More Q&A Spotlight
Don Lorenzo has been charged with renovating a 100-year-old Seattle house whose owner wants him to gut the interior but leave the outside of the building unscathed. Exterior walls have no insulation, no sheathing—and no signs of water damage.
“The owner wants to gut the interior and insulate the exterior walls from the inside while leaving the siding in place,” Lorenzo writes in this recent Q&A post. “Of course, when I insulate these walls I want to avoid creating new moisture problems.”
Lorenzo has been guided by an article about a similar situation in which Martin Holladay recommends an air space between the back of the siding and the new insulation. Lorenzo’s twist on this idea is to use a Dörken product called Delta-Dry immediately behind the siding t0 create a kind of interior rainscreen, which he says would accomplish much the same thing.
Dörken’s technical team recommends this approach, but Dörken doesn’t officially endorse the use of Delta-Dry this way.
“Another thing that bugs me is I don’t fully understand how water exits the wall after it trickles down the rainscreen and reaches the bottom plate,” Lorenzo adds. “Unlike a typical rainscreen in new construction, there are no weep holes. I think the answer is ‘the same way it always has’—it collects on the plate until it evaporates or leaks out through gaps in the siding. Does that sound right?”
Lorenzo’s appeal for feedback is where we start this Q&A Spotlight.
What you get is higher cost
What Holladay’s article suggested was to create an air gap by applying strips of wood in the corners of the stud bays, then adding a layer of rigid foam and, finally, air-permeable insulation in the form of open-cell…
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10 Comments
"Generally, I try to approach this type of work by recognizing that some day the siding will probably be replaced. Any work we do should lend itself to that process by being easily removed or added to."
That's very good advice - which is useful even with new construction. Anticipate the eventual replacement of the building components that will wear out first and make that process easier.
The concept of "shearing layers" is one of the most practical ideas I encountered in architecture school... and one that we probably didn't spend nearly enough time on. https://en.wikipedia.org/wiki/Shearing_layers
Brendan,
How Buildings Learn is a fantastic read for anyone interested in architecture or Construction.
Unfortunately I went to an architecture school that imparted next to no practical knowledge, and I've been playing catch-up ever since.
Oh architecture school. I have to be careful not to start ranting! Reading How Buildings Learn was definitely the exception, not the rule in terms of practical knowledge! And it was a personal recommendation from a professor, not something that was assigned to my whole class, although it certainly should have been!
Brendan,
Surely things must have improved a l0t since then. John Straub and Joe Lstiburek made a big difference t0 how building science is taught in Canadian schools. Hopefully a lot of the other nonsense has disappeared. I'd better stop too.
The detail pictured above was a typical detail for retrofitting historical houses in NOLA after hurricane Katrina. The building code officials would not allow any original siding or trim removal.
Good to know, Armando. If you have any more info on that, please send it my way.
Thanks again to everyone who took the time to weigh in on this!
Ben's Tyvek weep system seems logical, as long as the bottom of the Tyvek isn't visible poking out between the siding boards.
The spacers-and-rigid-foam method seems like the consensus favorite here on GBA. However, our contractor may ultimately opt for the spray-foam approach, in which case we'll just need to choose a product to create the air gap.
It seems like the jury is out on the importance of foam permeability, but I'm inclined to agree with the posters who think it's not a critical issue.
Something else we discussed in the Q&A was the potential for the foam insulation to absorb water that gets past the siding. My understanding is that the risk is pretty low, and it's probably overkill to add an additional layer (e.g. Tyvek) just to protect the foam. Martin has an old article that's probably relevant:
https://www.greenbuildingadvisor.com/article/using-rigid-foam-as-a-water-resistive-barrier
Finally, I wanted to mention this article by Kiley, which I overlooked in my initial research. Her post adds a fireblocking detail and proposes priming the back of the siding for additional protection.
https://www.greenbuildingadvisor.com/article/managing-water-and-insulating-walls-without-sheathing
I'm currently renovating a 120 year old New England house and there was definitely concern about insulating old walls and trapping moisture. Turns out the previous owners had done some work that involved standard fiberglass batts. But was this optimal for dealing with any wall moisture? We later realized that there was also a Tyvek rain screen between the horizontal cladding and cedar siding. Before realizing there was a rain screen, the plan was to leave an air gap behind the new insulation, using a FHB article as a guide https://www.finehomebuilding.com/2021/04/27/managing-water-when-insulating-old-walls. Ultimately these steps, and any air gap seemed unnecessary once I understood the role of the rain screen. Like the previous owners, we decided on sticking with standard batt installation but 'upgrading' to mineral wool for its better drying properties if moisture does get introduced. There's more work to do, so I'm still interested in best practices, and if my assessment of the rain screen does allow an old house to be conventionally insulated with batts filling up stud bays.
This is a comment I left on the article by Kiley linked above that is relevant to this post:
When renovating a house with no sheathing I installed felt paper in the stud bays, stapled to the sides of the studs and top and bottom plate, then had spray foam installed. The felt kept the insulation away from the siding and the foam provided a good air seal at the studs and plates. You can't really properly flash existing windows with this technique but it is about the best you can do.
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