can serve as a foundation for most structures. It’s also worth noting that these techniques can be used to insulate your garage, playhouse, treehouse and most other garden buildings.

As you continue reading, I’ll delve into more specific instructions for each particular type of garden building.

Step-by-step process to insulate your garden building

Step 1. Build a supporting frame in which the insulation sheets will sit

Prepare to lose some of the interior space within your garden building. Insulation and finishing can take up between 30 & 120mm. The supporting frame should be thick enough to house sheet insulation such as PIR boards. These come in a number of different thicknesses so you’ll need to decide how thick you want the insulation before you buy the timber battens for the frame. If you have a small garden building then the thinner insulation might be the preferred option.

garden room interior works

Step 2. Add a moisture barrier to protect the internal walls and build-up of mould

A moisture barrier or vapour barrier is easy to install and doesn’t have to be expensive. For a 2.5m tall roll, you can expect to pay about £24 for a 25m length. If you are using PIR boards the vapour barrier should be installed over the top but if you are using a soft, wool-type insulation then the barrier should be installed before the insulation.

Another option if you want to buy locally is this vapour barrier from Wickes. It is ideal for the job and is very reasonably priced.

Step 3. Cut and fit the insulation

PIR boards are easily cut with a sharp knife or panel saw. The boards will be cut to fit in between the supporting frames. When cutting to size ensure they are a perfect fit. There should be no gaps left anywhere within the wood frame.

PIR boards also act as a great moisture barrier.

If you are using wool insulation like Rockwool then this can also be cut with a sharp knife.

Here are the Celotex (or Kingspan) boards I used on my project. Celotex and Kingspan and basically the same product.

Kingspan insulation boards

These are the Rockwool insulation slabs I used on my shed conversion project. They are perfect acoustic and thermal insulators.

Rockwool RW45 insulation slabs

For those who are keen on maximising both thermal and acoustic insulation, consider using products from Galaxy Insulation & Dry Lining. Their Rockwool wall insulation not only excels in thermal performance but also offers excellent acoustic insulation, making it a great choice for garden buildings where you might want to block out external noise. Whether you’re planning to use your garden building for music practice, as a home office, or simply as a quiet retreat, Galaxy Insulation provides an effective solution for making your space more comfortable year-round.

Step 4. Replace windows with double-glazing units

Replacing the windows and doors on your garden building will dramatically increase thermal efficiency by up to 50%. Double glazing can be expensive but this step should not be ignored.

Step 5. Cover the insulation with a suitable sheet material like plasterboard

I finished the interior with plasterboard, then skimmed it with plaster for a nice clean and smooth finish. This could then be painted for a presentable finish. Depending on your preference, you could also finish the walls with softwood cladding or a sheet wood material.

plasterboard ebay
Standard square edge plasterboard

Step 7. Use insulation boards to cover the roofing sections

The same insulation boards can be used to line the interior of the roof. PIR boards can be glued into place and then covered with sheet materials or cladding.

Roof insulation job garden room

Step 8. Insulate the floor with a suitable underlay

Assuming you have already built your garden building, the best way to insulate the floor is to install a thick thermal underlay and laminate flooring. A sub-layer floor can be installed with PIR boards but this will raise the floor significantly which might not be ideal for your build. If you haven’t built your garden building yet then you have a great opportunity to build a properly insulated sub-base which I will describe in more detail below.

Floor insulation job garden room

This completes the basic process for insulating your garden room, shed or summerhouse, so let me go into some more detail about each section.

Insulating the roof and walls

You may be limited with the thickness of insulation you can use on your walls and roof. The slimmest insulation available can be as small as 3mm, but this isn’t going to be effective. If possible, use a 25 – 50mm insulation board or wool. I’ve used both before and either can be effective. The wool insulation can also provide an acoustic barrier which can be good if you are using your space for music production, DJing or using noisy power tools.

roof insulation

Read on below to see the different types of insulation I have previously used in my garden buildings.

Insulating the windows and doors

There are a few key ways to improve thermal insulation in your windows and doors.

  1. Replacing the doors and windows for double-glazing units is the best way to insulate your building. You can easily fit house-quality (uPVC) windows and doors into your garden building. I have fitted these windows and doors to my really simple shed.
  2. Enhance your current windows and doors with secondary glazing, a cost-effective solution that you can install yourself. It can be customised to fit any window or door shape and can boost thermal efficiency by up to 65%. You can find cut to size Perspex secondary glazing at Cut My.
  3. You can also use rope caulk to help seal any gaps in the window and door frames. This will help prevent heat from escaping through obvious spaces in your building.
garden office 2

Insulating the floor of your garden building

This can be tricky if you are unable to access underneath the building. If this is the case the only real option is to insulate on top of the existing floor. This will mean raising the floor which can also create a problem. The best way to approach this is to lay a thin (but good insulator) 3-4mm underlay and put a reasonably thick floorboard down. I have previously used a gold standard thermal underlay before which is perfect for this situation.

installing gold underlay garden room

If you can access underneath the building then the best idea would be to install a thick 100m foam board insulation sheet. These can fit in between rafters or a small sub-base can be built to house the sheets. It doesn’t have to be 100mm but the thicker the better.

Floor insulation garden room
100mm insulation boards between rafters

Here is an image of my garden room build. I installed an extra layer of foil covered bubble wrap underneath the floorboards for an extra layer of insulation.

floor insulation

When insulating a garden building floor. try and add a number of layers of insulation for the best results. Warm air will be trapped between each layer for better thermal efficiency.

Read all about the different types of shed bases here to get an idea of what is possible when trying to insulate your shed base.

How to insulate a shed floor

A shed floor will differ slightly from my garden office build. You might not have access underneath so the only option is to insulate on top of the floor. You can do this effectively, but the floor’s thickness will need to increase. This will mean a step up into your shed, which can be fine if you’re ok with a little less headroom.

In the above image, I’ve also used a slightly cheaper insulation bubble roll under the floorboards. Foil backed insulation rolls are perfect where space is tight. I’ve used these insulation rolls as an addition to other insulation boards. They also help create a vapour barrier to prevent moisture from rising up through the floorboards.

It’s also a great idea to install a good thermal underlay if you are planning on installing laminate flooring in your shed, summerhouse or garden office. I decided to use this gold foil-backed thermal underlay which is a great insulator. It’s a bit more expensive than other types of underlay but it’s worth spending the extra for that additional thermal insulation.

Gold thermal underlay

This thermal gold underlay will cost you about £75 for 15m2 and is thicker than a standard 3mm underlay. This one is 5mm thick and covered with a gold foil moisture-proof barrier. Laminate flooring can be installed directly on top without the need for any adhesive.

laminate flooring on thermal underlay

Now my garden office has two layers of thermal insulation which is ok for most of the year. If you have the option to install the foam boards before your garden building has been constructed then I would highly recommend it.

What type of insulation is best for a garden building?

Garden buildings typically have thin wooden walls and are therefore not great insulators. To help improve this we can make them thicker, add foam or wool insulation and create a watertight, airtight barrier. Here is a cross-section view of a well insulated garden building.

Garden room wall cross section

I have successfully insulated two garden builds now. I used different insulation for each project based on research into the best types to use in each situation.

For my first project, I needed an insulation that could provide both thermal and acoustic insulation. The best product for this application is a Rockwool RW45 insulation slab. These come in 3 different thicknesses, 50mm, 75mm and 100mm. You will need to decide which is best for your project. How much interior space are you willing to sacrifice to insulate your building?

Here is an image of the 50mm Rockwool RW45 shed insulation being installed in my DIY garden shed.

insulation in shed

The second project was to insulate my BillyOh Kent garden office. For this, acoustic insulation wasn’t a big priority so I decided to go with a 25mm Celotex insulation board. The thinner 25mm insulation was selected because I didn’t want too loose much of the 4x3m interior space.

The other great thing about this board is that it also acts as a vapour barrier, preventing moisture from reaching the inner walls, which in this case was a 12.5mm plasterboard.

Celotex insulation boards in garden office
25mm Celotex insulation boards in my garden office from BillyOh

The walls for each project were covered with plasterboard and plastered for a perfect seal and smooth finish. If you don’t go with the foil-backed sheet insulation then it’s a good idea to seal your walls with a vapour barrier which will help prevent mould and moisture from building up behind the plasterboard. This should be installed on top of the insulation before the plasterboard is fitted.

plastering garden office

How to insulate a garden room

A garden room or garden office is normally designed to house a thicker layer of insulation within its walls, floor and roof. That’s why you will find it much easier to insulate a garden room than you would a shed or summerhouse.

The walls of a garden room will normally be made up of 89mm thick timber battens. This gives you more room to play with insulation options. For the best results, I would recommend using both a foam board (like Colotex) and a Rockwool insulation. This way you will get the benefits of the thermal protection on the foam boards and the acoustic insulation of a wool product. You should be able to fit a 50mm foam board and a 50mm wool insulator into the 89mm gap.

Starting from the interior, here is a list of materials that make up the walls of a well insulated garden room:

  1. Paint on 12.5mm plasterboard
  2. Timber frame constructed from 89mm C16 timber
  3. Within timber frame – 50mm Rockwool RW45
  4. Within timber frame – 50mm Celotex boards
  5. 18mm OSB 3 boards
  6. Breathable membrane
  7. Furring strips
  8. Cladding – Ideally a redwood cedar cladding at least 18mm thick
cross section garden room walls

The floor should be similar to that listed above with a timber frame housing 100mm foam boards, topped with 18mm P5 flooring chipboard. P5 boards are ideal for this application due to their moisture-resistant qualities.

A garden room roof could take many different shapes, sizes and styles but here is a look at a well insulated pitched roof.

well insulated pitched roof

Further insulation can be added to the interior but this will depend on the overall look and feel of your personal project.

How to insulate a shed

Sheds are typically constructed from panels made out of a thin cladding, fixed to a thin timber frame. This can make it tricky to insulate. My advice would be to bridge this gap with a thicker timber batten and create a larger gap that can house the insulation boards.

In most cases, a flat-pack shed will have framed panels constructed with 25x25mm batten timber. If you attach a 25x50mm batten timber on top of this you will seal the gap and create this perfect void on the panel to which you can instal some 50mm foam insulation panels.

Insulate garden shed guide

Red markers indicate additional batten – Blue indicates where thermal insulation boards are added

25x25mm batten can be added on top of the existing frame where there is no join. Insulation panels can then be cut to size using a saw and inserted into the gaps. It’s then easy enough to install panels over the top of this to seal and finish the interior.

Interior walls can be finished with plasterboard, chipboard or plywood board for extra strength and thermal insulation. Again, thicker boards will provide better insulation. It’s also worth noting that a small gap between the boards can be a good thing. Warm air can get trapped in this small gap which will increase the thermal efficiency of the building.

Garden sheds are typically made with thin 11mm spruce wood. Not a great insulator! There are however many options for increasing the thermal efficiency of even the thinnest of sheds. Rhino Sheds offer a range of buildings that can easily be insulated and converted into a workspace, studio, gym, games room or whatever your imagination can muster up.

How to insulate a summerhouse

A summerhouse will differ slightly from a shed or garden office in that it will likely have more windows and larger doors. As described above, 25% of the heat will be lost through windows and doors. But, as there are more on a summerhouse you can expect to lose up to 50%.

Windows and doors can be upgraded with double-glazing glass panes which will help reduce heat loss. Another great way to prevent heat loss it to add seal strips around the doors and windows to ensure air cannot easily travel through gaps.

The rest of the insulation process will be similar to that of insulating a shed. The walls, flooring and roof can have insulation panels fitted and sealed. The interior can be finished with cladding, wooden sheet materials or plastered and skimmed as I did in my garden office.

An insulated garden summerhouse opens up a number of opportunities for its use, and they look pretty amazing too. Whether it remains as a relaxation space or you decide to use it as an office space, you’ll have far greater flexibility with this insulated space.

Planning permission for an insulated garden building

Just remember, if you are planning on insulating your garden building to sleep in, you will need to apply for planning permission. Even if you are thinking of sleeping in there a few times a year, you will need planning consent from your local council authority. You can read more about planning permissions for sheds in my other post. Click the button below to view the full article.

How to keep your garden building warm

With the addition of insulation your garden room will now be able to retain heat much more efficiently. You now have to consider how you’re going to produce the heat in your outdoor building. To do this cost effectively you’ll likely want to avoid expensive electric heaters and consider a wood burning stove or even a gas fired heater.

If you already have a good source of wood then a wood burning stove is going to be a sensible option. It will have the highest up front cost but will be inexpensive to use in the long run. The fire might take a little while to get going (15-30 minutes) and will require attendance every hour or so but will produce plenty of heat all day long.

A portable domestic gas heater will give you instant heat which is easy to control. Gas can be sourced locally and a single bottle will last you many hours. Typically a 15kg butane gas bottle used on a low setting will last 147 hours and 73 hours on a medium setting.

In comparison, an electric heater will cost more, especially with energy prices as they are now (Jun 2022). You can read more about costs in this useful article.

How can I insulate my shed cheaply?

If you are trying to achieve this project on a budget then there are ways to do this. If you are looking to save some money then consider using packing materials and recycled clothing to insulate your roof or walls. Polystyrene is a great insulator and if you can save enough it can be used to pack out the walls and roof cavity. This can however be a time consuming process. Ask friends and family to collect it for you or look on FB Marketplace for any freebies. You can also look in your loft for any excess insulation. I’ve been in many lofts with way too much insulation stuffed in. (Not sure if builders have over compensated for something?)

For my garden office I was stuffing the ceiling void with clothes, packaging, soft insulation and polystyrene from deliveries which makes for great insulation. We also used some recycled packing from our frozen meal delivery service which was amazing. It’s made from recycled jeans. You can just see it in the top of this image.

insulation in roof of garden office

Bubble wrap can also act as a good insulator. The small bubbles trap warm air and with a few layers can be very effective. A layer of foil thrown in the mix is also very beneficial although I’m not sure how many rolls you’d need!

Old clothes can be utilised to insulate your garden room but they will perform better when shredded. I have previously used recycled packaging materials made from shredded jeans.. They are a great insulator and never in short supply.

BillyOh Kent Garden Office
BillyOh Kent Garden Office

What is the best material to line the inside of a shed with?

As you can see I have converted a shed into a warm workshop and a garden building into a functioning office. So depending on your desired use you may wish to have different materials to line the inside.

An office will look best when finished with skimmed plasterboard, but a workshop or gym might benefit from something stronger like plywood or OSB boards.

That’s all for now. Insulating a garden room, shed or summerhouse doesn’t have to be complicated or expensive and can give you many returns. I hope you have found this guide useful and will now be able to make better use of that garden building.

Thanks for reading.

Why not try one of my other related posts:


  1. Excellent helpful article! What is your review of the BillyOh log cabins? They’re the cheapest I can find and look great for the money, but wondered if the build quality was up to scratch?

  2. Can’t thank you enough for your shed articles. I’m moving house and it’s likely that I’ll be buying something in the region of a 20×10 – your information has prepared me!

  3. Thanks for the article.

    Are you in the UK? And was 25mm PIR sufficient? Asking because I’m in the same boat.

    • Thanks Martin. Yes, I’m in the UK. 25mm is ok. It holds the heat pretty well but obviously the thicker the insulation, the better. I would have gone for thicker but I didn’t want to lose too much of the interior space.
      It’s also good at keeping heat out during the summer.
      Hope this helps

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