Vented Attic and Roof Plus Exterior Insulation
The basic question is as follows “Is it possible to do a vented attic +roof assembly with exterior insulation?” but it has a series of internal questions built into it…Thank you for everyone’s input.
This question comes after reading a lot of the material on this site – it has been very helpful but I am still stumped on this item. They began brewing in my mind as I was thinking about how to make my home progressively more efficient as I make periodic improvements to it. In particular, the following line of thinking began after I read these articles (https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work & https://www.greenbuildingadvisor.com/article/a-new-look-at-conditioned-attics) and about how I could accomplish this at my home.
My Current Condition:
I have a small 1350 sqft home with a 2*4 framed roof built in the early 1960s in SF Bay Area; the home currently has gable exhaust vents, soffit/fascia intake vents (pictures attached) and it is currently insulated with loose blown-in insulation about 6-8″ thick and has no baffles at the intakes. My current HVAC ducting is in this hot attic.
My Immediate Goal:
My roof is due for replacement and I wanted to retrofit the roof and attic with roofline insulation and exterior roof insulation when I redo it. The goal is to bring the HVAC into the conditioned envelope for now. Ideally, I would like to do the work myself as much as possible – not sure if this is realistic. I am no expert at this kind of work, but I am willing to put the work in and I have friends and family who have done work like this before and are willing to help me.
My Long Term Goal:
I would ideally like to get my home to generate all the power it needs from the PV system (the home is south-southwest facing with the majority of rooflines facing to the E/SE/S/SW/W) with almost no solar obstructions (trees, other homes, etc. To accomplish this goal, I plan to get PV on the roof, Battery storage systems, replace gas appliances with electric/heat-pump systems (water heater, cooktop, heater, etc), aggressively insulate and air tighten the home over time, encapsulate and insulate the crawl space, and add a fresh air ventilation system.
My Current “Main” Concern:
As indicated by the title query, I am uncertain on whether exterior insulation can be applied to a vented roof using the venting setup detailed below as I have never seen it done or seen it detailed anywhere before. The driver for this concern is the distance from a future ridge vent to the initial framing and vent baffles after we account for the entire roof assembly thickness.
My “Plan” for the Venting and the Roof:
The plan on venting is as follows. I would closeup my existing gable exhausts and shift to a ridge-vent system (seismic/shear needs permitting) with baffles running on the underside of the roof running from existing intakes to the ridge peak, with dense blown-in insulation applied to the underside of the baffles on the interior and with 5/8th drywall on the underside of the roof rafter framing. If needed I would consider extending the roof rafters by sistering on 2*2 or even a 2*4 if possible (seismic/live load/dead load) permitting. I would also replace the existing attic access, with a new insulated dropdown door, and also add some subfloor to the attic floor to the ceiling joists to use the now insulated attic for some limited storage and to place an HRV or ERV in the future. I will be seeking guidance on the structural items prior to moving forward with any modifications.
I would like to work with the soffit/facia venting setup I currently have due to cost and complexity issues. Is this possible, or am I working with an irrational expectation/assumption? In the future, I plan to replace my windows & doors and also add mineral wool insulation to the exterior walls, followed by either stucco or some form of horizontal siding – when I do this, I would add a continuous boxed soffit intake as seen on more modern-built homes and recommended in the details I have seen for vented roof assemblies.
If exterior insulation is possible, then the plan for the exterior insulation is to repair and replace existing roof sheathing as needed, apply a fluid-applied or peel-n-stick WRB (exact spec is TBD and I am open to recommendations), then install 1-2 layers of mineral fiber insulation (1.5/2/3/4 inches total depending on structural feedback), tape the exterior layer seams (again, haven’t seen mineral wool taped so not sure this is possible – open to input), followed by an air gap using either 1*2 or 1*3 furring strips (undecided on right furring to use) with a continuous bug screen (can be removed when boxed soffit is built), and finally another roof sheathing layer, with a traditional tar paper + shingle roof on top. I was initially considering a standing seam roof, but I couldn’t justify the added roofing costs to myself (is this true, or a false assumption?).
An Alternate “Plan”:
I could also shift to an unvented roof assembly as detailed in the article above and other sources (Fine Homebuilding and ProTrade Craft articles and videos), but I am not certain I have the skills to make this switch on my own and would probably need to hire a contractor to perform this work as most schemes I have seen would require the roof system to be reframed. I am also concerned about having an unvented roof given the sun exposure my roof has; my understanding is that the roofing material takes a beating when done with an unvented roof. Also, my understanding is that when you switch to an unvented roof, you need to tackle the exterior walls at the same time – if so I would also need to wait a few more years before doing this project so I could save the additional cost of this larger scope change.
I have tried to be nice and polite – wasn’t able to satisfy the “brief” requirement; my apologies for the lengthy post. I look forward to your responses.
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