Should I offer to pay?

If you feel awkward then get him a voucher - maybe John Lewis / Waitrose and explain that he has been so helpful and this is a treat.

I always offer to pay unless I am doing an equitable favour back. That way I am sure they don't feel taken advantage of. If he refuses payment maybe bring him some baking or a plant or something he will like to say thank you?

Oh definitely pay what you feel you could afford or a small percentage of what you would have paid a professional
It also depends on your finances if he’s a rich man and you aren’t too well off just a small amount and a bottle of wine if you’re equal footing a bit more if he’s out of work and you re feeling ok yes £50
Lucky lady I don’t know anyone who would offer small jobs free or for small ammount

I wouldn’t bother with a voucher, offer him cash, depending on how long he is doing it and what you can afford.
You will know by his reaction if he is hoping for something, or genuinely doing it as a favour.
Never know why a voucher is viewed as more appropriate than hard cash.

Try find out how much a professional would charge , and decide from that amount what is reasonable to give . Otherwise , think £50 would be a nice sum . I would absolutely insist that he takes the money when offered as this is quite a big job to be doing for nothing . Sounds a very kindly person .

Did you know that this can be done for free with Government Insulation Grants ect . Warm Home did mine for free when it needed ‘ topping up ‘ to required depth just last winter .

Dibbydod offers the same suggestion as I would; ask around for an approximate estimate of the work being done and offer something near that. Laying loft insulation is hard work!

I agree Chewbacca it is hard work and there is a lot to be said for having someone you trust doing the work. I would check how much it would cost if you employed a tradesman kangaroo73

What's the insurance situation regarding work being done in your house? Tradesmen have insurance but your helpful nieghbour may not. What if he God forbid, falls through the floor or electrocutes himself?
Sorry to be a downer but it's worth discussing then you can bring up the subject of payment.

I would be very wary of causing offence. Perhaps sound him out with something like 'this is a big job, how would you feel if I were to offer you something for your trouble?' and watch closely for his reaction. If you pay him it changes the relationship, and he may not want that.

My neighbour does some jobs for me, last week he stopped two dripping bathroom taps, was a bit of a faff as they needed new cartridges so he went to 2 different shops for the right ones, climbed in the loft to turn the cold water tank off and replaced the cartridges with a bit of a struggle to unscrew the taps. The new bits were nearly £20 and with petrol etc I gave him £50 in an envelope he didn’t even look. Texted me later to say thanks very much it was very generous. I figured if I could even get a plumber out for such a small job it would of cost much more.
I think £50 would well received by your kindly neighbour.

£50 sounds ok to me but I might personally choose the voucher route. Someone wondered why..well to me that is more like a gift than a payment & keeps thing more on a friendship level.

I have been on both sides of this, and this what I suggest saying: This feels like quite a big job to me and I would like to offer to pay......he will say no.....you say: I do understand, and I am so grateful to have you as a neighbour and would hate to offend you......could I make a contribution to a charity that you support?
Doesn't suit everyone, but for some is very welcome indeed. I have one neighbour who is delighted that I do this.

Then a small gift (especially if you are up to offering home made cake or similar) can be a nice thing to do.

There are companies who supply the insulation and install it for free, the cost is covered by a government grant which is not means tested.

Cash in an envelope is what I'd do, maybe tucked inside a thank you card. I was reading (Money Saving Expert, I think) that it's safer to give cash than gift vouchers at the moment. More convenient for him too, to be honest.

i think that at least £100.
not as job, would be much more, but as a favour.

i presume you can vouch for his bona fides.
see other thread, re scammers.

Thanks everyone for the advice. Re the latest government grant, as far as I’m aware it isn’t completely free. I think they pay two-thirds and you have to pay the rest