Spray foam insulation

Totally agree with Gumbo on this one. Unfortunately people believe the sales person when they say it's airtight and has good insulation properties.

I'm not sure about heat loss calcs, etc. but can speak from my own experience. We got an old house/new roof spray foamed back around 2014/2015 by a company in Roscommon.

It made a huge difference for us. While probably not air-tight, you wouldn't feel a breeze in our attic spaces. I did use 25mm insulated slabs on the flat ceilings and 50mm on the vaulted ceilings to help, but I feel the main insulation is provided by the spray foam.

Cardboard inserts were stabled between the rafters to ensure the 50mm gap underneath the roof membrane.

The only downside for us was the mess from shaving the excess away when slabbing a vaulted section of ceiling. I'd also be wary of any ceiling downlighters - I presume most are LED now so heat is not an issue, but the old style ones would quite likely overheat.

I used open cell foam on the roof. And closed cell in the cavity which is quite a bit more expensive. Depending on cost, I'd consider closed cell for the roof if I was doing it again. A lot more rigid.

"If what you wish to do is not shown here, then do not construct it, as it will not be in Compliance with the Building Regulations."

That is incorrect the TGDs show a prima facia way of complying with the building regulations, but does not preclude you from adopting a different approach to comply as shown in bold below

taken from par F of building regulations TGDs

Where works are carried out in accordance with the guidance in this Document, this will, prima facie, indicate compliance with Part F of the Second Schedule to the Building Regulations. However, the adoption of an approach other than that outlined in the guidance is not precluded provided that the relevant requirements of the Regulations are complied with Those involved in the design and construction of a building may be required by the relevant building control authority to provide such evidence as is necessary to establish that the requirements of the Building Regulations are being complied with.

I've heard a lot of conflicting opinions recently as we are planning to get our attic converted. I've gotten quotes from several attic conversion specialists. Some swear by spray foam explaining that is has better thermal insulation properties than rigid board. They go on that they've done 100s of conversions with it and that will provide an architect's inspection and certificate of compliance.

Other attic conversion specialist said they will use rigid board, when I asked why not spray foam they say it's more expensive/messy and rigid board works just as well. One guy did say he'll never work it because it'll rot the rafters over time.

It's hard to know what to believe but I'm getting the impression spray foam if done right and following regulations is good but that there is a lot of people out there not doing it right/cutting corners. I think we're going to go with an attic conversion specialist who will use spray foam so I'm going to be questioning him on it and if I don't like what I hear I'll be asking that he use rigid board instead.

Ask the prospective specialist for previous air tightness test results and how were they achieved. Bare in mind that, imo, the building regulation pass / fail criteria for air tightness is still pretty poor in comparison to best practice such as the passive house standard.