Does a Summerhouse Need a Vapour Barrier?

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During the construction stage of a summerhouse, there are many aspects to consider. As well as the exciting features such as heating and electricity, it’s also important to understand whether a vapour barrier will be required.

Summerhouses are becoming an increasingly popular choice for a garden room and can be converted to make the perfect garden office or gym. For use as an additional room throughout the year, insulation and heating are going to be required to lock in warmth and keep electricity costs down. When we add heating & insulation to a summerhouse, creating a sealed structure opens the door for potential issues with damp.

Thankfully, taking additional steps during the construction and planning phase can keep the summerhouse structure ventilated and protect against damp build-up from daily use. One of the first questions to ask is whether a vapour barrier or breather membrane will be required. Both of these play an important role in protecting insulation from moisture build-up and keeping water outside your garden building.

If you are going to be installing insulation yourself, be sure to understand whether you also need to install a vapour barrier before lining the walls. When purchasing an insulated summerhouse, explain to the manufacture how you will be using the building and whether you will be installing a heater for the colder months of the year.

How does a vapour barrier work?

Installing insulation and heating into a timber building such as a summerhouse is an effective way to prevent heat loss. Unfortunately, this also makes it difficult for moisture to escape and over time this moisture can build up and lead to damp and mould growth. Insulating a summerhouse often seals up all of the areas where air would naturally ventilate and also creates an unventilated space where hot and cold air meet.

Inside a warm summerhouse during the winter, evaporated water is in the air as water vapour. When the warm air meets colder air (inside insulated walls), condensation takes place and the vapour becomes liquid. Over time, the water droplets can build up and lead to moisture issues in the building.

A vapour barrier is an impermeable plastic sheeting placed up against the warm side of insulation. This stops vapour from passing through the wall lining and into insulation, where it can cool and form condensation. Insulation that is full of moisture will not perform as efficiently and can lead to mould in the surrounding timber framing. 

For a vapour barrier to be effective, it must create an impenetrable seal on all of the walls of the summerhouse. Several systems are available depending on the type of insulation used, but the concept is always the same.

When does a summerhouse need a vapour barrier?

If a vapour barrier is so good, you may be wondering why your newly purchased summerhouse does not already have one installed. The answer is because not every summerhouse requires one and installing a vapour barrier unnecessarily may prevent air from circulating effectively.

A vapour barrier is required when a summerhouse is insulated and heated. Both of these create a scenario where warm air can come into contact with colder air inside the wall of the building and lead to condensation. Installing a vapour barrier prevents moisture from entering the wall. Without any airflow into the wall, moisture cannot enter and dampness is prevented. We are aiming to achieve airtightness in all of the summerhouse walls.

A vapour barrier is not required for a summerhouse without insulation that will not be regularly heated. Without a major difference to the temperature outside of the building, and nowhere for the moisture to get stuck, condensation will be reduced. As the walls are not sealed, the air is free to pass through as the wood breaths. Installing a vapour barrier against the cold exterior wall would prevent the wood from properly ventilating and may lead to moisture build-up. In summerhouses without insulation, the wood can ventilate naturally, as moisture is not stopped from moving around the interior. Just be sure to install some sort of passive ventilation system to prevent an excessive build-up of moisture.

Which type of vapour barrier should I use in a summerhouse?

The type of vapour to use will depend on which method has been used to insulate the summerhouse. 

With foiled backed insulation boards

We always recommend foil-backed insulation boards for a summerhouse when the budget allows, due to their easy installation and high R-Values. The good news is that the foil backing on PIR insulation boards is sufficient enough to act as a vapour barrier, without the need to install any separate sheeting. 

As there cannot be any gaps, the edges of the insulation boards, where they meet the timber, should be covered using aluminium foil tape. The result will be a vapour resistant covering all of the walls throughout the summerhouse.

With wool insulation roll

While thick insulation rolls are also great for sound insulation, they do not have any moisture-resistant backing and a separate vapour barrier is required. Most vapour barriers are made from Polythene and come in either green or clear colours. A standard vapour barrier will come on a 50m2 roll which is plenty to cover a small summerhouse. Several different thickness options are available, starting at 0.1mm and thicker options are best for use in the floor to prevent damp rising.

Vapour barriers are available to purchase from most homeware stores. Be sure to check the details of the product to ensure it stops the passage of moisture. A breathable membrane is different to a vapour barrier as it lets moisture pass through. 

Guide: How to insulate a summerhouse

Preparing the walls before installing a vapour barrier

We like to take an extra cautious step when insulating and lining a summerhouse. Before installing insulation, we treat all of the interior walls with a clear wood preserver. While this may not necessarily be needed if the vapour barrier is installed correctly, the belt & braces approach gives us peace of mind.

A clear wood preserver is used as a base coat before painting exterior summerhouse walls and we always have some leftover. A good quality wood preserver protects against damp, rot and fungal growth. This extra step should prevent us from needing to remove the interior wall lining in the future due to any moisture issues and maintain the longevity of the building.

How to install the vapour barrier

With foil backed insulation boards, installation is easy. Simply apply aluminium foil tape around the edges of the insulation where it meets the timber framing. The resulting interior will have minimal timber on show and the insulation will appear as if it is joined together throughout the room. 

For wool insulation roll, the vapour barrier is stapled into the timber framing using a heavy-duty staple gun. Best practice is to start from the top and cover all of the way around the interior before working downwards. If a tight fit can be achieved in the corners, a single piece can be used to wrap around all sides of the interior walls. Where the vapour barrier joins with another piece, overlap them both and use polythene joining tape to maintain an airtight seal.

Our Choice!

Polythene Vapour Barrier

For use within structures to prevent the penetration of moisture into surfaces and to improve the airtightness of buildings. Perfect for use with insulation roll in a summerhouse.

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Should I use a vapour barrier in my summerhouse floor?

For summerhouses built on a concrete base, it’s recommended to install a vapour barrier into the concrete base. This will prevent moisture in the soil underneath from rising. Whichever way damp gets into concrete, it always leaves through the surface. This can lead to rising moisture causing damp in a summerhouse floor. By installing a vapour barrier into the concrete base, the amount of moisture will be reduced.

Protecting a summerhouse floor from dampness is best achieved by adequate ventilation underneath. A gap between the floor and concrete base will allow fresh air to pass through and avoid water building up.

How is a breathable membrane different?

When shopping for a vapour barrier, there are plenty of options to pick from including membranes. A membrane is a different product with a slightly different use to a vapour barrier; be sure not to get them mixed up.

A breathable membrane is placed on the cold side of the insulation rather than the warm side with a vapour barrier. The aim is to prevent water and rain from outside from passing into the insulation, whilst allowing vapour to pass through and evaporate. The big difference with a membrane is its ability to let vapour pass through.

Breathable membranes are installed earlier during the construction process before the exterior cladding has been applied. By allowing water vapour to evaporate, the exterior side of the wall can remain ventilated and at the same time prevent rainwater from soaking the insulation.

Most pre-built summerhouses will not feature a breathable membrane, they are more commonly found on self-builds and garden rooms with cladding. Don’t worry that there’s no membrane in your summerhouse; tongue & groove exterior walls are unlikely to let much moisture through if treated correctly. A good quality exterior wood treatment enhances the weather resistance of exterior wood.

Do I need to leave an air gap for a vapour barrier?

A vapour barrier does not require an air gap between the polythene and insulation. The barrier should be installed tightly against insulation. An air gap is used on the opposite cold side of the insulation where it meets the exterior wall. A small air gap allows the air to circulate without getting stuck between the insulation and timber.

How does a vapour barrier affect ventilation?

Moisture control is important for any building, especially when you are going to be in there for long periods, as with a garden office. Most pre-built summerhouses which do not come with insulation installed will not have any ventilation system. This is because they are designed with casual use in the summer months in mind, and often not conversion into a garden room used daily. Naturally, there will not be complete airtightness and the building will mostly be able to ventilate itself through small gaps in the single lining of wood.

When we install insulation and a vapour barrier, we restrict the airflow, leaving nowhere for it to naturally ventilate out of the room. The result can be high humidity inside and an uncomfortable environment. An easy fix is a passive ventilation system in the form of a small air vent on the wall. Vents allow humid air to leave the building and be replaced with fresh air. For larger buildings, 2 air vents can be used, with one placed high for warm air to exit and the other placed lower down to bring in cooler air.

Buying guide: Best wood treatment

Summary

Installing a vapour barrier is an important step when creating a fully insulated and heated summerhouse. The aim is to provide an impermeable layer to prevent water vapour from travelling from the warm to the cold side of the insulation where it can form condensation.

Using foil-backed insulation boards is the easiest way to achieve a vapour barrier without needing to install one separately. If the vapour barrier is being installed separately, the process is simple and can be completed within a few hours. Before you begin, we recommend treating the interior timbers with a wood preservative to take a cautious approach to protection from the weather conditions throughout the yet.

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