I’m building a new single-story single-family house with an unvented attic and am looking for input on how to address my attached unvented garage attic.

The home is located in Northern California in climate zone 3-C (Marine), so mild weather year-round.

The house attic is insulated with R-42 open cell foam on the roof deck, and there is currently no insulation in the garage attic.

The wall between the house attic and the garage attic is drywalled and insulated with R-30 open cell foam on the house side.

The garage has fire sprinklers, so the garage ceiling will need to be drywalled.  Fire sprinklers are also in the garage attic as the space will be used for storage. The roof is 12:12 which allowed for 6′ tall attic storage space accessible by folding ladder. 

The garage will be used for my workshop and misc storage. The space will be unconditioned, however I‘m considering insulating walls with R-21 fiberglass before drywalling to stabilize temperature during summer/winter. I was not planning to insulate the garage ceiling at this time. 

A heat pump hot water heater and small utility sink is located in the garage. Garage concrete slab has 10 mil vapor barrier underneath.

I’m looking for input on whether it is necessary to add some natural ventilation in the garage. There is currently no vents, not even in the exterior walls. I was considering adding a gable end vent in the garage attic however, the designer does not support this idea as the house is very modern with minimal penetrations, and the garage gable is very visible from street. The house/garage has no eaves, so eave vents are not an option. The metal roof has already been installed, so adding roof vent/vapor diffusion port is not an option.

I’m considering the following three options:

A) Basic Venting – Vent garage attic with modest gable end vent and vent garage space with exterior wall vents near foundation (similar to crawlspace/foundation vents). No spray foam insulation on roof deck and no fiberglass insulation in ceiling. Considering installing vents in garage ceiling to allow fresh air to flow from garage up into garage attic.

B) Unvented Insulated Attic – Treat garage attic same as house attic by installing open cell spray foam on roof deck and gable end. Storage space in attic triggers need to drywall attic storage walls and ceiling to protect spray foam from fire danger.

C) Unvented Uninsulated Attic – Leave as is. Monitor and implement option A or B down road if issue.

I was originally planning to go with Option C (leave unvented), however I’m now leaning toward option A (Basic Venting). Option C is potentially problematic given that condensation and mold could accumulate in garage attic (similar to unvented, un-spray foamed cathedral ceiling). Although moisture content of garage is relatively low compared to house living space and limited moisture should get into garage attic, it seems risky. Option B would make sense if I planned to turn the garage into living space which I do not. Spending thousands of dollars on spray foaming the roof deck does not seem like a good use of resources. Option A addresses some of potential mold issue and is cost effective.

Would appreciate feedback on the best strategy to implement. Thanks!