Sealing around windows - how much difference do these tape systems make? My windows will be sealed against a plywood box (sealed at the corners) back to the frame using steel straps, with the small gap (about 5mm) sealed with expanding foam. Will the addition of these tapes make a significant difference or would a bead of silicone do just as good a job?

There are alternatives to silicone sealers which are designed for the job such as Siga Primur but it costs about £10 a tube, it supposedly has an indefinite lifespan. I got a few tubes of it for places like under the threshold strip of the outer doors where using tape isn't practical.

My windows came pre-fitted in the TF panels from the factory and had expanding foam around them. I have cut back the excess and sealed with normal silicone, each window takes a tube. I will then finish off the job with Siga Corvum 12/48 tape, all the windows will take 5 rolls in total, which cost me €156 off ebay.de

The way I see it is that you only get one chance at it so do the best job you can.

[Edit] It's really up to you to decide if it is worth taking air tightness seriously. The house I currently rent is reasonably well insulated but has very leaky windows and doors, I know from practical experience that it is easier to keep it warm when it is -5 and still outside than it is when it's +5 and windy, so I know that air tightness matters.

But to what level? There is a school of thought that says once you get to 1 ACH any improvement on that will only see very marginal financial savings and unless your house is on a very exposed site subject to regular gale force winds then spending money to get below that level will not see a return.

As I see it having made the investment to build a fairly high spec house then I am prepared to spend a little extra (less than £1,000) to ensure that I get the highest level of airtightness I can reasonably achieve. So I will do the extra work on the windows and doors and pay attention to other details such as using a thick rubber self sealing tape anywhere a screw has to penetrate the airtightness membrane. And the biggest cost item will be a blower door test to verify the work.

Whether the extra expense will have a payback I don't honestly know.

borpin, ME500 is shown on the Tremco-illbruck UK site, perhaps they can advise on distributors.

Here is a study carried out in Norway comparing different airtightness sealing methods around windows which may be of some (limited) assistance.

web.byv.kth.se/bphys/copenhagen/pdf/040-2.pdf

However expanding foam or silicone wasn't used in the test although they did test a proprietary expanding foam tape which is used by some of the companies in the window industry, but the results were not very promising.

I know that normal expanding foam is not airtight, Hilti used to sell an expanding foam CF 812 which was generally considered to be airtight but for whatever reason it no longer appears to be available.

My own feeling on expanding foams (not backed by any proof) is that whatever level of air tightness they may have is greatly reduced if you cut off the skin on the surface.

I didnt use window tapes but I did build tight seals between all junctions round the windows.

Hi Chris P,
Thanks for the air tightness comparisons good to know where to spend our money. Another Illbruck product, FM617 claims to be airtight up to 600 Pa's but there appear to be no parameters by which to judge this.

Sorry, I was quoting from memory FT, looking at the tube now what it actually says is: "unlimited durability, resistant to ageing" If only it was beauty treatment , they would make a fortune!

Environmental/chemical/UV degradation of plastics, endless push-pulling by wind loads, fingerprints on surfaces, quite apart from movement. Stickiness just cannot endure for decades, I believe. years ago I was impressed by pressure tester Paul Jennings saying he finds airtightness degrades starting at 6 months in, tapes lifting for no apparent reason etc.

@Cav8andrew Soudal flexi foam

Following this up I found these 2 videos http://www.youtube.com/watch?v=74HnLT9S3io&feature=plcp and http://www.youtube.com/watch?v=xH8SfIooXYo

Initial investigations show the SWS appears to not be available in the UK. I've emailed them and will see what comes of that.

The 'glueing' of the window to a plywood box at the end of the first video intrigues me!

Hi borpin, thanks for the link as for one thing it gives me the air tightness component of the foam, but good to see the process of the whole system for when we fit our large windows. Not sure about the rest of the system but the flexi foam element is readily available, just taken delivery of 12 cans, from IronmongeryDirect.

Posted By: Cav8andrewHi borpin, thanks for the link as for one thing it gives me the air tightness component of the foam, but good to see the process of the whole system for when we fit our large windows. Not sure about the rest of the system but the flexi foam element is readily available, just taken delivery of 12 cans, from IronmongeryDirect.

I emailed them and had a call this morning. They are in the process of introducing the system to the UK so the tapes etc are not readily available. They are going to advise me on this project though so I'll keep to up to speed.

I pointed the guy in this direction so he may well appear!

Posted By: fostertom

Posted By: borpinThe 'glueing' of the window to a plywood box

But more often you'd want the box in place before the window arrives - late as usual. Is that foam going to really be the sole airseal between the ply box and the masonry - e.g. rough-texture conc blockwork not nice smooth bricks as illustrated? Would it do that better than a generous Compriband-type tape compressed by the fixings of the ply to the masonry?

Possibly, but in my case it would be sealing against the steel frame so a nice smooth finish! I think this was also related to SIP type construction as well. The person I spoke to said the biggest issue is that we put windows in from the outside rather than the inside like the rest of Europe.