How do you install Compriband _under_ windows?

[Alan Ambrose]

Just supposing you leave 5mm nominal gap around windows (this is a timber frame building). I get that you can install Compriband on the sides and top, but what about the bottom? Won't it just crush and leave a bigger gap at the top? 5mm temporary or permanent packers maybe?

[craig]

As you would on the sides and top, you should retain packers all around the frame and not install without any. The compriband should be on the outside aspect of the frame, packers every 150mm from corners and then every 300/400mm and I also recommend under every mullion if possible, as it helps the frame from dropping/twisting.

 

 

 

[saveasteading]

@craigmay correct me. I only know Compriband from major Civil Engineering works, eg bridges, basements and sea defences and long ago.

The band had to be compressed using a special machine exerting huge pressure,  was inserted or cast in, and then would return to shape over many hours and continue to exert pressure indefinitely. It was intended to seal joints and cope with future movement. By hand it barely compressed under a thumb.

From what I understand there are umpteen products for different situations thus for every case it can be the appropriate strength to not crush.

 Ie it will keep pressure on both sides of the gap.

 

BUT I've a suspicion that many installers use cheap and quite flimsy foam strips.

I came across a supplier who seemed to have a wide range of products for the window industry, which was encouraging. I asked what was the difference between theirs and Compriband, and they insisted theirs "is Compriband". 

I suspect the trade name was being abused as a generic term.

[craig]

It's a generic name for compressed foam that expands, originated in Austria in the 50s or 60s if memory serves me correct. Some are very good (Illbruck for example) others are just 💩

Edited Monday at 16:44 by craig

[Nickfromwales]

1 hour ago, Alan Ambrose said:

Just supposing you leave 5mm nominal gap around windows (this is a timber frame building). I get that you can install Compriband on the sides and top, but what about the bottom? Won't it just crush and leave a bigger gap at the top? 5mm temporary or permanent packers maybe?

The gap will be dictated by the supplier, for eg Norrsken wanted 10mm each side, and 10 - 15mm above, and to be able to pack out 3-5mm underneath to level the units. 

 

Don't order any products until you have these particulars  

 

FWIW, why Compriband? FM330 foam and then something like clear CT1, applied after the foam has cured and been cut just shy of the surface, does a splendid job and is (afaic) easier and more robust. I prefer to have a continuous bead of mastic 360 degrees around the unit to provide sealing / weathertightness etc. You can also CT1 across the face of the packers, whereas with Compriband you have to stop and start in between the packers which is a bit shite.

[craig]

8 minutes ago, Nickfromwales said:

FWIW, why Compriband?

• Breathable
• Water repellent
• Acoustics
• Thermal

Are just some of the key benefits and why it is the preferred option for most suppliers/manufacturers.

[saveasteading]

It appears to be that the Illbruck product is Compriband and other manufacturers are adapting the name as a generic.

[saveasteading]

8 minutes ago, craig said:

why it is the preferred option for most suppliers/manufacturers

The reputable ones anyway.

 

Aerosol expanding foam is banned on my projects without express permission. It (esp the cheap varieties) is the preferred solution of the bodgers.

Even the more flexible varieties should be used with knowledge and skill.

[Nickfromwales]

13 minutes ago, saveasteading said:

The reputable ones anyway.

Are you challenging me to a dual sir?

[Alan Ambrose]

@Nickfromwales

 

Re: clear CT1

 

I saw recently that the clear version is not intended for external use. Not sure why not - UV maybe or maybe a different formulation?

 

See:

 

 

 

Edited Monday at 17:53 by Alan Ambrose

[saveasteading]

30 minutes ago, Nickfromwales said:

challenging me to a dual sir?

I have no doubt you are reputable and have a proven solution. 

I've had badly used foam cause extensive damage and leave a bodge behind. 

[Nick Laslett]

Here is how I did my windows. 
 

 

[Nickfromwales]

1 hour ago, Alan Ambrose said:

@Nickfromwales

 

Re: clear CT1

 

I saw recently that the clear version is not intended for external use. Not sure why not - UV maybe or maybe a different formulation?

 

See:

 

 

 

I wouldn't leave it on show! I'm referring to using it for fundamental sealing and weatherproofing, only, with decorative rain screen meeting the frames of the units so hiding the 'seal' behind.

[SteamyTea]

Most foams are polyurethane based, but can have other polymers added during manufacture, or diffused post manufacture.

There are a number of different polymers added 'to the mix'. Acrylics are the most common, but polyolefin and ethylene propylene diene monomer are often used.

Those two can control expansion times, water and weather resistance.

Generally these slow expanding foams are open cell as it is hard to compress the air reliably in close cell, but also it makes them more dimensionally stable over a greater range of temperatures and air pressure differences. So best to create a secondary seal with a water and UV resistant sealant both sides, and tape if you can.

 

I don't know the cost of a good product, but suspect it is a tiny part of the overall fitting cost.

Edited yesterday at 10:12 by SteamyTea

[saveasteading]

2 hours ago, SteamyTea said:

slow expanding foams are open cell as it is hard to compress the air reliably in close cell

In  case this is relevant or of interest.    From my experience, limited to non-glazing uses.

 

1. The Compriband I knew in structures was close celled and fully closed on the outer surfaces. 

 

2. In constructing hundreds of steel buildings, every one had lots of profiled foam fillers to close off the ends where there were crowns in the cladding. It came from a single source from our cladding supplier. One of our clients happened to be a manufacturer of foam sheet and board for packaging. The owner examined a piece of the roofing material and expressed huge satisfaction that it was closed  cell, saying it was a superb product and he was pleased to have it in his building.  He added that it was an unusually high quality and relatively expensive. Thereafter I took more interest and looked at all foam fillers from other sources, and the difference in quality was clear.

 

I suppose I am saying that there will be all sorts of similar products out there, with differing performances. The wrong one may save the installer £30 on your house, but you will pay many times that in heat loss and deterioration.

 

[SteamyTea]

33 minutes ago, saveasteading said:

The Compriband I knew in structures was close celled and fully closed on the outer surfaces

 

On 05/01/2026 at 16:40, saveasteading said:

The band had to be compressed using a special machine exerting huge pressure

It was probably integral skin PU.  Car steering wheels and office chair arm rests are made from it.

Made hundreds of different tools to mould in over the years.  Can be tricky to mould right as it is very temperature sensitive to  get the correct skin thickness and consistency.

Extruding it would be pretty cheap, once the machinery and dies were made.

[craig]

1 hour ago, saveasteading said:

may save the installer £30 on your house

 

I can assure you, they absolutely hate having to do it and will charge to apply it. I prefer to source it direct, that way I know the install is with products I have discussed. They would rather not apply it, they'll try and offer alternative solutions but compriband, airtight pu foam, airtight membrane is the way to go. 

[SteamyTea]

5 minutes ago, craig said:

they absolutely hate having to do it and will charge to apply it

Does it make taking windows out harder as well?

[craig]

Not really, the compriband is stuck to the window frame and only expands to the building.

[Nick Laslett]

26 minutes ago, craig said:

 

I can assure you, they absolutely hate having to do it and will charge to apply it. I prefer to source it direct, that way I know the install is with products I have discussed. They would rather not apply it, they'll try and offer alternative solutions but compriband, airtight pu foam, airtight membrane is the way to go. 

For me that was the joy of self building. Never having installed windows, I thought compriband was great. Followed the YouTube video, what is not to like. 
 

Here is my playlist of many window install videos. 
 

https://youtube.com/playlist?list=PLMqBydLyN5jH5KYILUMXeVpz3dljZLrPE&si=zTzode7Uz_pSIjb5

 

Edited yesterday at 18:52 by Nick Laslett

[Nick Laslett]

On 05/01/2026 at 17:51, Alan Ambrose said:

@Nickfromwales

 

Re: clear CT1

 

I saw recently that the clear version is not intended for external use. Not sure why not - UV maybe or maybe a different formulation?

 

See:

 

 

 

For what it is worth, OB1 Clear is intended for exterior purposes if you need a clear version of this type of adhesive. 
 

https://ob1original.com/uk/wp-content/uploads/2025/10/OB1-Multi-Surface-Construction-Sealant-Adhesive_Silver_Clear_TDS-1.pdf

[saveasteading]

1 hour ago, SteamyTea said:

taking windows out harder as well

it will be fitting the space exactly, including any lumps and bumps, eg mortar courses, so will require a hefty tug. it will then start to expand again as it has been released, so won't go back in.