Sloping lean to roof insulation question

4 Sep 2009
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United Kingdom
Hi all,

We are having a sloping pitched roof on our rear extention and the architect has put 100mm insulation between 200mm rafters, then below the rafters a 50mm insulation with vapour control & drywall built in, Kingspan Insulation K17. Then an additional 12.5mm plasterboard on top of this.

I asked the architect if this could be changed to 150mm in the rafters then a vapour control barrier then 12.5mm plasterboard but in his usual 1 line how dare I question him response gave the answer "as per the spec".

So the question is could this be changed as I have suggested or should I go with what the architect recommends? Also is there really any need to add 12.5mm plasterboard on top of Kingspan K17?

Thanks in advance

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Re the extra plasterboard, could this be for fire regs. perhaps.
Re the architect, he is in your employ - ask him - don't be intimidated, if he needs reminding just reaffirm his position in the play of things. If there is a building warranty issue that's something you'll just have to live with I'm afraid...pinenot :)
The 50mm insulation underdraw is to prevent cold bridging or shadowing occurring and is quite pertinent.

As for two layers of plasterboard, not a clue unless there is the need for additional fire protection i.e. 1 hour etc.

Ask your architect for the reasons and remind him not to be so rude. These guys like to feel very important you know.
Im not afraid to ask the architect questions, but I am biting my tounge in the interest of getting the project done. My usual response to such an arsehole would be to tell him in no uncertain terms what I think of him and ask if he had a problem would he like to meet me face to face and be so rude. He will know exactly how I feel once the project is finished and reviews are left on him over the internet.

Currently after asking the same question 3 times, and wording it differently we are back in full circle with a "refer to spec" answer literally that is the extent of his e-mail.

RE the double boarding I have worked on sites on and off for years as a tiler and im now a plumber and bathroom installer. Structural isnt my area of expertise but its unusual for things to be double boarded. Ive run this past my builder and plasterer that I use on jobs and the answer im getting back is that the Architect is an arsehole and covering his own arse, which I pretty much thought anyway.
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Thanks for the explanation regarding the cold bridging. That makes perfect sense. Now why couldn't the Architect have just said that in his reply????

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