New Double Stud Wall Assembly with Rock Wool & Smart Vapor Retarder

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We’ve been inspired to test a new double stud wall assembly on First Steel that may be used in future projects like reNEWBOLD. There is a 2.5 minute video below walking through the basics of the wall assembly. You will find more details below the video to feed your inner building science geek’s thirst for knowledge and discussion.

If you’d like to see this wall in person before we close it up, please attend our mid construction event tomorrow evening between 4PM and 8PM. Use the Facebook Event to RSVP and find out more details. We hope to see you there. As a side note, anyone interested in the reNEWBOLD project should strongly consider attending as one of the homes is a beta model for the homes in South Philly.

New Double Stud Wall Details

That’s it for the assembly. I’ll try to address some additional detail I didn’t have time for in the video.

The rock wool insulation is very unique product that is used a lot in Europe and also in the US in commercial fire rated and sound proofing applications. What’s great about it is how well it completely fills the cavities. It does not droop or sag at all like typical fiberglass batts. It also does not absorb moisture and will not produce mold which makes it perfect for installing during construction and prior to being completely dried in.

In our first test houses, we found it was easiest to frame up the entire house with the exterior stud walls and treat the interior stud wall just like the other interior framing. That allowed us to get the house tight with the ZIP sheathing product and come back to insulate the exterior wall at a relaxed pace once the main framing of the house was completed.

I am still a bit torn on the effectiveness of the vapor retarder, especially when we have the ZIP sheathing and vented rainscreen on the exterior keeping us dry and a thick layer of drywall with multiple coats of paint on the interior. I know people will chime in saying I’m wrong not to have this interior vapor barrier and hopefully I will learn more from the comments in this post to help make a decision on future projects.

Special thanks to Greg La Vardera of Lami Design who has been inspirational in testing out this wall assembly. That’s all I’ve got for now. Let’s leave the rest to the comments.

36 Comments

  1. Chad, that really looks like an awesome assembly.

  2. cyndi says:

    Would love a home made that well, esp. in the north!

  3. Jack says:

    Chad, thanks for sharing your process in this video — much more informative than a detail drawing. I think this wall is an easy leap for builders accustomed to the old “code wall” since the exterior sheathing and window installs are all unchanged. Nice having the vapor barrier protected from future penetrations (e.g. homeowners nailing holes to hang artwork). A few questions come to mind:

    1. Do you do a blower-door test prior to installing the final layer of insulation? Seems like you might want to identify any air barrier problems before burying the MemBrain… Or now I’m re-reading this and wondering if the Zip system provides your air barrier at the exterior, with the MemBrain only being used for vapor control.
    2. Does the MemBrain product have a “front & back”? If so, then installation in this manner would be a bit tricky since it comes off the roll for a traditional installation on the interior side of the wall. (I’m thinking it probably doesn’t matter, and the MemBrain must be smart in either direction.)
    3. Why fiberglass instead of more Roxul on the interior side?

    Cheers!

  4. Jack says:

    OK never mind on my question #3…. I thought you were using ordinary Knauf, but now I see that it is EcoBatt. Seems like a good choice for lower embodied energy and better indoor air quality, plus it should be easier to fit around your romex, etc.

  5. Chad Ludeman says:

    It’s also a lot cheaper ;)

  6. Mike Cartwright says:

    Great video guys! Just getting into sustainable design and have been taking a few on-line course. In a recent class on envelope design we discussed the proper location of the vapor layer. From what I have learned the location inside the interior wall is fine. A quick calculation of the wall assembly temperature profile would suggest the assembly temperature will not go below the typical indoor dew point of 40-45F until you get into the outer wall on a 20F day. Even if you do get water vapor into the assembly it can not get into the insulation so you should be fine.

  7. Jay says:

    Green construction is definitely an important area for the history of construction. The problem is that not enough developers see the value in trying to go 100% green. Great to see videos like this with people in the industry really worried about our planet.

  8. Rob Fisher says:

    So if I am understanding this correctly you are achieving R-39 with no exterior rigid insulation? If so that seems pretty impressive for this rather straight forward approach. Why the change from the previous double stud, cellulose and exterior rigid insulation wall? And are you still using a rain screen detail? If so what is it? I suppose with the metal siding you would not need it but I would think you would still use it with the other wall claddings. I would also like to hear more about the smart vapor retarder considering it is the third vapor retarder in this assembly, I wonder how much it actually does?

  9. Chad Ludeman says:

    Hi Rob. Excellent questions. First, we never did actually use the exterior rigid insulation. It turned out to be much more expensive than the straight double stud wall. We seem to be constantly experimenting with insulation types and honestly we are not set on any one insulation for all of our homes. We are set on air tightness and eliminating thermal bridging in all projects though.

    We are still using rainscreens. With the corrugated metal cladding, there was no need for an additional rainscreen as the corrugations provided most of the gap we needed from the exterior of the wall. Lastly, I agree that the smart vapor barrier is a bit redundant but others do not so I wanted to try it out once. I’m not the expert here, but you can find great articles at the Building Science website for starters. Thanks for the comment!

  10. Michael Tuso says:

    Hi Chad,

    I am planning a high R exterior wall for a home addition in the (sometimes) wintry mountains of Va. Double stud looks really good for insulation, sound blocking and wiring cavity. I have been thinking about a variation on your design 1. Use 2″ Roxul Comfortboard IS 4′ x8′ panels between the walls instead of 2″ AFB Roxul. 2. Use metal studs for the interior studs. Metal studs should conduct less sound and dont twist or warp but thermal bridging might be an issue. Any comments are appreciated !

  11. Chad Ludeman says:

    Mike – This sounds like a good wall to me. We have considered doing metal studs on the interior in order to achieve a better finish on our interior walls. It is separated from the exterior enough to not have thermal bridging be a concern. I’m not an expert on metal framing assemblies, so you might want to double check more with more knowledgeable people or documentation.

    As I recall, we priced the 2″ IS board and it was far more expensive than the AFB which is why we did not use it. It would save a bit on install, but not nearly the amount of the price difference… Good luck!

  12. Michael Tuso says:

    Hi Chad, have you had any problems with the interior studs warping and affecting the inner wall surface ? Since the studs do not have plywood or OSB sheathing on the other side, there isnt much to keep them in alignment. Some 2x4s look like airplane propellors after they dry.

    Metal studs that have a U or C cross section would require batt insulation that is wider than the insulation intended for wood studs. From the Ecobatt web site I see that batt insulation is listed in the correct widths for metal studs – dont know how available it is.

    Damp spray cellulose insulation might be good for the inner stud wall, but Im not sure it will adhere to the MemBrain vapor retarder that lines the exterior-facing plane of the stud cavity.

    Why did you choose Ecobatt instead of Roxul for the interior stud wall ?

    Thanks,
    Mike

  13. Michael says:

    Interesting stuff Chad. Did you price out several different methods before landing on the double wall? Just curious to see how a 2×6 with expandable foam might stack up. Keep up the good work.

  14. Chad Ludeman says:

    We price out different assemblies constantly. We are not huge fans of spray foam due to the large carbon footprint and cost. It would be more expensive to go that route and we would have a lower R value overall with more thermal bridging. It also doesn’t allow your wall to breathe which there is a debate on constantly on whether that is critical or not.

  15. NIce wall and good job on the service cavity. Some items I would like to note to greatly improve air-tightness of this wall:

    The airtight vapor variable membrane should be connected to the airtight layer on the floor below, to the ceiling and windows, otherwise the service cavity will get bypassed at those points -(of course I recommend INTELLO Plus that I import – as it is stronger, doesn’t tear and works really well with Pro Clima tapes).

    Some ideas how to solve these airsealing issues in the following blogpost (an using INTELLO as a blow-in mesh):
    http://www.foursevenfive.com/index.php?main_page=page&id=15&p=724

    Of course these means you either need to use a little bit more lumber, or apply the airtight layer over the rockwool before building the service cavity wall in front of it.

    Some of Greg La Vardera’s details (swedish and new USA walls) make connecting airtight layers from floor to floor easy – this and other solutions to deal with beams, floor connections can be found here:
    http://www.foursevenfive.com/index.php?main_page=page&id=15&p=313

  16. Dave D says:

    What is the R-value of the Roxul AFB? I don’t see it listed on their website. I know its a higher density aimed at STC ratings and fire resistance, like the Safe n Sound. Wouldn’t the higher density compromise it from preventing heat loss?

  17. Chad Ludeman says:

    Good question. We calculated that it was about R-7.6 or so. I don’t think the increased density lowers the R-value. It’s kind of like when you cram a 2×6 batt into a 2×4 wall. You only get the insulated value of a 2×4 wall or the same R-value per inch even though it’s higher density. Not sure if that reference is clear or helping…

  18. Mike S says:

    Chad, It appears you are using double 2×4 for the front and back wall at 24″ and 2×6 for the party walls? Is that correct. Are you using a double wall construction for the bearing party walls as well. What does the insulation look like there?

  19. Chad Ludeman says:

    Hi Mike – We are using 2×6 on 24″ centers for the party walls. We are just using batt insulation there for sound proofing as both sides are inhabited. Even on unihabited party walls, we do this as there will be a building there in the future and we can’t give up that much floor area and cost to justify a double stud party wall… Also few UL rated double (or quadruple) party wall details.

  20. Dave D says:

    Chad-
    I get your logic but I’m just going by what Roxul says on their site. They mention the higher density in their safe n sound actually does have a negative impact on heat loss. I’m not sure if this is the case with the AFB. I’ve used the safe n sound in some exterior walls where there was machinery/hvac to deal with. having done it in winter I can say that the rooms felt warmer despite it not being the comfortbatt.

  21. Dave D says:

    Finally found a tech doc with the R value, Thermal Performance:
    ASTM C 518 (C 177) R-value/inch @ 75°F 4.3 hr.ft².F/Btu

  22. Dave D says:

    It looks like the electric is run behind the interior studs but in front of the vapor barrier. Is the wire stapled to the stud?

  23. Brian Beaulieu says:

    Thanks for posting this! I’m really glad I stumbled upon your video as I intend on utilizing the same wall detail for my home being constructed this summer. One concern I have is with the walls being so thick, that the inside surface of the zipboard will build up condensation (cold sheathing issue) and OSB hates water. How did you handle this? Did you strap the exterior to provide a rain drainage gap?

    One additional question: I assume that you taped the seams of the membrain? I didn’t see this in the video.

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