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Musings of an Energy Nerd

Plan Ahead For Insulation

Important decisions shouldn’t be left to the last minute

At this stage of construction, all of the insulation details had better be nailed down. Whatever you do, don't wait until the last minute to decide how you'll be insulating your walls and ceilings.

For decades, designers and builders of wood-framed homes didn’t spend much time thinking about insulation. The usual approach — still followed in much of the U.S. — was to fill the stud bays with fiberglass batts, and, once the ceiling drywall was installed, to unroll some fiberglass insulation in the attic.

Because of this decades-long legacy, it’s not unusual for a designer, builder, or homeowner to post the following question on Green Building Advisor: “We just finished framing, installing windows, and roofing. Now we have a few questions about the best way to insulate.”

My usual reaction is, “Really? You’re asking now?”

These decisions need to be made early

In many cases, these insulation questions are posted a few weeks too late. Why?

Why would builders leave these decisions to the last minute? Well, some builders think, “We’ll either fill the stud bays with fiberglass batts — or, if the homeowner wants to pay for an upgrade, we’ll just use spray foam.” The trouble with that approach is that a wall with insulation between the studs has a lot of thermal bridging through the framing. If you care about this thermal bridging, you really need to include exterior rigid foam insulation or exterior mineral wool insulation — or frame a double-stud wall.

So this important decision — about how to address thermal bridging — has to be made at the design stage, before the walls are framed.

You have to plan ahead for roof insulation, too

In most climates, cathedral ceilings are also best insulated with exterior rigid foam. But the decision to install rigid foam has to be made before the roofing is installed.

True, it’s possible to thoroughly insulate a cathedral…

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5 Comments

  1. C. B. | | #1

    small suggestion
    Martin, could you modify "it’s usually too late to install any rigid foam on the exterior side of the wall sheathing" to "it’s usually too late to install any rigid foam or rigid mineral wool on the exterior side of the wall sheathing".

    For the future homeowners starting the process who have found GBA (like me 3 years ago), just want to make sure all of the options are listed for them to consider.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Mineral wool is mentioned
    C.B.,
    I didn't want to repeat myself too many times, but my article does make the point you're talking about. I wrote, "If you care about this thermal bridging, you really need to include exterior rigid foam insulation or exterior mineral wool insulation."

  3. Clara Kim | | #3

    Unfortunately, we had a gray
    Unfortunately, we had a gray haired architect who wasn't interest in significant green energy design. I sent him GBA information, I got a consultation report from Transformations, and I discussed my net zero goals. He assured me that double stud walls would be way more expensive than some rigid insulation and spray foam. He made complicated roof designs and two rooms over exterior space. Ultimately we decided to walk away because the aesthetic was not a good fit and I couldn't get over my anxiety about how he was handling green energy issues. It was an expensive lesson on the design process. We're working with a new architect now who works closely with Transformations. I feel optimistic we'll get it right this time.

  4. Lucy Foxworth | | #4

    Important article
    Martin,
    I know you don't necessarily control what Taunton decides to do, but some articles are too important to be restricted. This is a prime example. This is precisely what pulls people in and makes them subscribe, but if they don't see it, they don't know what they're missing.

    Every once in a while an article like this should be open to everyone - both for information and as an example of the content you get when you subscribe to GBA prime.

    It's a great article. I've been here for a number of years and research things pretty thoroughly and I still am kicking myself about some of the renovation, insulation choices I've made.

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Lucy Foxworth
    Lucy,
    Thanks for your kind words. GBA will be publishing this week's "Musings of an Energy Nerd" blog on the free side of the paywall, as a "GBA Prime Sneak Peek" article.

    Here is the link to the version of the blog that is accessible to all readers, even non-subscribers:
    GBA Prime Sneak Peek: Plan Ahead for Insulation

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