External insulation for boarded Victorian roof

[isabel smith]

I plan to discuss this issue with the local conservation officer and the surveyor specifiying our remedial works. However, I want to be "well informed" prior to these discussions. Roof covers a rectangular single storey Victorian schoolroom. The roof timbers (3 A-beams, 4 purlins, rafters and sarking planks all in excellent condition) are visible from inside the room. The walls are random stone 500mm. The roof extends externally to form generous eaves and over-hang at gable end. I do not want to change the basic structure of the roof in any way nor alter the inside appearance. Can I add external insulation without compromising the "good health" of the roof? What materials would be most suitable? Are air gaps needed? If 'Yes' where in the assembly should these be and what ventilation path is required? Are there any good examples out there of external insulation being successfully added to this kind of roof?

 

[chuckey]

The problem is that adding insulation to the top of the sarking planks will make the roof thicker and upset the tile line especially on the gable ends if not hipped. The basic method would be to nail on new (counter) battens on top of the joists, then fill this new gap with insulation, then your sarking cloth, then battens then re-tile with eave ventilation. Unless your roof is radicaly different from the norm, the amount of headroom your insulation has is a miserable inch. There is a breed of thin insulation materials genericaly known as multifoils which claim to be wonderful for this situation, though even these need an air gap over the top of them. There is a lot of debate how (and even if) this stuff works. The Goverment has chickened out of approving them, leaving this up to local building inspectors to do. If you have the time read all 311 articles in this link :roll: :-http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=125&page=1#Item_0
Frank

 

[isabel smith]

Thank you for that. Have read the multifoil debate. Impossible to judge! - so best not go that route. Is the problem mainly about how to fill the gap at the gable end if thicker insulants are used? Is this a structural issue that is hard to solve without getting in the stone masons or predominantly an aesthetic one?

 

[chuckey]

Without knowing how your gable ends are finished - impossible to say. Until corrected, I think there are three basic arrangements:- one, Sarking boards protrude beyond wall (supported by purlins), gable end board nailed on to end of purlins/edge of sarking boards(facia board), edges of tiles covered in lead flashing. Two, tiles finish flush with outside edge of wall and gaps mortared up. Three, gable end wall extends above the tiles and capped. The tiles are water sealed to the parapet wall, with lead flashing. Providing the parapet wall is high enough this third system can be used with a "thicker" roof with minimal problems.
Frank

 

[isabel smith]

Thank you again for the response. Gable end construction is type one on your list. Why would thickening the sarking boards from above with a layer of breathable insulation be a difficult/harmful thing to do in this situation? What are the limiting factors?

 

[chuckey]

All your facia boards boards will have to be wider , or more accurately taller to cover the increased distance between the sofit board or what ever the bottom edge of the FB lines up with, and the, now higher position of the tiles. in principle its straight forward but mouldings should really match the originals and the increased amount of the wood work could make the house look top heavy.
If you are going to do this there could be an arguement to go with 2" (or more) counter battens so more insulation can be used, but again the "top heavy" look?
Just thought the gutters will have to be raised as well (and the down pipes lengthened).
Not trying to dampen your adour, but unless the roof has to be re-worked anyway, its an expensive project.
Frank