Insulating Detached Garage with Loft - What is the Best Approach?

I'm in the planning stages of my garage remodel. The biggest steps are insulation and electrical / lighting. I'd like the members' opinion on how to best insulate the space. My goal is to make the space more comfortable in summer and winter. I will be heating as needed with propane heater, but the garage will go days without being heated.

Location: Climate zone 4, minimal snowfall amounts

Detached 1.5 car, unfinished. Basically it's one big room. The loft has no existing venting. The existing roof is fairly new and in very good shape.

There is a full loft or attic made of floor boards atop the ceiling joists, but there is no airflow separation between attic space and main space... the floor boards for the attic space provide storage, but there are gaps at the walls.

I don't want to separate the main space from the attic space. I want to leave the floorboards as-is, and keep the opening to access the attic open.

I plan to insulate the attic and walls (and windows, doors, etc.) as a single "heating zone." Is it best to keep the garage as a single zone? Any issues with moisture that I should be concerned with?

From this link:
http://www.finehomebuilding.com/how-to/articles/a-crash-course-in-roof-venting.aspx



"Option 1: Insulate below the roof
The most conventional approach to insulating a roof is to put all the insulation below the roof deck. This approach is especially prevalent in retrofits when the existing roof is in good shape but the attic is being conditioned"


I'd like to expand this idea to "the whole space is being conditioned."

Thanks for your tips and suggestions!


EDIT: I realize there are similar threads here on GJ within the first few pages of this subforum. I have not seen specific application of an existing unvented space, and I need to evaluate options to insulate, hopefully without modifying the existing structure.

I think you need to fully close off the attic area from the main floor (garage area) and fully insulate both spaces. I think you will need to vent the attic/loft by some means. Could you install gable vents and finish the attic ceiling to allow for roof venting?

If your heating the lower area and not using (heating) the upper floor i would close the two off because all your lower heat will escape to the top floor.

I have a similar set-up but loft is vented and fully closed off from the lower floor (currently have a stairway that will be closed off with entry door).

My primary usage area is the garage so i wanted that tight. Both areas are to also be heated/cooled seperately. I have R19 in the walls and R30 between the floors with no moisture barrier. I have not yet started insulating/finishing the loft area.

You can check out my build thread to see how i set mine up.

Thanks Dave! I've seen your build and it's impressive. If I were building from scratch, I would be doing separate zones. My space is a lot smaller (20x16x8'6" lower space, attic space of 20x16 but the roof meets the walls so there is no additional 'wall height' in the attic portion, just maybe 6' center height to the roof spine).

Volume, main area: roughly 2700 cu. ft.
Volume, attic: roughly 960 cu. ft.
Total ~ 3660 cu. ft.


I want to avoid separating the attic from the main space. The attic 'floor' is just floor boards nailed to the beams and not sealed in any way from the space below. There are gaps on the sides where the attic 'floor' meets the roofline / walls. It will be a LOT of work to get the spaces separated.

The main space's 'ceiling' is the beams, with the attic's floor boards going across. I want to maintain that look of the old wooden structure, and utilize the space in between the beams for lighting and long item storage.

If I insulate the ceiling, I will lose a lot of valuable overhead space. Insulating and enclosing the ceiling will drop the lighting down another 4+ inches which will cut me down to 8' working height. Now, I can recess the lights into the space between the beams.


I want to insulate, but don't know if I can just follow the above example of a non-vented space if it is the entire garage.

Short version: cut and fit rigid foam between the rafters.

Long version: It sounds like your objective is conditioned storage on the decked ceiling joists. I think that's reasonable. I also think it's reasonable *not* to try to isolate the attic storage space from the garage space. It's no different from insulating a room with a cathedral ceiling or creating a conditioned attic (treating the entire garage as the "attic"). Why isolate, seal, insulate and condition the garage space, then turn around and do it again for the attic? I suppose a case could me made to condition the storage attic less aggressively than the garage, but even if it used less energy, what's the payback period?

How is you roof framed? Rafters or trusses? How deep are your rafters or truss upper chord members? What's the slope of the roof? If yours is like may, you may have insufficient depth to easily and inexpensively insulate to R-38 (IECC 2009) between the rafters. You may also have an issue with insufficient depth above the top plates for insulation equal to or better than the wall's insulation. Implementing a venting approach will reduce the available depth for insulation.

Given your desire not to re-roof, the general approach shown in the image you posted from Lstiburek's article is the one I favor. Of course, the magic bullet for unvented roofs is expensive closed-cell spray foam between the rafters.

There is a similar, but more affordable option for DIY's: cut rigid foam to fit between the rafters. It's not really practicable to have a contractor do this, it's fussy and time consuming. In a nutshell, you cut the foam smaller than the rafter bay and seal the gaps with spray foam. Here's a link to get you started: How to Retrofit a Roof with Insulation. Read the comments and the check out the link to the original question and comments that prompted the "How to Retrofit a Roof with Insulation" column.

Assuming R-6.5 for polyiso board, you'll only need 6 inches to achieve R-38. Frugal types will seek out used polyiso board. Here's a company specializing in reclaimed rigid insulation: Insulation Depot and there's always craigslist.

The main concern I would have with an unvented attic (aka hot deck, aka cold roof, aka hot roof, aka insulate the underside of the roof itself) stems from some reading I did (probably on the building science community website or in a fine homebuilding article) was the need for a vapor barrier on the EXTERIOR side of the roof sheathing when using spray foam on the underside of the roof deck.

The reasoning being something along the lines of: it dews or rains, you get moisture on shingles, then the sun comes out. That mosisture is driven through the shingles, through the somewhat permeable typical felt paper, through the sheathing, and is trapped between the impermeable closed cell spray foam and the sheathing, causing the sheathing to rot.

Would traditional fiberglass batts be capable of blocking that much moisture? I doubt it, but I would look into it to be sure.