Unvented roof on an occasionally heated building: is roof insulation necessary?
Hello all,
I am building a small workshop (170 sq ft) that will normally be unoccupied in climate zone 2. It has a simple sloped roof (unvented enclosed rafter assembly) built with 2x4s and insulated with rock wool (r15) in the rafter bays. I have attached a photo for clarity.
I have read Martin Holladay’s article on “An Unvented, Superinsulated Roof” which suggests that an unvented roof assembly like mine needs rigid foam on the roof sheathing to prevent condensation on the underside of the roof sheathing in the winter. I then read IRC 2018 R806.5 (requirements for an unvented enclosed rafter assembly) which states that an interior temperature of 68F is assumed in suggesting a minimum of R5 foam board on the roof side to prevent condensation on the underside of the roof sheathing.
As the winters are mild here, I don’t see much point in heating it unless I am working inside. The workshop is just for hobby work so the occupied hours will be limited. This has brought two questions to my mind:
1. Do I need to bother with the rigid insulation on the roof sheathing if the interior temperature will only sometimes be “warm” compared to outside (and never at night time which has the lowest outdoor temperatures)?
2. Is there some reason that I should consider heating the unoccupied building in the winter? There is no plumbing to consider. I do intend to keep tools inside, but I can’t think of a reason that being left in the above-freezing cold would harm them.
Thanks for reading. Any feedback is very much appreciated!
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Replies
The "R" in "IRC" stands for residential, and has built in assumptions that the space is both conditioned continuously and occupied at some high duty cycle.
The shop building is not residential space.
So, no, there is no absolute requirement to heat the space when unoccupied or to install the exterior foam to protect it from the chronic interior side moisture drives of breathing / cooking / bathing human occupants that aren't there.
Dana - thanks for your reply.
Although it is not a "requirement" to install insulation board or to heat the space - will omitting this step in the roof assembly be likely to cause condensation to form on the underside of the roof sheathing if I am occasionally heating (which would give the air more capacity to hold moisture) and occupying the space? Should I plan on adding a dehumidifier in the winter months to avoid this from happening?
If it matters, I have tried to build this workshop like a house - tight - with zip sheathing, fluid applied flashing, and caulking - in order to keep heat, humidity, and bugs outside.
Unless you're generating a lot of moisture in the indoor air, the roof deck is not at risk.
Wintertime air is normally too dry (in absolute dew point terms) for room dehumidifiers to have much effect. Ventilating with outdoor air when it's cold outside would lower the indoor humidity in winter, whether actively heating or not. If building tight, plan on ventilating it when using it in any season, but active dehumidification beyond that is a cooling season issue, not a heating season issue.
Thanks for your explanation. It is much appreciated!