How To Insulate a Basement

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External Insulation of basement.

The DryDirect guide for insulating basements including:

  • How to insulate a basement?
  • External Basement Insulation
  • Internal Basement Insulation
  • Basement Insulation and Waterproofing
  • Basement and Buried Roof Insulation.

Basement living spaces, plant rooms and storage areas generally need to be insulated in order to protect the room from heat loss, control condensation and maintain a suitable living environment.

Although a buried basement structure will often maintain a steady temperature due to the insulation effect of the surrounding ground, it is important to remember that there will be a cooling effect due to moisture and groundwater surrounding the basement. 

Basement roofs and decks are also often subject to fluctuations in temperature which can lead to condensation in poorly insulated basement spaces.

A method for calculating U values in basements is given in BS EN ISO 13370

How to insulate a basement?

External Basement Insulation

Given a free choice, the best method of waterproofing a basement is externally.

External insulation offers lots of advantages as it allows for the whole structure to be kept warm. This greatly reduces the risk of condensation from warm, moist air in the basement hitting a cold surface.

The design of the insulation can be included in the waterproofing design. For concrete walls, rigid external insulation boards can be used to line the formwork and oversite before the application of a pre- applied external membrane such as DualProof. This arrangement allows for direct contact between the waterproofing and the structure which is the most reliable method of external waterproofing. 

On an open site, insulation can also be placed externally after the formation of the structure and application of the waterproofing. This  is useful for masonry, stepoc or blockwork walls where a liquid applied waterproofing system such as Deuxan 2c is used.

While it is possible to apply the insulation to the structure, then waterproof the outside of the insulation using a liquid applied system, this is a less reliable method. Movement in the joints between the insulation boards mean that there is a greater risk of installation defects and waterproofing failure.

Internal Basement Insulation

Even though external insulation is the preferred option, in the UK a majority of basements are insulated internally. There are several reasons for this.

Many new UK basements are formed by underpinning an existing structure. With this method of construction it is not possible to insulate, or waterproof, externally.

In addition, due to high land values, many new build basements are constructed right up to the site boundaries using modern piling techniques. Again this means that it is not possible to insulate externally.

As a result, an internal approach to both insulation and waterproofing is required.

There are a few more risks with internal insulation, specifically.

Uneven insulation, it can be difficult to maintain a continuous internal insulation line around internal structures such as spine walls, intermediate basement floors and structural columns. Cold spots from uneven insulation is often referred to as cold bridging.

Using insulation internally means that the structure behind the insulation will be colder. This means that there is a greater risk of condensation from warm wet air hitting a cold concrete walls.

The risk of condensation can be reduced by designing in a vapor barrier on the warm side of the insulation.

Basement Insulation and waterproofing.

External Insulation will be exposed to moisture in the ground. This means that any insulation used must be moisture resistant and not break down or lose it’s insulation performance when wet. The insulation also needs to be strong enough to support any loads from the structure. The Danosa TR100 XPS insulation offers great performance when used externally.

Internal insulation must be closed cell in order to limit the transmission of water vapour through the insulation layer. It should also be rigid enough to withstand internal loadings from finishes, walls and screed. 

Internal insulation can be used as a structural spacer in a cavity membrane system on the slab between the drainage channels. Danopren TR50 50mm XPS insulation works well here.

Basement Buried Roof Insulation

Waterproofing basement roofs is one of the trickiest areas of basement design. Waterproofing and insulation build up have to work together to keep this challenging area dry, warm and free from condensation.

First, we always recommend waterproofing the outside of the buried basement roof using a liquid applied system, direct to the structure. Applying the waterproofing to the structure is much more secure than applying to the insulation. There will be less risk of movement and failure and you will be able to joint and lap correctly to upstands and external waterproofing system. A great basement roof waterproofing system is Koster NB4000 as it is really sturdy, trafficable and UV stable.

This then means that the roof build up becomes an inverted roof, so the insulation is on the outside of the waterproofing layer which creates a few more challenges.

Even waterproof, closed cell insulation like Danopren XPS should be protected against the cooling effect of flowing water. Inverted roof design allows for a water flow reduction layer – also know as WFRL. This slows the flow of water through the insulation layer and reduces the cooling effect.

Depending on the final finishes a studded geotextile membrane can be applied like Delta Terraxx for hard landscaping or Delta Floraxx Topp attenuation membrane for green roof build ups.

I hope you find this guide useful with your basement insulation project. We will be happy to help with design, product selection and even installation on your basement insulation project.