GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

another insulation dilemma

thefinn2003 | Posted in Energy Efficiency and Durability on

hi. i’m building a workshop in colorado springs (zone 5b is it?). it’s a 700 sq ft box with a 12 foot tall 2 in 12 roof using i joists. i’m presently framing it and revisiting my insulation plans.

my drawings approved by the building department show exterior foam (4″ xps on the roof and 2″ over 2×6 walls) with r19 batts in 16oc stud bays, but i’m starting to become very skeptical of fiberglass batts. are they really that bad? should i be looking at other alternatives to filling stud/rafter-bays? will canned foam between batts and lumber make things better? this place will be my livelihood, so i’m trying to build for durability and efficiency, but cost is something of an issue.

also, the shop may only be heated to 50 degrees in the winter, but often brought up to 70 for extended work periods. is this temperature cycling a potential moisture issue?

i’m a perfectionist and i’m doing all the work myself, so i’m fine doing lots of detailing. i’m also vehemently anti-air-conditioning. this isn’t phoenix.

regards – ian.

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.


  1. iwatson | | #1

    Fibreglass is fine IF it is installed properly. However, it is notorious for having poor installs because it lacks rigidity. It’s much easier to get a good install with rockwool, so for the marginal extra cost I would recommend it. Also, since you are DIYing it it is much nicer to work with (easier to cut, not near as itchy).

    Don’t do batts on to- of spray foam. The foam will not have an even surface so batts won’t fit properly and you’ll have voids.

  2. Tim_O | | #2

    Are you intending to humidify at all? If you are trying to keep humidity up 40-50%, like a house, you may want to run the calculations on the wall sheathing temperature relative to dew points. With a 50*F interior temp, the typical rules of thumb for interior to exterior insulation may need revisiting I would think. I don't think the temperature cycling is as important as the sustained 50* temperatures, as this will cause the sheathing temperature to drop and potentially condense.

  3. Expert Member
    Akos | | #3

    An in-between option is to go for high density fiberglass batts. These are still cheaper than MW but hold their shape much better and easier to get a class I install.

    For a roof with I-joists regular fiberglass batts are the easiest as they can deform around the flanges, with MW you have trim the batts otherwise there are big gaps. Assuming your rafters are 16"OC make sure to order 16" actual width batts for a snug fit because of the thin web, with standard batts which are 15" wide ones there will be a gap.

  4. walta100 | | #4

    Consider the DIY option of using the free blower from Home Depot and covering the wall with netting and blowing in cellulose. It will fill all the voids in the I joists.

    Avoid the caned foam!

    What will the roofs total R value be?


  5. DavidDrake | | #5

    Walta's suggestion (#4) is a good one, but try to avoid the HD blowers if at all possible. They are very underpowered. The one my local HD store had to rent was also beat up beyond belief.

    I was able to rent a much more powerful blower from an independent building supply store for less than the HD blower. It made the process pretty easy, and the learning curve wasn't steep. Total cost (not including my labor) was less than fiberglass batt.

    DIY dense pack is more work than fiberglass, however. There's several threads here that discuss the process.

  6. thefinn2003 | | #6

    the roof is presently planned to have nominal r-39 insulation - r-19 batts and r20 foam. my thought was just to shoot canned foam into the gaps, not install over spray foam. sounds like people tend to like blown cellulose better?

    1. andyfrog | | #7

      From what I've heard blowing cellulose is not as bad as people might think, although what other people said about getting a good blower does sound like it'd be critical for having an easier time. And also I heard something about using sealant or glue or something to make tacking up netting easier.

      This is all second hand info (no personal experience) so definitely take with salt.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |