Airtightness around windows is a key component to thermal insulation and energy savings | The Library

Install Bostik's Airseal system products to achieve airtightness around window frames and achieve the first line of defence against heat loss in your home

What can I put into the gaps and joints around window frames to keep the cold out?

Why a window frame? Because this is often a weak point in the insulation of a house.

Before we answer this specific question, first some basic principles about insulation and airtightness.

A well-designed house will have sufficient insulation on all faces (roof, walls, and floor slab), and it will be designed to prevent heat from escaping. There will be a lining to prevent water from getting in. Insulating materials and air gaps (e.g. double-glazed windows) will reduce the amount of heat that can escape.

Insulation keeps things warm in winter and cool in the summer.

We all know the consequences of poor insulation. Your house will feel too cold in the winter and too warm in the summer. And then comes the electricity bill.

The problem of poor insulation becomes personal when it begins to affect your bank account!

Airtightness is the first line of defence. A good heating system will create a warm, healthy environment inside your home. The last thing you want to do with that warm air is lose it through weak points in your insulation system.

That’s where the window question comes from. Yes, you can close your windows. But if there are gaps around the edges, your home will still leak heat.

Can I insulate the gaps and joints around the frames of my double-glazed windows?

Yes, you can.

Enter the Bostik Airseal system, a concept that provides products on three levels of airtight construction: airtightness, thermal insulation, and driving rain tightness.

All three types of protection are equally important. The first concept, starting from the inside, is Airtightness.

You cannot leave your windows open constantly during the winter, because your heater will be working overtime to maintain a comfortable temperature. Similarly, if it’s a hot summer and you’ve got the air conditioning on, you’ll need to close the windows.

But if you have leaky window frames, shutting the windows won’t solve the whole problem. Heat will always look for a path to somewhere cooler. And wind on a winter’s day will find gaps to force cold air into your house.

If there’s a gap through which air can leak, it will find it, and it will take heat with it.

What is the best way to insulate window frames?

Bostik’s Airseal range provides you with a range of products that focus on sealing the joints around windows. This system is made to eliminate those weak points in the airtightness of your building. The products consist of 7 tapes, a coating, and flexible PU foam.

The complete Airseal range

Discover our Energy Saving & Airtight

This article will focus on four key components of this system:

BOSTIK T660 AIRSEAL WINDOW TAPE

Is a modified polyamide film/nonwoven material combination. The full-surface self-adhesive underside enables easy, full-coverage installation of the flashing tape for plastering or painting over. Bostik T660 Airseal Window Tape Duo is moisture-variable and can therefore be used both inside and outside. It is airtight, and it resists driving rain, so it can be used to prevent the penetration of moisture from the outside.

BOSTIK T665 AIRSEAL WINDOW TAPE DUO PLUS

This is the same product as T660 but with the addition of an adhesive strip on the opposite side to allow other products to be easily fixed to it.

BOSTIK P605 AIRSEAL FLEX FOAM

Our one-component professional extremely flexible thermal- and sound-insulating polyurethane gun foam. It is the most airtight foam available on the market.

BOSTIK H670 AIRSEAL COATING

Is a high-quality durable elastic sprayable and brushable airtight coating for seamlessly sealing joints and gaps between various substrates.

How to seal window frames?

BOSTIK T660 AIRSEAL WINDOW TAPE DUO & BOSTIK T665 Airseal WINDOW TAPE DUO PLUS

Step 1: Prepare the surface

The surfaces onto which the tape is to be applied must be free from oil, grease, and dust.

Note that low temperatures can lower adhesion. Test beforehand to check that the adhesion will be adequate.

The base must be sufficiently load-bearing. Highly porous or cold or humid surfaces may have to be pre-treated with a suitable primer to enhance adhesion.

BOSTIK T660 Airseal Window Tape Duo can be installed down to -10 degrees Celsius. However, at such temperature, make sure there is no moisture, ice, or hoar frost on the surface onto which the tape is to be installed.

Please note: the additional adhesive strip of the BOSTIK T665 Airseal Window Tape Duo Plus can be applied at temperatures only as low as +5 degrees Celsius.

At low temperatures, the time during which the bond cannot be loaded will increase. We recommend that you perform pre-tests to ensure adequate adhesion. We also recommend you preheat the tape to +15 to +25 degrees Celsius before applying it.

Step 2: Apply the tape

This is a summary only. Please refer to and carefully follow the installation instructions provided with the product.

Cut the Bostik T660 Airseal Window Tape Duo (or T665 Duo Plus) to the required length, allowing about 5 cm extra. Pull off the cover film and carefully install the mounting strip to the back of the window, applying some pressure.

Repeat this procedure on all sides of the window requiring sealing, for both the indoor and outdoor seal. Before you apply the indoor seal, the space between the window and the wall can be filled with Bostik P605 AIRSEAL FLEXFOAM insulating material using a foam-dosing gun. (See the next section.)

Put the window into position and fasten it. Then remove the remaining film cover, attach the tape, and press it fully onto the window surrounds. Overlap the tape joints.

BOSTIK P605 AIRSEAL FLEXFOAM

Step 1: Getting ready for application

The temperature of both the environment and the substrates must be within the range of -15 to +35 degrees Celsius. However, we recommend a range of +15 to +25 degrees Celsius.

The temperature of the canister must be within the range of +5 to +30 degrees Celsius. However, we recommend a range of +15 to +25 degrees Celsius.

Before using it, shake the canister vigorously – at least 20 times.

Step 2: Applying the foam

Hold the canister with the valve turned upwards and affix the adaptor on the tread. We recommend a universal PU foam gun.

Turn the canister upside down and apply the foam. To regulate the flow of the foam, loosen the valve at the back of the handle. Fill the cavity to 70% of its volume. In a low-humidity environment, lightly spray the foam with water. The foam will expand to fill the rest.

Joints wider or deeper than 4 cm should be filled in multiple layers, waiting 15 to 30 minutes between applications and lightly spraying the foam with water after each application.

Use spacers and wedges to hold the frame in place for about 24 hours, until the foam has fully cured.

Once it has cured, BOSTIK P605 AIRSEAL FLEX FOAM can be sealed or plastered over.