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Best DIY exterior wall cavity insulation for 2×6 wall with exterior rigid foam

jgfornetzero | Posted in Green Products and Materials on

Hi everyone!

I’ve been searching long and hard for the best way to insulate my future home’s exterior walls with the available sub contractors and cost effective materials available in my area. I definitely want to include a thermal break between the studs and sheathing in some form. The path we’ve gone down to date is to have exterior insulation on the outside of our sheathing of a 2×6 exterior wall.

To give context, I’m in Yakima Washington. I’ve been told having OSB or plywood sheathing is most likely a requirement due to seismic considerations. It’s also worth noting there are not any dense pack cellulose contractors in my area willing to do the job, which was the direction I wanted to go… that leaves my contractor suggesting a dense pack fiberglass as option. It’s my understanding that dense pack options mean I cannot do it myself without a commercial blower which I don’t believe I can rent in our area. I’d like to save on labor costs for the wall cavity insulation if I can to keep the budget where it needs to be.

All things above considered, this limits dense pack options unless the materials costs are greater than a subcontractors labor plus their charge for materials. The options I’m debating between are listed below, these materials are in sequential order going from exterior to interior:

1) Stucco exterior finish, XPS rigid foam at 2″ taped w/ taped seams (DIY), Tyvek Drainwrap w/ taped seams (DIY), plywood sheathing (DIY), 2×6 OVE framing, dense packed fiberglass cavity, finished gypsum board (DIY).

2) Stucco exterior finish, InSoFast EPS panels at 2.5″ (DIY), Tyvek HomeWrap w/ taped seams (DIY), plywood sheathing (DIY), 2×6 OVE framing, dense packed fiberglass, finished gypsum board (DIY).

The Questions:

– Thoughts on using Roxul ComfortBatt or high density fiberglass in either of these walls since I can to the labor myself and save on the subcontractor labor costs?

– Thoughts on the Stucco finish with the exterior rigid foam? Looking to do something different than fiber cement lap siding. Open to other ideas, especially DIY friendly ideas or something more cost effective 🙂

Feedback is much appreciated and advice is much appreciated!

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  1. GBA Editor
    Martin Holladay | | #1

    Stucco can be a tricky cladding. Before you decide to install stucco over exterior rigid foam, you should read this article: To Install Stucco Right, Include an Air Gap.

  2. Dana1 | | #2

    Dense packed fiberglass is a good option, but not usually amenable for DIY installation.

    Most batt installation companies can install it for less money than it cost YOU (a lower volume buyer, non-contractor) to even buy the stuff. But there are definitely good /better/ best insulation contractors. Be sure to inspect and fix any imperfections before closing it all in. R21 fiberglass is pretty good if installed will, R23 rock wool is better on several fronts (fire retardency, lower environmental footprint in manufacturing, etc) but probably more expensive. R20 fiberglass is still pretty good, and manufactured in higher volumes than R21, ever since R20 became code min under IRC 2009. If there's a big up charge for R21s or rock wool, go with the R20. The difference might even buy you another half inch of exterior foam.

    Use foil faced polyiso and NOT XPS on the exterior, since it's both somewhat higher performance and MUCH lower environmental impact in your stackup & climate.

  3. jgfornetzero | | #3


    Thank you for the feedback. I have been considering a board and batten finish or maybe even fiber some modern cement panels if available in our area. I think either of these options may be a little more budget friendly as well.


    Appreciate your response. I completely understand your preference for Polyiso over XPS. What about an EPS product like InSoFast panels for the exterior sheathing (link)? I've been looking at these and really like that they solve the issue of needing a rain screen/gap between the sheathing and siding.

    Would really to hear about any experience folks may have with using EPS on the exterior.

  4. GBA Editor
    Martin Holladay | | #4

    For an overview of many of these issues, see How to Design a Wall.

  5. Dana1 | | #5

    EPS is blown with the same low impact blowing agent (pentane) as polyiso. As XPS loses it's HFC blowing agents, over time it's performance becomes the same as EPS. In Yakima (US climate zone 5B) it takes a minimum of 2" of EPS to provide adequate dew point control at the sheathing when using only standard interior latex paint as the interior vapor retarder.

    EPS is orders of magnitude more vapor open than foil faced polyiso, and potentially an issue under stucco, but as long as the stucco is vented both bottom & top it should work. It's not clear that InSoFast would provide sufficient venting for the intense bursty moisture drives of stucco. It provides a drain space, but not much vent space, and at 2.5" it's still more vapor permeable than dry CDX or OSB. It would probably be fine in semi-arid Yakima, but it's not a substitute for a true 3/8" or greater vented gap.

  6. jgfornetzero | | #6


    What if I went with a different finish on the InSoFast EPS, something like board and batten or fiber cement panel siding option. Would that be better since the moisture drive would be less? I would think a 3/8" gap could possibly be foregone but I don't have any experience or reference data that suggests this.


    I've thought about looking into Roxul products in the exterior wall cavity and as a continuous insulation on the exterior walls. I've heard it is quite expensive but if there is significant cost discount available that would be interesting. Guess I could call and ask rather than just wondering :) I do like their products from what I've heard, seems like they are well liked and that they are considered greener building material in comparison to other options.

  7. Sal_123 | | #7

    Joshua, if you don't mind, would you share the stucco system you plan on using? I had a similar assembly planned with an exterior "outsulation" of 2.5" XPS (satisfy dew point condensation, thermal break over 2x6 stick framed walls, zone 5, new construction) with final stucco exterior. Take heed of Martin's remarks, this could be a risky build. After extensive reading, talking to local stucco and masonry contractors, getting input from guys smarter than me, including 2 local engineers, I gave it up. Not one person said it was a good idea. Not sure what stucco system you have planned (three coat, single coat, EIFS), dependent on the weight, have you considered the issue of wall creep? Did you have an engineer detail your fastening system? Care to share it, would love to hear what they specified for 2" XPS. What made you choose Drain Wrap over stucco Wrap? I have just started to review options (again) for this part of my project.

  8. JC72 | | #8

    I wonder if Roxul would give you a discount if you went with Comfortbatt for the interior studs and Comforboard 80 for the exterior insulation. Stucco can be applied over Comfortboard but it requires a backer board to create a rain screen gap and I don't know how expensive that is in your area.

    See pg 69 of their install guide.

  9. jgfornetzero | | #9


    We have been planning on an EIFS type system that deals with moisture (see here here), but all the details haven't been finalized. We are also seriously considering InSoFast EPS 2.5" exterior panels with smooth Hardie fiber cement panel siding instead which is often used with commercial projects. It looks clean, different than stucco but clean (examples). As you are, I've been concerned with the stucco details as the subs in our area rarely deal with exterior rigid foam. With a fiber cement paneling we can do the work ourselves and make sure the details are right.

    I would be very interested in hearing from anyone with InSoFast experience with exterior rigid foam applications. Curious if the drainage ridges do the trick or if a gap on the exterior of the sheathing would still be required?

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