Clearance between chimney and insulation
Hi again.
I am rebuilding the roof for an old stone cabin, and at the same time I have to build the chimney.
The roof is composed of the following layers (from bottom to top):
- Timber rafters with Expanded polystyrene between them
- 16 mm osb board
- Thick rubber weatherproof membrane
- 2″ ferrocement layer with weatherproof aggregate
- Flagstone roof tiles
I am planning to line the chimney with a flexible steel liner surrounded by rockwool. The chimney itself will be built of bricks, concrete blocks or stone.
And my questions are:
How would you solution the problem of the clearance between the chimney and the flammable roof components (the osb board, the membrane and the polystyrene boards)?
Will I have also to keep a gap between the ferrocement decking and the chimney?
Thank you again, guys.
Miguel. http://casapollo.bajatec.net
Replies
Interesting situation...
I am thinking that a lot depends how much rock woll between the liner and the masonry and how thick the masonry body is.
I had read about five years ago that the new codes were making provision that if there was a minimum of 12" masonry between fire channel and combustables, that the normal 2" spacing was no longer required.
Then for the past three years I have seen nothing to that effect.
Rock wool is better than the masonry as an insulator...
Can you fill a couple of inches between the combustables and the masonry with rock wool, then trim around it? It is framing that typically is held back 2", but trim can be in contact.
you can contact the chimney with the cementious roof, as far as fire protection goes, but the fhjalshing detail should not be tight there froma spatial POV. The masonry mass of the chimney will grow and shrink with temperature and would bust a tight flashing loose so it should be a base/counterflash type. How you will secure it to the deck and waterproof it is an unknown to my mind.
how old is this? Polystyrene insulation, OSB decking, and cement based roof coverings are not that old....
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Hi there, thanks for the prompt answer!
Well, the existing roof is very old, but providing that it is not insulated or even waterproof, it needs to be rebuilt using new materials.
The clearance issue can't be only a problem of mine, there must be thousands of buildings with similar conditions (timber rafters, plywood decking, rubber membrane) and a chimney through the roof. There must be a solution!
Thanks again.
Miguel.
Well, the typical would be to design the size chimney you will build, then frame the new roof with a hole headered off for that size and location allowing 2" of space between framing lumber and the masonry.Then build the chimney with flashings appropriate for the roof pitch and type of roofing materials. The space betwen is usually left void but could be stuffed with rock wool and then trim to hide that.I am curious about your decision to use flexable SS liner for a new chimney instead of clay flue liners. You must have a reason.
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