Tape, silicone etc please explain

[Selfbuildsarah]

We are juggling window quotes at the moment. What fun!

Can someone please explain the taping, silicone stuff to me? The company we favour has silicone as an optional extra. Do we need this? We will not be hands on but I would like to understand the process. Thank you

[JohnMo]

The ideally for me, airtight low expansion foam in void between window frame and wall. External depends on external finish - you need to explain this?

 

Inside insulation on the internal wall return butt up to window frame. Then some method of airtightness - tape etc.

[Selfbuildsarah]

Timber cladding on some walls, render on others. Why would the silicone be an optional extra? 

[JohnMo]

10 minutes ago, Selfbuildsarah said:

Timber cladding on some walls, render on others. Why would the silicone be an optional extra? 

No idea, but makes you wonder about sealing the cavity around the window as well, have confirmed they are doing that? If the don't you will have a thermal bridge, leading to additional heat loss and damp walls in those areas.

 

What does your architect drawings show?

[Temp]

14 minutes ago, Selfbuildsarah said:

Timber cladding on some walls, render on others. Why would the silicone be an optional extra? 

 

Did you ask them to quote including installation? If you did then I agree this is a bit odd. 

 

My guess is you didn't so they are just trying to up-sell you some sealant. Whoever is going to install them will have their own preferred make and method so I'd probably decline the silicone. You can buy the stuff everywhere.

 

Does the quote have the right VAT rate...

 

If the quote did include installation I believe the whole thing should be zero rated for VAT to you. So check if the quote includes VAT. If it does then you need to tell them its for a new house under construction and should be zero rated. Supply your planning reference number and ask them to requote.

 

If it doesn't include installation the quote should include VAT which you reclaim when the house is finished.

 

 

Edited Wednesday at 14:40 by Temp

[craig]

It's about how it is worded, it is likely to be part of the wording for renovation replacement but if you could paste the entire wording for context, that would help. Generally speaking though, on a new build, it's inputting the windows, fixing method, PU foam, airtightness tapes and externals left to others.

[Nickfromwales]

On a current project, Norrsken fenestration, installed by Elite, the “silicone” I challenged. 
 

The fitters then pulled out tubes of high quality Sika mastic, and I pointed out that wasn’t silicone, but a brother of CT1. Very good stuff and this was used to create a fundamental weather seal around the outside of the frames, but which would live out of eyesight behind the external render system. 
 

@Selfbuildsarah, if you are hands off, then consider the merits of a completed installation, but first let us know what the cost is for “silicone”. 

[Selfbuildsarah]

Thank you Nick. 19 windows and 5 doors from Norrsken. We are hands off. Timber frame, part render, part timber cladding.

 

Quote from Norrsken

 

“We normally quote for external fine silicone, assuming foam + rough silicone install. The only reason its’ optional is that it is done after install and some customer choose their builder to do this later and also not all other suppliers include this in their quote hence we like to specify this price.

 

External fine silicone is not required when using compriband tape so if you had a discussion with your architect then you can let us know during order process as this is one of the points my colleagues will discuss with you and they are the same price. I attach a document that might help you or your architect. 

 

Newbuild timberframe should be absolutely fine for compriband.” 
 

 

 

[Selfbuildsarah]

Forgot to say. Having just watched a YouTube video I think I understand now. Such a huge chunk of the budget so I hope we will be pleased with the result.

Thanks for your interest in my post!

[Nickfromwales]

25 minutes ago, Selfbuildsarah said:

Forgot to say. Having just watched a YouTube video I think I understand now. Such a huge chunk of the budget so I hope we will be pleased with the result.

Thanks for your interest in my post!

I’d say do the cosmetic silicone via other after the external rain screens are 100% complete.

 

A few folk employ a “mastic man” at that stage to do all the external mastic / frame sealant etc, and these folk eat / sleep / breath sealant and mastic so do a good job; essential this isn’t fecked up as it can’t be easily removed and reapplied. Like REALLY can’t. 

[Selfbuildsarah]

I got the impression it was part of the Norrsken installation. Have I got that wrong. I liked the look of Compraband. Was there a reason to avoid?

[Nickfromwales]

5 minutes ago, Selfbuildsarah said:

I got the impression it was part of the Norrsken installation. Have I got that wrong. I liked the look of Compraband. Was there a reason to avoid?

It’s down to the finish of the reveal. You don’t really want CB on display, and some will use a fat bead of mastic to cover it.

 

Theres fitters and then there’s fitters, but Norrsken tend to attract / employ the best from what I’ve seen to date.

[saveasteading]

On 26/09/2025 at 22:01, Nickfromwales said:

I pointed out that wasn’t silicone,

Can you either explain or point me to a reliable source of information, even a reliable manufacturer's page, of what are silicon, mastic, CT1, etc chemically,/ property-wise and their uses and abuses.

I mean in general.

I admit to saying silicone as a generic term and I should know more about it.

When I replace the blackening and failing shower base seal, fill failed gaps between old weatherboard and aluminium windows and so on, it seems I shouldn't be grabbing any old tube form my stores.

Would a good starting point be to only buy big branded stuff and select from their range?

 

[saveasteading]

Compriband is stunning stuff, and many a reservoir, bridge, big building has it in crucial locations. It has a dimension memory and expands with force to fill a gap, and won't fail in sunlight.

I suspect cheaper products, and very much cheaper products,  are often substituted in domestic work.

[Nickfromwales]

1 hour ago, saveasteading said:

Can you either explain or point me to a reliable source of information, even a reliable manufacturer's page, of what are silicon, mastic, CT1, etc chemically,/ property-wise and their uses and abuses.

I mean in general.

I admit to saying silicone as a generic term and I should know more about it.

When I replace the blackening and failing shower base seal, fill failed gaps between old weatherboard and aluminium windows and so on, it seems I shouldn't be grabbing any old tube form my stores.

Would a good starting point be to only buy big branded stuff and select from their range?

 

 

A big subject, so I’ve started a unique thread for it.