Just working out how much rigid insulation I’ll be needing for the roof/walls of my little Outer Hebrides project. Looks like it will be in the region of 59 sheets (2.400 x 1.200mm). Is there a rule of thumb or way to estimate how much 100mm silver aluminium tape I will need to get?

Could I suggest either that you don't use foil tape at all - use air-tightness tape - or that you buy one roll of foil tape from a number of different suppliers? I have had very mixed experience with the adhesive on foil tape. Much of what I have bought in the past I would not bet on for 10 years, let alone 60 (assumption for life of air-tightness tape).Re the length of tape I tend to draw it out if I don't just seriously over-buy, but I think if you allow 4.8m (1.2 + 1.2 +2.4) per board and then measure the whole perimeter of the room(s) and add it all up and then add 10% you should be about right.

As above from redbeard, foil tape is, generally useless. Some of mine fell off overnight. I did buy some "cold weather" foil tape. That was quite good.

Airtightness tape is much better, though im sure as its become more widespread, theres crap versions of that too. The stuff i used as bonkers sticky!

I've used mountains of foil tape in the floor and ceilings and found it totally fine. Was left up on the ceiling for a year before I boarded and it was still stuck solid. As for estimating how much, I just bought a load of it and when I needed more just went and got it, no method in my madness.

Just any old stuff from Screwfix worked for me, some has backing paper some doesn't. When you get in the zone and your technique down it just flies on.

Logically, if the boards are a bit dusty from trimming, or storage then adhesion can be an issue with foil tape, and if you try to use for bridging an angle, or gap it is easily punctured.

I also used this Tyvek tape, bought a box of the stuff - brillant stuff, sticky and tough!

I used aluminium foil tape on my aluminium coated VCL and it stuck really well (cannot get it off easily well), have sections still exposed and 5 year later all good.

However used exactly the same tape on PIR boards in the garage and it was very hit and miss if it actually stuck.. would say 90% was fine the other 10% I had to clean the contact areas.

I found that the stuff from SF was very poor. Not very sticky, very thin and fragile aluminium, but didn't come cleanly off the paper either. Perhaps they use varying sources then brand themselves.

Otoh tyvek is super sticky.... and yet easier to use.. I got it at TP where they also had some unbranded... they assured me it was the same, and it did seem to be.

The dearer stuff was better value.

If you have any left you will use it elsewhere.

Quantity? In the perfect world you use 3.6m per board on large areas. Being diy you should have minimal waste on boards, by using offcuts, but more tape.

5% waste on board, 15% on tape?

I bought some of this the other week very similar to the stuff I bought 5 or so years ago. Nice and sticky and thick enough to be robust when installed.

https://amzn.eu/d/04YOmIBN

my technique wash hang the foil on your belt. You cant cut it clean and then get it off the backing paper, you need to rip it. So id pull it out to length, fold it over on itself and then tear it. That leaves a rough/slightly exposed edge and you can simply peel the packing paper off. Its still a big job due to the square footage but i found it fairly satisfying.

I used screwfix foil tape and it was terrible, very thin and not very sticky. Then i tried SuperFoil which I found in Selco, more expensive but very robust and sticky.

You get what you pay for and would use superfoil again.

It’s hard to estimate a quantity as I found I used more in certain areas.

I bought loads, by the box full, ended way cheaper overall. Ended up with about 8 rolls left, now have none. It's also great for overwrapping insulation on pipes and will keep rodents off.

9 minutes ago, JohnMo said:

I bought loads, by the box full, ended way cheaper overall. Ended up with about 8 rolls left, now have none. It's also great for overwrapping insulation on pipes and will keep rodents off.

Is that why pipe lagging has foil outter and foul tape? Does that thin piece of foil mean a rodent won't try to chew?

1 minute ago, Oz07 said:

Is that why pipe lagging has foil outter and foul tape? Does that thin piece of foil mean a rodent won't try to chew?

Think mostly for external use and allows insulation to stay dry and not be UV damaged. But also rodents don't like chewing through aluminium that much, they will buy a simple first line of defense.

Also use bait stations, which are the best method.