Converting 3rd floor to unvented, conditioned living space — insulation dilemma
My house was built in 1902 and includes a 3rd floor that has always been used as a conditioned living space. The furnace was located above collar ties/ceiling on this floor with all insulation being blown-in cellulose on top of collar tie flat ceiling framing and batts behind knee walls. Furnace didnt pass inspection and was removed along with ductwork. My bottom two floors are zoned, each with a new energy efficient central HVAC system.
There was significant damage to plaster on 3rd floor and I recently finished removal of all ceilings and walls. We plan on the 3rd floor being a master bedroom suite with a walk-in closet, bathroom and bedroom and were hoping to vault the ceilings as opposed to a lower flat ceiling and installing a mini-split system with 2 indoor units to condition this space (approx 800 sq ft).
The roof framing is very complex. It is a 12 pitch hip roof with 6 hipped dormers. From attic floor to ridge is 16′. Insulation was removed along with plaster.
I am located in the city of St. Louis, which I believe to be zone 4a.
Based on all the research I have done it seems that in a roof deck with such complex framing and so many dormers such as the one I have, ventilating it properly via ridge and soffit vents becomes nearly impossible.So… I decided an unvented attic would be my best option. I have gotten several quotes and simply cannot afford spray foam as an insulation option for underneath roof deck between rafters. The thing is… I would bite the bullet and go for it IF any of the many options these companies gave me presented an incredible r-value upon completion. My rafters are old true 2X6 boards. Even when filling it completely with closed cell insulation my r-value doesnt even meet the current standard according to 2015 code for attics.
After obsessing over it for too long I have decided to do with the cut and cobble method many have spoken about on this site.
I still have a few questions regarding specific recommendations for my application:
1. Should I leave an air gap (or baffle) on main parts of the roof where this is feasible (continuous air flow) prior to installing my rigid insulation? Or should i apply rigid insulation directly under roof deck?
2. I am still unsure if I will flash and batt or use the stacked rigid foam application, which is preferred?
3. There will be storage tucked into every knee wall and i therefore intend to insulate all the way to the eave for a proper seal in attic space, how do i address the eaves? The roof frame sits on the solid, 3-wythe brick exterior walls- where should i place my soffit dam in terms of the brick exterior walls? Inside edge where brick wall below meats the interior space, middle of the brick wall or above the outermost layer of brick?
4. With the cut and cobble method which rigid insulation is preferred and in what order should it be installed?
5. Prior to installing my first layer of rigid foam insulation where should I seal with can spray fiam?
I know this topic had been addressed and I apologize for being redundant but every house presents a unique situation and I am hoping someone will be familiar with a situation such as my own and offer advice for movnig forward
GBA Detail Library
A collection of one thousand construction details organized by climate and house part