Where to spend time and money on cathedral ceiling?
I am putting an addition on my own home and feel like I have absorbed enough information from here to be dangerous! With so much information I feel like I have gone down a rabbit hole and cant see out.
Our house was built in the 1960’s and has 2×4 walls with paper faced batt insulation, 1×6 sheathing, stucco exterior and 2×8 rafters (2/12 pitch) with batt insulation, cathedral ceilings and 1×6 sheathing and strip venting along soffits only. The mechanicals and windows were replaced throughout about 8 years ago. We are in climate zone 7. We are on relatively tight budget, and will be self performing alot of the work.
The addition will be 6×18 and will enlarge the master bedroom and provide space for a master ensuite. The new construction will be 2×4 walls, tyvek drain wrap and 2″ of exterior XPS insulation with 3/4″ strapping and fiber cement siding. The ceiling assembly is 2×8 rafters to match existing and avoid increasing total height due to property bylaws which would require more engineering/permits to avoid. It is designed as a bulk fill 7″ closed cell spray foam unvented assembly (Cathedral ceilings need an effective R28.5 for code here so interior strapping would likely be required) . Plywood would be used for the sheathing and rockwool R14 cavity insulation. Foundation will be a frost wall with 2.5″ of exterior XPS. These assemblies seem to give us the most cost effective way to meet code.
I have received alot of push back on the ceiling assembly as lots of people want to avoid unvented ceilings and the risks involved. This assembly will also need to be engineered for the city to allow this to be done.
So my questions are:
-Is this the best option? Given that the remainder of the house is still only at a maybe R20-24 ceiling. Would this money being spent on spray foam be better off spent on bringing the remaining roof assembly to code by doing an overroof over the new and old together? The total roof area is 700 sq ft vs the 100 sq ft for the addition.
-Would something like standing seam metal on strapping be an effective way to minimize any issues with the unvented spray foam ceiling.
-If going the spray foam route should I convert the entire existing roof assembly in the master bedroom into the unvented portion as well? The interior ceiling will be removed anyways so in theory it makes sense however the amount of spray foam required is definitely cost prohibitive.
-Since we are replacing some portions of the exterior cladding, do we continue exterior insulation further and insulate the walls better because we are already doing portions of it? This would be alot less work, then completing an overroof.
-Finally do we just keep it simple and plan on doing the new portion as designed (Because it will be less time and money) and tackle the remainder of the house another time from above and outside when/if the funds and desire may be there to do it all.
Thanks in advance for your help!
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