GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Detached Garage Insulation and Venting Query

brandon! | Posted in General Questions on
Following up to my previous post, Attaching 1″ Rigid Foam Under Hardie Plank Lap Siding. I’m seeking advice on my insulation approach for a Zone 5 (Chicago) detached garage with a ridge vent. In particular, are rafter vents required (see attached photo) if the attic space remains open and unconditioned; e.g., we do not build knee walls or finish/seal the rafters? 
 
Our garage will be similar to the photo in that our main/lower space will be conditioned as I use the garage as my DIY wood/workshop year round. However, we are not planning to build out the attic space. Instead, we’ll simply lay OSB/Plywood over the joists to allow for storage (e.g., sleds, camping gear, etc.) My current insulation plan is as follows:
  • Roof (Completed)
    • None. Builder wasn’t flexible on this.
  • Ceiling
    • Drywall
    • Rockwool batts (R30) in the joist cavity
  • Walls 
    • 1/2″ fire rated drywall and/or plywood (French cleat system)
    • 2×4 studs w Rockwool batts (R15)
    • 1/2″ OSB (Completed)
    • WRB (Completed)
    • 1″ rigid foam board (R5) (Completed)
    • Hardie siding (Completed)
  • Heat
    • Comfort Zone Ceiling-Mounted 7,500-Watt Fan-Forced Industrial Heater

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. walta100 | | #1

    In zone 5 your choices are vented attic or conditioned attic you need to pick one.
    Add the vents or move the insulation to the roof line.

    If it were my shop I would add the roof vents and a heat pump set to the lowest heating set point its software allows. Plus keep the 7500 Watt heater hooked to a app based control. The HP will heat the garage using ¼ of the electricity that the shop heater will and give you AC if you chouse to run it. The HP keeps the shop at say 55° when you want to use the shop you turn on the shop heater remotely and it quickly heats the shop to a nice temp before you arrive.

    Walta

    1. brandon! | | #2

      Thanks Walta. If I’m understanding you correctly, my current approach is considered a vented (unconditioned) attic and is OK, e.g., the attic will be entirely open from the soffit vents to the ridge vents. The ceiling insulation is for conditioning the main garage space, not the attic. While I plan to place OSB over the joists to allow for storage in the attic, they will not extend over the soffits. In other words, they will NOT impede their venting function.

      Thanks for the heat pump tip. I’ll investigate it though it may be more than I need – I’m used to working in an uninsulated, leaky, unheated garage...

  2. pstephens1 | | #3

    I'm in zone 2A and am planning a conditioned/unvented detached garage attic with 3" of polyiso on the roof deck. If you're only insulating the ceiling then you'll want to vent every thing as Walta says.

    I'm also planning to condition with a heat pump (haven't decided on conventional/ducted vs. mini split yet) but I'm not sure how long the recovery time will be without the resistance heater. I may need to plan ahead on the winter and summer days I want to use the shop space.

  3. brandon! | | #4

    Follow up question to all. We've installed the ceiling drywall, blocking to reinforce the joists, knee wall studs (for joist reinforcement only, not closing in the attic), an attic ladder AND now trying to finish insulating the ceiling. I'm using Rockwool 1.5 inch x 2' x 4' Comfortboard 80 directly on the ceiling drywall with 3.5 inch x 15.25" x 4' Rockwool Comfortbatt R15 on top of it. Both will be primarily covered by 5/8" plywood to allow us to store a few things up there. I say primarily because the insulation that extends beyond the knee wall studs (towards the exterior walls) will be left uncovered. Likewise, the insulation that will be added to the attic walls on either peak end will have the Rockwool 1.5 inch x 2' x 4' Comfortboard 80 mounted (vertically) with construction adhesive. I realize the insulation levels are not to recommended amount for zone 5, but they're what's feasible for the space, the cost and the actual use - It's a garage detached garage versus attached or a room in the house.

    My question is:

    At the very edges where the soffit vent air enters the attic, is it necessary to place a small foam board piece between the Rockwool and the opening to direct the air upwards or is it okay if it just spills into that open space over the uncovered insulation and then up to the vent - The Rockwell will not be extended over the opening or blocking the opening. Basically I want to make sure I'm not creating a potential mold situation. I've attached some photos to give a sense of what the space looks like. Thank you in advance.

  4. FrankD | | #5

    If you are insulating the attic floor, you don't need any insulation on the attic walls. The attic will be cold anyway.

    It is a good idea to have some kind of baffle to prevent wind from blowing through the batts at the soffits. The baffles could be anything. You can buy pre-made cardboard or plastic baffles, or cut them from scraps of OSB, foam, etc.

    Unless you can get a really good deal, ComfortBoard is usually a pretty expensive insulation. You might be able to get 5.5" Rockwool batts for quite a bit less than your board+batt combination.

    1. brandon! | | #6

      Thanks so much for the clarifications and suggestions! Will make things much easier and hopefully less expensive!

  5. walta100 | | #7

    The appeal of rockwool is lost on me.

    I like cellulose it is an almost 100 post-consumer recycled product cost much less for the same R value and most big box stores will lend you the blower to install it for free.

    If you are going to store stuff in the attic, consider building a platform.

    Like Frank said insulating the attic walls is a waste of materials.

    You will need some baffles to prevent the insulation from blocking your vents. Have a look at this article.
    https://www.finehomebuilding.com/project-guides/insulation/baffled-by-baffles

    Walta

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |