Replies

1. 
   turbo_rowan | Dec 22, 2015 11:33am | #1

   I found some answers here, for anyone else who's wondering.

   https://www.greenbuildingadvisor.com/blogs/dept/musings/cut-and-cobble-insulation

   I'd still be interested in hearing specific stories or feedback.

2. GBA Editor
   Martin Holladay | Dec 22, 2015 12:01pm | #2

   Rowan,
   It looks like you managed to answer your own question. When used on an unvented roof assembly, the cut-and-cobble method that you propose using is risky. The article you discovered includes a few failure stories.

   To learn about the right way to do things, read this article: How to Build an Insulated Cathedral Ceiling.

3. 
   turbo_rowan | Dec 22, 2015 12:33pm | #3

   Hi Martin,

   I have no idea if it makes any difference - I'm not doing a cathedral ceiling - it would be just insulating the rafters and using the space as otherwise unfinished storage.

   Also, I see the major failure story was new construction on a flat roof?

   Can you elaborate on the risks? I'm really (REALLY) not sure I can afford the spray foam option.

   Thanks so much for your time and input!

   ****EDIT - I see the major problem seems to be improper air sealing causing condensation and subsequent rot - so I guess my question becomes, whether it could work with proper air sealing.

   I do have a follow up question though - if I can somehow make the spray foam work financially, is it going to bite me when I eventually have to replace the roof? Given that the inspector will most likely require that the original planks be removed and replaced with plywood?

   Thanks again!

4. GBA Editor
   Martin Holladay | Dec 22, 2015 12:40pm | #4

   Rowan,
   If you are "just insulating the rafters," you are still creating an insulated sloped roof assembly -- also known as a cathedral ceiling. You still have to follow the rules if you don't want damp roof sheathing and rot.

   I'm not sure from your description whether this top floor of your house (an attic-like space) has any insulation on the floor.

   If not, it means that your house has no ceiling or roof insulation whatsoever.

   If you do have some insulation on the floor of this room, it doesn't change the rules about how to insulate your sloped roof assembly.

   You noted the link to the story of the low-slope roof with a failed cut-and-cobble job. Did you also notice Kohta Ueno's description and photos of a more steeply sloped roof with a failed cut-and-cobble job? Ueno's comment appeared as Comment #7 on that page.

   I'm not sure from your description whether you think that it matters that you aren't planning to install a ceiling. Because of your comments, it's worth pointing out that building codes (and common sense) require rigid foam to be protected by an interior layer of gypsum wallboard (drywall), for fire safety reasons.

5. 
   turbo_rowan | Dec 22, 2015 01:00pm | #5

   Hi Martin,

   No, I hadn't planned on doing the drywall, and after reading the problem cases, and with your comment on the code requiring drywall, I guess I need to find another alternative. There is a very poorly distributed (and thin) layer of balsam wool on the ceiling of the top floor and where the top floor has a sloped ceiling, the same balsam wool nailed up, and that's it. The house walls didn't have any insulation either, and we've been adding it as we remodel (dense pack cellulose, FWIW).

   What if we skip the "hot roof" idea, and go with mineral wool batts on the ceiling and behind the kneewall, and spray foam where the rafters have been drywalled over, and just leave the top of the house unconditioned - since it's unvented, are we still going to have issues? As I said originally, I have a couple of contractors telling me different things...

   Thanks very much for the information.

6. GBA Editor
   Martin Holladay | Dec 22, 2015 01:06pm | #6

   Rowan,
   It would be good to know your location or climate zone.

   If you decide to leave the top floor of your house on the outside of your home's thermal envelope, there is no need to vent the top floor.

7. 
   turbo_rowan | Dec 22, 2015 01:18pm | #7

   Hi Martin,

   We're in Madison, WI, which is (if I read the map right) zone 6?

   So - I want to make sure I'm not misinterpreting here - closed cell foam where there is sloped ceiling against the rafters, and mineral wool across the ceiling would be a viable approach with no venting other than the gable vents?

   Just to be complete - the house is 3 finished floors, with standing headroom above the third floor - but the finished third floor has sloped ceiling which is against the rafters.

8. GBA Editor
   Martin Holladay | Dec 22, 2015 01:22pm | #8

   Rowan,
   I think that you should read this article, which discusses insulating a complicated thermal boundary that includes kneewalls and sections of sloped ceiling: Insulating a Cape Cod House.