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Is polystyrene insulation any good for a shed?

Before insulating a shed, it's important to make sure the building is water-tight and damp free. Treating the interior walls with a wood preserver is an extra step to provide peace of mind. Insulating using EPS or PIR boards is our recommended option. Both of these allow for easy installation and provide great results.

gardenroom.guide - How to Insulate a Shed - Garden Room Guide

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Using insulation inside a shed can provide greater control over the temperature and reduce heating costs. A shed can be insulated affordably and completed over a weekend. The timber shed has been a feature of most gardens up and down the country for many years. More of us are spending an increased amount of time in our sheds than ever before. The garden shed has gone from just a location to store tools to an extra room in the garden. The bad news is that most garden sheds are not designed with comfort and temperature control in mind. During the winter the temperature can be the same as outside and in the summer the temperature inside can be extremely warm.

Reasons to insulate a shed

If you’re planning on spending a lot of time in your garden shed, insulation is a great idea to keep the heat inside as well. The ideal room temperature is 20 degrees Celsius. For using your shed throughout the year, we need to aim to stick within a few degrees of this temperature to create a comfortable environment. Some of the uses where insulation is a great idea are when the outbuilding will be used as a garden office, man cave or pub shed. All of these activities are fairly stationary, meaning you will soon feel the chill on a cold autumn or winters day. Protecting fragile equipment is another reason to insulate a garden shed. Electrical items in storage can be affected by extreme temperatures which could cause damage. The idea behind insulation is to reduce thermal transfer through the sheds walls, roof & floor and reduce variation in the temperature. A shed can also become extremely expensive to heat due to how quickly warmth can be lost. The wall thickness of most sheds is around 8mm thick, providing little resistance to heat being lost. The answer for most will be to install an electrical heater inside the shed, but without insulation, the heat will be leaving nearly as fast as it comes in. Using a 1.5kW electrical heater for 7 hours per week (1 hour per day) can add approximately £90 onto the average annual electricity bill.

Heater Power 1.5kW Energy Price 16.3p/kWh Usage 7 hours/per week Price £89.30

Guide: Heating a garden building

Before insulating a shed

Before starting, we need to ensure our shed is suitable for being insulated. Many sheds over 8 years old can be damaged or warped, making it difficult to insulate when there are other underlying issues that can cause heat loss.

Interior framing

To insulate a shed, it needs to have adequate framing on the interior. The framing allows insulation to be placed in between and sit comfortably in the sheds interior walls. Without wooden framing, it’s going to be difficult to install insulation and there won’t be anywhere to easily slot the insulation in. The thickness of the framing will also decide how much insulation can be placed inside. A thicker frame allows for thicker insulation to be placed in the wall. Begin by using a tape measure to confirm the thickness of your shed framing, many are 28mm. When we install insulation, we leave a slight gap between the insulation board and the exterior wall. 28mm framing will allow us to use 20mm or 25mm insulation boards in the wall.

Sealed & watertight

Before insulation is installed, the shed needs to be completely watertight and drought-free. Gaps in the wall will allow heat to easily escape and reduce the effectiveness of the insulation. Insulation and moisture create the perfect environment for damp in the walls which can be difficult to treat once the insulation is in place. Wood filler or sealant can be used to cover any small gaps. Be sure to check near the windows and doors as this is where gaps are most likely to appear.

Wood protection

A commonly missed out step is to treat the interior wood of the shed before the insulation is installed. While this step is often not mentioned, we believe it’s an important step to provide peace of mind and extend the longevity of the shed. Once the insulation is installed and the walls are boarded on the interior, it’s difficult to get access to them again as the boarding needs to be removed. Therefore you won’t know if the wall is damp behind it. Using a wood preserver such as Cuprinol Wood preserver will protect the wood against rot, fungal growth and insect attack.

Which shed insulation is best?

When choosing which insulation to use inside a shed, we need to consider the cost and performance required. A shed can be insulated very cheaply using bubble wrap or could use a premium option such as PIR insulation boards. Remember that investing in insulation with better thermal values can reduce heating costs in the long run if an electric heater is regularly being used.

Low cost: Bubble wrap & multi-layered foil

The lowest cost and arguably the simplest way to insulate a shed is using bubble wrap. This works due to the air pockets reducing heat transfer and therefore reducing the amount of heat that can escape. Foil-backed bubble wrap is best to provide additional insulating properties and can be purchased in large rolls. Many multi-layered foil products fall into this category and can be purchased as kits designed for use in sheds. This type of insulation can provide high thermal properties without taking up much space and makes a good solution where the framing of a shed is not very thick. Installation involves stapling or tacking the bubble wrap onto the framing of the shed. Avoid attaching directly to the cladding as this could cause damp issues. Once installed, the bubble wrap can be boarded over.

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Versatile: Polystyrene insulation boards

Expanded Polystyrene is an extremely versatile material that has been used in packaging and insulation for decades. EPS is very durable, meaning the thermal performance does not degrade over time. The material is resistant to rot, mould and moisture making it a great choice for use in garden buildings. The benefit of using EPS over PIR insulation boards is the cost-saving that can be achieved. Polystyrene insulation is around 20% cheaper which can add up when insulating a large shed. Installation is easy as the polystyrene boards can be cut into shape and place in between the framing of the shed.

Sound insulating: Insulation roll

Wool and fibreglass insulation roll is commonly used in home building and is great for temperature control. The insulation is bound tightly together and doesn’t suffer from problems with mould. Most insulation roll products also have great fire safety ratings, which is important if electrics are being installed into a shed. One of the benefits of insulation roll is the sound insulation which it can also provide, something that is difficult to achieve in a shed. As insulation roll is very dense, it can reduce the amount of sound travelling through walls. However, insulation roll can be more difficult to work with when compared to insulation boards. Unless sound reduction is a top priority, we would recommend using one of the other insulating materials.

Best performance: PIR insulation boards

Rigid PIR insulation boards are used in house insulation and provide very high thermal values for a high level of insulation given their size. Each board uses rigid and lightweight foam, allowing it to slot into position in between shed framing and not move. Getting a good cut is important to create a tight fit in between the framing. Lots of choice of thickness is available for PIR insulation boards. With a shed, we are going to be limited to the thinnest options depending on the size of the interior framing. 20mm or 25mm thickness will be suitable. Lots of different brands are available including Kingspan and Celotex. We recommend looking for the best value option as the insulation values will be similar. Installation is fast with PIR boards as they can be cut and slotted into place. The main drawback is their cost. A 25mm board can cost £20 at a size of 2.4m x1.2m. When it is being used for the sheds walls, floor and roof the cost can quickly rise.

Guide: Best choice of shed insulation

How to insulate shed walls

Before we begin, be sure to check the pre-requisites are complete and the shed is now watertight. If you have treated the interior wood, leave a few days in between for it to dry out completely. 1) To begin we need to get all of our equipment including the insulation, cutting equipment, tape measure, marker, aluminium tape and expanding foam. The aim is to cut each piece of insulation board so that it slots tightly in between the sheds interior framing. Sometimes the distance between the framing can vary for different sections of the wall. Use the tape measure to get the width in between the framing. A standard sized insulation board is 1.2m in width and will usually need to be cut down to size. With the marker and a straight-edged ruler, mark out where the cuts to the insulation board need to be made. 2) It’s best to position the insulation board with the length running upwards on the wall. Before cutting, the insulation board should be clamped firmly in place to avoid it moving during cutting and achieve a precise finish. Using the cutting tool, cut the insulation. We recommend using eye protection and a face mask as the insulation board can get everywhere. 3) The freshly cut insulation board can now be placed in between the interior framing of the shed. The insulation should have a snug fit and remain firmly in place. If there are gaps where the insulation has been cut, these can be filled using expanding foam. The result should be no gaps where the insulation board and wood meet. If you are leaving a gap between the insulation board and the exterior cladding, a small piece of wood can be placed at either end of the insulation board to prevent it from touching the cladding. 4) The aim of insulation boards is to achieve airtightness to prevent warmer air from moving around the edges of the board. To do this, we can use aluminium tape on all edges of the insulation board where it meets the wood framing or another piece of insulation board. Once the aluminium taping is complete, all of the points where the wood touches the insulation board should be covered up. The shed walls have now been insulated. After, we can use plywood or similar to board and decorate the interior.

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How to insulate a shed floor & roof

Insulating a shed floor is not always needed as most of the heat is lost through the walls and roof. If you plan to be in your shed during the coldest months of the year, the extra insulation could reduce heat loss further. Cold floors in a garden building tend to create a cold feeling in the room and can also cause a draft. Insulating a shed floor is best during the initial construction. If the shed is being installed on a wooden frame, the insulation can be positioned between the frame and the same process as the walls is followed. For a pre-existing shed, the best solution is an insulating underlay type of material, depending on the finish you plan on installing on the floor.

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The shed roof is just as important to insulate as the walls for preventing heat loss. As the interior temperature heats up, the heat rises and will leave through the roof of the building. During the summer, a black shed roof can also have the opposite effect and make the interior of the shed swelteringly hot. Insulation works to slow down heat transfer both ways. The process is similar to insulating the walls of a shed. The biggest difference being the framing is usually thicker on a shed roof so thicker insulation is able to be used. It’s extra important in the roof to leave a gap between the insulation board and the exterior cladding. The warm air rising from the interior will contain an increased amount of moisture and this needs to be preventing from getting trapped between the insulation and wood. As it reaches the cold side of the ceiling, the moisture in the air will turn back into vapour and create damp, particularly if the shed is regularly used during colder months. EPS or PIR insulation boards are the best options for the roof due to being lightweight and able to hold themselves in place without putting extra pressure on the building. We would avoid using insulation roll as it will need to be supported to stay in position. The same process as the walls can be followed where the boards can be cut and slotted into place. Pay extra care when taping the edges to ensure there are no exposed gaps.

Further info

Is a vapour barrier needed?

A shed can be insulated with just insulation, but you may choose to insert a vapour barrier before installing insulation. A vapour barrier is plastic sheeting placed against the warm side of the insulation. The idea is to prevent vapour from penetrating into the insulation where it will cool and form condensation, resulting in damp and moisture build-up over time. Using foil backed insulation board and covering the edges with aluminium tape provides an effective vapour barrier without the need for an additional polythene layer.

Guide: Garden building vapour barriers

Installing an interior wall lining

Once a shed has been insulated, the interior walls can be boarded and decorated. The choice of interior lining will depend on the finish required and the budget available. MDF – Medium-density fibreboard is a man-made wood with a smooth and precision cut finish. As an interior shed lining it is good value and can be purchased cut to the size required. Plywood – One of our favourite materials to board a garden building with, plywood is extremely strong and durable. The hardwearing finish makes a great choice if items are going to be hanging from the wall and additional strength is required. For best results, the wood can be sanded to a smooth finish and then painted. OSB – Oriented standard board is a versatile wood that can work well to line the interior of a shed. The board is constructed with opposing-orientation wood fibres to create additional strength and stability.

Electricity in a garden shed

If a shed requires electricity, it’s best to plan this before installing insulation. A first fit of the wiring will be completed before the insulation is installed. Holes are cut out of the insulation board & interior wood lining to allow wiring to run through and into the shed. A second fit is then completed to install sockets and lights inside. If the shed has already been insulated and boarded, electricity can still be installed using trunking for the wiring.

any electrical work should always be carried out by a qualified electrician.

Conclusion

Insulating a shed is a great idea if you plan to regularly use the garden building throughout the year. An insulated shed can be used for storing items that are sensitive to extreme temperatures or even used as a workshop. Retaining heat inside a shed is difficult without insulation and the heating costs can soon add up. The idea behind the insulation is to stop thermal transfer and reduce the amount of heat leaving the shed. Before insulating a shed, it’s important to make sure the building is water-tight and damp free. Treating the interior walls with a wood preserver is an extra step to provide peace of mind. Insulating using EPS or PIR boards is our recommended option. Both of these allow for easy installation and provide great results.

gardenroom.guide - How to Insulate a Shed - Garden Room Guide

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