Insulating a Shed with Celotex or Kingspan Boards

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Insulating a shed allows for use throughout the year with greater control over the temperature inside. Insulation boards such as Celotex & Kingspan are ideal for high efficiency and easy installation.

Garden buildings are a great idea for increasing living space at home without the need for a costly extension. While there are many premium options such as pre-built garden rooms and log cabins, a shed could be the perfect solution to create more space while keeping costs down.

Sheds are usually designed for storage, featuring a timber construction with no insulation and small windows. This leaves them exposed to temperature changes, such as freezing conditions during the winter and extreme temperatures in direct sunlight during the summer.

To make use of a shed as an additional space to spend time, we need greater control over the temperature. Insulation in the walls and roof can reduce temperature transfer between the interior and exterior.

Why Insulate a shed?

Garden buildings, including sheds, can be converted for a variety of different uses such as a garden bar or just a utility room with electrical appliances inside. Whatever the use, if you plan to spend time or keep electronics inside, control over the temperature is needed.

The average room temperature is around 20 degrees, so we should aim to keep as close to this as possible in the interior of a shed. This ensures a comfortable temperature if the building is used for extended periods throughout the day.

During the colder months of the year, if the shed has its own electrical supply, we can use a small electric heater to keep the shed interior warm. The challenge is that sheds don’t naturally hold heat very well when it’s cold outside and most of that heat is going to escape through the walls and roof.

A typical shed wall is constructed from timber between 6-8mm in thickness. This provides little insulation and quickly losses the heat being produced by an electrical heater.

With energy costs rising, electrical heaters are not cheap to run. Many of them feature a power rating between 1-2kW. Running a heater for an extended period when a shed is used daily can quickly add hundreds of Pounds to an average annual electricity bill.

Instead, it’s best to use the heater to provide initial heat, with insulation keeping the warmth inside of the building. Below is an example of the additional cost to a yearly electricity bill from running a heater for just five hours each week.

Insulating a shed with Celotex

Insulating a garden shed with a PIR insulation board such as Celotex or Kingspan can be completed as a DIY job. Several stages are involved, including preparing the building, installing the insulation and then boarding the interior walls. The full installation can be completed over a weekend and the garden building is ready to use after.

Preparation

Before we begin installing insulation, we need to ensure the shed is suitable. If we are converting an older shed, we need to ensure the structure is solid and weatherproof. Once the insulation is installed, it’s difficult to access the internal walls again and the last thing we want to do is begin pulling the interior boarding off from the walls.

Protect the wood

Timber sheds come untreated and the exterior walls are often treated as soon as possible after insulation. Before installing insulation, we also recommend treating the interior walls with a wood preserver such as Cuprinol Clear Wood Preserver (Amazon). This will protect the wood against rot, decay and insect attack.

Inside an insulated wall creates a place where mould can grow if water gets inside in the future such as from a small gap in the wood. Treating with a wood preserver will provide protection for the wood, creating peace of mind and longevity for the structure. As we mentioned, removing insulation in the future due to decaying wood can be a big job that we want to avoid.

Weatherproof the structure

Before installing insulation boards, the building must be completely weatherproof and watertight. Sometimes sheds can naturally have small holes due to knots in the wood. In older buildings, movement over time can create space where the panels join together. If there’s water getting inside when it rains, the shed needs some work before insulation can be installed.

Any gaps in the wood also create a place for heat to escape, reducing the effectiveness of insulation. We can use wood filler in the gaps to cover the wood and protect against water ingress. Be sure to check around the doors for any drafts where heat could be lost. Weatherstrips can be used to reduce gaps and create a tight seal when the door is closed.

Insulation board thickness

Insulation boards come in a huge number of thickness levels. Many start from 20mm and go all of the way up to over 200mm. A thicker board will provide better insulating effects, but also increase the cost of the installation.

The shed’s wooden interior framing will determine how thick the insulation boards can be. The insulation must be able to fit inside the framing and sit flush so that we can attach the interior boarding to the framing once the insulation is installed. We also want to leave a slight air gap between the inside of the exterior wall and the insulation.

For most sheds, the choice will be between using 20mm, 25mm or potentially 50mm insulation boards. The 25mm boards will provide a good level of insulation and be suitable for most sheds. While Celotex and Kingspan are some of the best options, a wide range of PIR insulation boards are available from other brands. If the shed is being insulated on a budget, be sure to shop around for own-brand products from homeware stores or builders’ merchants.

We also need to purchase aluminium foil tape that will be used later to seal around the edges of the insulation board once in position.

Guide: Cheapest ways to insulate a shed

How to install Celotex insulation boards

Once the shed has been prepared, we can go ahead and install the insulation board. It may take a few days for wood filler and preserver to dry out completely, so ensure everything has dried before beginning.

Shed walls

We recommend starting with the walls as the roof can be a bit more tricky.

To begin, prepare all of the equipment that will be used. This includes a tape measure, pen, cutting equipment for the insulation board, aluminium tape and expanding foam.

The insulation board is cut so it tightly fits in between the wooden framing and is able to hold itself in position without using anything to support it. Start with the tape measure to get the distance between the framing and use a pen and ruler to mark this out on the insulation board.

We recommend cutting insulation board with the length running upwards. As we want a consistent cut, clamp the board into position to avoid it moving. Any movement when cutting can create small gaps that are visible once the insulation board is in position. Be sure to use a suitable face covering and eye protection when cutting as the insulation board can get everywhere.

Once the insulation board is cut we can push it in between the framing of the shed. The aim is to achieve a snug fit where the board remains in place by itself. We don’t want any gaps around the edges that can allow air to work its way around the insulation.

If we do see gaps, expanding foam can be used to fill them and create a tight fit up against the wooden framing.

We repeat the process for each section of the shed wall until all of it is covered with insulation. To reduce heat and moisture transfer we want to create airtightness in the walls. To do this, we can use aluminium foil tape where the insulation meets the wooden framing or another piece of insulation board. 

With the aluminium tape installed, we should not be able to see any of the locations where the insulation touches another piece or the wooden framing.

Once the insulation is installed, the interior walls can be boarded. Plenty of different wooden boards are available depending on the budget and desired finish. Our favourite is plywood, but OSB and other similar boards can also be used. 

As the insulation sits flush with the interior framing, the framing can be used to attach the interior boarding on to.

Shed roof & floor

Insulating the shed roof and floor can be slightly more tricky, which is why we recommend completing the walls first. As heat moves around the shed it rises, leading to most of the warmth being lost through the walls and roof. Insulating just the walls will still result in lots of heat lost through the roof.

Remember that insulation works both ways. During the summer, insulation in a shed roof will have the opposite effect. Direct sunlight on a black roof can heat the shed up quickly, with the interior of the roof being hot to touch. Insulation can protect against high temperatures during the summer, keeping the building temperature at a manageable level.

Insulating the floor is less important, but still a good idea to retain maximum heat inside the shed. Garden buildings without anything covering the timber flooring can often feel cold as the draft works its way through.

Fitting insulation boards to the roof and floor

The process for insulating the shed roof is the same as for the walls. We need to cut the Celotex insulation to fit tightly in between the wooden framing and stay firmly in position. It can be more difficult to install but the process should be familiar to the walls.

One difference is that we can sometimes use a thicker insulation board for the roof. Many sheds will have thicker internal framing on the roof, creating a wider cavity where the insulation can be fitted. While using 25mm on the walls, we may be able to use 50mm on the roof.

Be sure to leave an air gap between the insulation and exterior wall to prevent moist air from becoming trapped and causing damp issues. Once the insulation board is in position, aluminium foil tape can be used to seal where the wood and insulation boards join.

Insulating the shed floor is not always possible and may prove difficult if the shed is already installed. The best time to insulate the floor is during initial construction when we can get easy access to the base and wooden structure beneath the floorboard. For sheds already installed, a better option may be to use an underlay material, depending on how the shed floor will be finished.

Useful info

Insulating a shed with Celotex or similar insulation boards shouldn’t be too difficult. While lots of insulation options are available, boards make one of the easiest solutions to install.

Lining the interior walls

Once all of the insulation is in position, we can use wooden boards on the interior of the shed to finish the walls. We like plywood as it’s very durable and hardwearing, making a good choice if additional strength in the walls is required. The plywood can be sanded down and painted for a smooth finish.

Alternative options include OSB which is available at a lower cost and also provides plenty of strength. MDF is also suitable and provides good value for money.

Vapour barriers

Vapour barriers are commonly discussed when it comes to insulating a garden building. They usually consist of a plastic sheet placed on the warm side of the insulation. The idea is to prevent moisture from penetrating into the insulation, where it can condensate and lead to damp issues.

The foil on foil backed insulation boards already does the same job, so a separate vapour barrier is not required. Instead, we use aluminium foil tape to seal the vapour barrier and prevent moisture from getting through where the insulation and framing join together.

Guide: How to insulate a shed

Summary

Insulating a shed with Celotex, Kingspan or any similar insulation board is a great idea. Once the shed is insulated, we get greater control over the interior temperature, creating a comfortable atmosphere throughout the year. Energy bills are reduced as the building can hold onto warmth and prevent it from escaping through the walls and roof.

Installation involves cutting the insulation boards to fit tightly between the interior framing of the shed. Once installed, we can cover the interior walls with wooden boarding such as plywood to create a smooth finish.

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