Spray foam roof insulation

[Wavewatch]

Hello , I am considering spray foam insulation for the roof ,(under tiles), of a 1920's house just purchased. There is no uderfelt present . I assume the foam would help bond the whole thing together , avoiding the task of removing the whole roof to felt and then re-tile.
I know about some of the benefits and advanages of this stuff ,( though I've just found out there is closed cell & 5 percent open cell!), but can any one tell my why not to use it , please? Has anyone gone down this route and regreted it and why?
Love the site

 

[big-all]

water that gets in cant evaporate easily and rots the timber if trapped

immposible to easily replace broken tiles

will add hundreds off pounds to the cost off replacing the roof when you get it done properly

 

[Wavewatch]

Thanks for the speedy reply, big-all ... sounds like sense. Have you come accross rotten laths and timbers as a result of this yourself?
I suppose rusty ,failing nails would be included here.
No matter what insulation is fitted ,any damp or water getting in should be sorted out well before hand ,I guess you'd agree.
I still have yet to find anyone who has had thier loft sprayed ,despite the huge amoubt of spraying options available on the web; this could well be a good indicator ! along with your advice , big-all.
Cheers

 

[mointainwalker]

I believe, from forum posts, once you get outside of members of the " Plastic-Loft Spraying Association" then nearly all will advise you not to do this unles it is a measure to ptreserve an "almost-dead" roof for a last few years for whatever reason.

Main problems are: condensation, lack of breathability ( different aspect of same problem ) , impossible access

 

[noseall]

Main problems are: condensation, lack of breathability ( different aspect of same problem ) , impossible access

...oh and completely fuc*s up the roof...