Insulating a summerhouse creates the perfect environment for use throughout the year. Done properly, insulation can reduce heating costs and maintain a comfortable temperature.
Many of us choose a summerhouse over a log cabin or garden room thanks to the better value for money and greater choice of models available. In fact, many summerhouses are available to customise to fit the size & space in your garden. Unlike log cabins, summerhouses are designed with tongue & groove cladding and internal framing. This makes them quick and low cost to assemble.
The external walls on most summerhouses are 12mm thick, which can lead to heat loss in colder months and heating up too fast in the summer. So why insulate a summerhouse?
Ideal room temperature is 20 degrees celsius. To make a summerhouse a habitable room during all months of the year, we need to be close to this. A comfortable temperature is key to be able to use your building as a garden office, gym, pub or games room. Leaving equipment in a room with a poorly regulated temperature could lead to damage e.g. if computer equipment comes up against freezing conditions.
A summerhouse without insulation is also extremely expensive to heat. We recommend electric heaters for warming up a garden building. If your summerhouse is drafty or loses temperature fast, the heater is going to be in a constant battle to keep the temperature stable.
Using a 2kW heater for just 10 hours per week (2 hours each week-day) can add £170 to an annual electricity bill. Insulation can increase efficiency and reduce cost.
| Heater Power | 2kW |
| Energy Price | 16.3p/kWh |
| Usage Time | 10 hours/per week |
| Cost | £170/per year |
Using insulation can also prolong the life of your summerhouse (and contents) by preventing temperature changes from damaging the structure over time.
Pre-requisites
- Waterproof & draught free
A combination of insulation and trapped moisture is going to lead to damp and mould in a summerhouse. Before work is started, be sure to inspect the building for any signs of water ingress or draught. A new summerhouse should already be perfectly watertight, but once the insulation is installed, it’s difficult to go back and check.
For summerhouses which are several years old, small gaps may show where the building has aged. A sealant or filler will take care of these and should be left to set in place in advance.
- Framing
The framing inside a summerhouse creates the structure in which the insulation will be placed. Adequate framing is needed to allow insulation to be fitted in-between and boarding to be secured on top. Measure the thickness of the framing as this will determine how thick the insulation inside your walls is going to be.
A minimum of 50mm will be adequate for a good thickness of insulation. Our roof framing was just over 60mm in thickness. For this, we used 50mm insulation boards and left space for a small 10mm air gap between the insulation and roof. The spacing between framing should not be wider than 1.2m (ideally far less) to allow for insulation board to fit correctly.
- Wood treatment
While this step is often not mentioned when insulating a garden building, we had some leftover wood treatment which we decided to put to use. Before painting the exterior of our summerhouse, we used Cuprinol wood preserver as a base coat. This is important to provide protection against rot and decay. We had plenty left so gave the inside of the cladding a covering. Once the insulation is installed, it will be difficult to access the inside of the wall again. Treating the wood gives peace of mind against mould build-up and ensures the longevity of the building.
Which type of summerhouse insulation is best?
The type of insulation which should be used inside a summerhouse is open for debate. Each has its own benefits. We used a combination of insulation boards and Rockwool inside our summerhouse to realise the benefits of both. But sticking with one type is usually easiest to work with and more cost-effective.
PIR Insulation Boards
One advantage of PIR insulation boards over other types of insulation materials is their low thermal conductivity. This means that they are highly effective at reducing heat transfer, which can help to lower energy costs and improve the energy efficiency of buildings. Additionally, PIR insulation boards are fire-resistant and can help to improve the fire safety of buildings.
Used widely in house building, rigid PIR boards (polyisocyanurate insulation) provide extremely efficient insulation. Each board contains a closed-cell rigid foam, with an aluminium foil exterior to reflect heat. The material has a high fire safety value, which is important when used with electrics in a summerhouse.
PIR boards are offered in a different choice of thickness, but provide excellent thermal properties even when thin. Boards range from 25mm to over 200mm. For a summerhouse, 50mm is ideal board thickness for locking in heat, with an R-value of 2.25. Insulation boards are lightweight and their rigidness can hold them in-place tightly between framing. Popular brands include Celotex and Quinthermm.
A downside of insulation boards is they can be expensive, especially when insulating the walls, floor and ceiling. Sometimes local builders may have some for sale which has been leftover from large building projects.
Overall, insulation boards are our favoured method. We also recommend using expanding foam and aluminium tape to seal any gaps with insulation board. More on this later.
Rockwool insulation roll
Rockwool and similar (fibreglass) types of thermal insulation roll are great for both temperature and humidity control. The wool insulation is bound tightly together by resins which also give some waterproofing qualities. As Rockwool is a mineral product, it does not suffer from mould and provides good fire protection, making it a great choice for use in a summerhouse.
One of the benefits of Rockwool over insulation board is added soundproofing. The increased density makes it difficult for sound to penetrate through insulated walls. This is one of the reasons we used insulation roll in the walls of our garden office. Glass wool is slightly different from Rockwool and tends to cause increased skin irritation. Handling both should be done with the correct protection. Compared with rigid boards, insulation roll provides slightly better value for money. However, takes longer to install and adds increased complexity when fitting between framing.
Multilayered foil
Reflective multifoil insulation products such as SuperQuilt are often recommended for a quick solution to insulate garden buildings and conservatories. The interior of the foil is made up of many layers of insulating material to reduce heat transfer. The outer layers contain aluminium foil to reflect heat back into the building, with an R-value of 2.5. Once installed, foil can typically be compressed to less than 10mm, making a good solution for where the framing is thin or space is tight.
Multifoil is rolled out to the length of the wall and then stapled to the framing using a heavy-duty staple gun. Secondary framing needs to be installed on top of the insulation to be able to board over with plasterboard or plywood after. While providing good insulating qualities, the increased framing required leads us towards alternative solutions which are easier to install.
Insulating summerhouse walls
Before beginning the installation of insulation, the pre-requisites including adequate framing and wood treatment should be taken care of. Treating the wood should be done several days in advance to give the walls time to dry out.
Equipment required: Insulation boards, tape measure, cutting equipment, pen, expanding foam, aluminium tape.
1) Firstly the space between each baton needs to be measured accurately. In some summerhouses, the spacing can be different in each section of framing and should be measured separately. Insulation board should be installed with the length running up the wall so we can cut each board as big as possible.
2) With the framing measured, use a pen and straight edge ruler to mark where the cuts to the board will be made. Insulation board should be clamped firmly in place when cutting to get a precise incision.
Cut the insulation board using cutting equipment. Be sure to wear a safety mask and eye protection.
3) The precisely cut insulation board can now be placed in-between the framing. If leaving a gap between the exterior wall and insulation, a piece of wood can be used to create a separation. The process should be repeated throughout the summerhouse walls.
The insulation board should fit firmly in-place between each section in the wall. If you accidentally cut a small part too short, don’t worry as this can be addressed after. Once complete, none of the exterior wood should be on display and the insulation board should sit flush with the framing.
4) Rough cuts and slight inaccuracies in the insulation board and framing can lead to occasional air gaps around the insulation. To address this we can use expanding foam and aluminium tape.
For larger gaps, using expanding foam to fill the gap and push the insulation board tightly against the framing. Wait for the expanding foam to dry and cut away any excess. Expanding foam can be avoided if there are no large gaps.
Aluminium foil tape is used to achieve airtightness and prevent moisture from getting between the insulation and exterior wall of your summerhouse. Foil tape is placed around the outside of each insulation board, covering both the board and framing. Once complete, the exposed wood framing is covered by foil tape.
5) The walls of the garden building have now been insulated to reduce heat loss. The interior walls can be lined and prepared for decorating.
Plenty of options are available for the interior walls of a garden building including Plywood, plasterboard and tongue & groove boards.
Complete guide: Heating a summerhouse
Insulating summerhouse floor
If you plan on using your summerhouse throughout the winter, insulating the floor is an additional step to increase warmth. While most heat is lost through the walls and roof, a small amount is lost through the floor. A cold floor can also create a draft and cold feeling in the room.
The best time to insulate a summerhouse floor is during construction. Insulation boards can be placed between the timber framing in the foundations before floorboards are put down on top. The process is the same as insulating walls and the thickness of the insulation boards will depend on the framing in the base. Boards should be cut for a snug fit and foil tape used for airtightness.
If your summerhouse is already installed, insulation can still be added to the floor. Insulating underlay can be placed underneath laminate flooring to reduce heat loss.
Insulating summerhouse roof
Installing insulation in the roof is very similar to the walls, with a slight bit more complexity. Insulating the roof is essential as a huge amount of heat is lost through the roof as heat rises throughout the day. In the summer, black felt on the roof also causes heat to penetrate the summerhouse and make the temperature inside too warm.
The only downside is losing the detail of the roof rafters on the inside. For this reason, many log cabins are built with the insulation board on top of the cabin. With your summerhouse likely already built and roof already fitted, its easiest to fit the insulation between the rafters on the ceiling.
Care must be taken to leave a sufficient air gap between the insulation and the roof board in a summerhouse. The warmer and with higher moisture content rises throughout the day into the ceiling. As it reaches the colder underside of the roof, the vapour turns back to water and creates condensation. Ventilation should allow air to flow in on one side and out of the other side of the roof to reduce condensation and moisture levels.
We recommend using PIR insulation boards for a summerhouse roof. The lightweight and rigid design allow them to be easily fitted in between rafters without putting additional strain on the building. The same process can be followed from when installing insulation boards in a summerhouse wall. Packing Rockwool into the roof will make it difficult to maintain enough of an air gap to keep the roof ventilated.
Further Considerations
Do I need a vapour Barrier or breathable membrane?
While neither is a requirement, both can provide additional comfort and enhance the longevity of your garden building. Whether to use either depends on how much of the summerhouse has been constructed and the type of insulation used.
A vapour barrier is plastic sheeting placed against the warm side of the insulation. This prevents vapour from penetrating into insulation where it will cool to condensation and lead to dampness over time. If using insulation board and taping the edges, the foil will act as a vapour barrier to block moisture.
A breathable membrane is placed on the cold side of the insulation to prevent rain and water from passing through to the insulation. At the same time, vapour and moisture build-up can still pass-through and evaporate. A membrane is installed between the framing and external cladding, making it difficult to install in a pre-built summerhouse.
Both are inexpensive to install. For an easy solution with an already constructed summerhouse, foil-backed insulation with taped up edges should be sufficient.
Guide: Installing a summerhouse vapour barrier
Which wall lining should I use?
Once the insulation is complete, interior walls can be lined and boarded. The choice will depend on the budget and finish required in the room. Some popular options include:
Plywood – A robust and hardwearing finish suited to most summerhouse use cases. For workshops and gyms, plywood provides a strong lining and can be used if mounting accessories e.g. shelves onto walls. Plywood can be sanded and painted for a nice smooth finish.
Plasterboard – Great for garden pubs and offices to create a homely feeling inside. The plasterboard will be required to be skimmed once fitted.
OSB – While not our choice, OSB is used where a low-cost option is required. The board can be painted easily and works well for a garden gym.
Tongue & groove – An expensive option, T&G provides a great finish which gives the feeling of a cosy log cabin.
How much does it cost to insulate a summerhouse?
Insulating a summerhouse can be expensive initially. The idea is that reduced running costs and comfort over-time will off-set the initial costs. 50mm PIR insulation board can be picked up for near £40. Buying in bulk or seconds can reduce cost.
Brands including Celotex and Kingspan are most expensive. Many builders merchants will sell less well-known brands which offer similar R-values for a lower cost.
If insulating a small garden office 10×8 summerhouse, we would estimate 12 sheets at a cost of £480 for insulation boards. Additional costs including foil tape and tools required will increase the cost further.
Foil insulation and fibreglass roll insulation can both be picked up for less.
Does my summerhouse need ventilation?
Spending a long time in your garden room with windows and doors closed is going to lead to moisture build-up in the air. This is less of a problem in sheds as they often have some gaps near doors and are not insulated.
Over time, without adequate insulation, moisture can begin to cause rot and damage the structure of the building. Condensation can also be noticed on the windows when a summerhouse does not have enough airflow.
Opening windows and doors often is the easiest way to ventilate airflow but is not going to happen often in the winter. Installing a small passive wall vent will allow the air inside to be replaced with fresh air from outside.
How do R values work?
The R-value is the measure of how much resistance there is to heat flow through a given material and thickness. A higher R-value indicates better insulating properties. A thicker piece of the same brand insulation board will have a higher R-Value.
A Celotex PIR board has an R-value of 2.25. A 100m version of the same product has an R-value of 4.5.
When do I install electrics?
The installation of electricity needs to be considered before insulating a summerhouse. Typically, the wiring will run between the insulation board and the cladding. A first fit should be carried out before insulation is installed. Space is cut through the insulation board for the wires to run through and then again through the interior boarding.
If a summerhouse has already been insulated and boarded, trunking will be required for installing electricity sockets.
Summary
Insulating a summerhouse creates a warm and comfortable atmosphere which is habitable throughout the year. In the winter, insulation reduces heating costs and stops heat from being lost. In the summer, insulation helps a summerhouse keep cooler for longer.
A summerhouse should be properly prepared for insulation with adequate framing, wood treatment and any gaps sealed.
While different options are available, we recommend foil-backed insulation boards for the easiest install and greatest insulation values.
How will you be insulating your summerhouse? Let us know in the comments below.
Great article. I have a summerhouse and am looking to insulate it. My framing thickness is just over 40mm. What insulation would you recommend?
I was looking into 30mm or 25mm kingsman?
Thanks
Yes, both sound like a good option. Be sure to leave the gap on the cold side.
A really good read, thanks. I have a question, I have bought PIR boards for my roof insulation which I will leave an air gap between the roof and the PIR board (the roof is covered with EPDM), but how do I create air flow in the gap to avoid moisture or does it not require this if it’s all sealed?
Cheers
Helpful article, thanks. Is there any point in insulating the walls of my summerhouse when it has single glazed doors and windows?
The insulation will still noticeably improve heat retention, but most of the heat will then be lost through the windows & doors instead of the walls. Consider adding an extra layer to the window using a material such as perspex.
Great article!
I have a query on PIR boards and breathable membrane.
If I have a breathable membrane fitted on the walls do I need to still leave a gap between the PIR and the membrane or can it push right up to the outside walls?
thanks
Thanks for this excellent article. I’m ordering a summer house to act as an outdoor office / relax space. The only question I have is – what size gap do you recommend between the PIR board and the external wall? Would even 5mm be enough?
We recommend leaving a minimum of 10mm. Slightly more if you have space without compromising insulation thickness.
On a prebuilt log cabin with 12mm thick walls would it be possible to insulate from the outside? As 3 sides are not on show the aesthetics of a nice exterior are not as important as the internal space. Is this possible?
Insulating the exterior will be difficult as you’ll need something to cover it such as weatherproof boarding. We recommend sticking with insulation on the interior side of the walls.
Hi, I’ve really been struggling to work out the best way to insulate the roof. I’ve read warm roofs are better than cold roofs but i don’t have space to put loads of insulation above the rafters. Could i put say 50mm directly on top of the raftersdeck and also 70mm between the rafters?