In an excellent article titled “Understanding Vapor Barriers”, the author puts it this way:
Two seemingly simple requirements for building enclosures bedevil engineers and architects almost endlessly: 1) keep water out 2) Let water out if it gets inJoseph Lstiburek
I’ll add a third requirement, which seems equally as critical when evaluating the ‘vapor profile’ of the building envelope (walls & roof): 3) How to give your insulation the best chance at regulating inside air temperate, i.e. cool in summer and heat in winter.
The task sounds simple but the approaches to solutions are highly variable: build the envelope in a way that allows the materials therein to do this work of moisture and air control. We always want to control the airflow and in order to do so, we want to minimize leaks and eliminate any stress on small cracks that might easily become big cracks under pressure. This is where we return to the building science debate of airtight vs breathable house. I read up on this subject months ago before any of this even made sense. But, technical information like this is rarely meaningful until you have a tangible way of applying it. As our wood interior framing approaches completion, it was time to turn our attention back to this investigation in order to dry-in our structures. Once the roof decking is complete, i.e. applying plywood to the top of rafters in preparation for the roof, you need to weatherproof it with some sort of barrier.
Now, this is where the Blox really shine. They function as air barrier, weather barrier and insulation. Add the lime-based plaster to the exterior and you have a wicking material sucking moisture out so hard you best have a year’s supply of hand lotion on hand just to handle the stuff. Add clay plaster to the interior and you’ve got another moisture control layer working on your inside. No further materials needed. Compare that to my stud walls and I need several more layers.
As you can see, we have a couple different vapor profiles going on, that each require a slightly different combination of materials.
What I learned is that sometimes it’s the architect who specify the products, other times the builder has a strong POV. And, I’m sure in other cases, the builder defaults to the roofer’s recommendations. Most houses use house wrap like Tyvek. But in the passivehaus community, there’s a new category of high performance ‘smart membranes’ that are designed to do things like reflect sunlight, manage moisture transfer at the molecular level and make your house so darn air tight that you could shatter a window by turning your range hood exhaust on. Hopefully this won’t happen to me, but I have read horror stories on forums about these kinds of tragedies.
So, back to the building science discussion. The article by Lstiburek summed up much of the information I’d read in bits and bobs on other sites. The descriptions and examples of vapor profiles helped me begin to wrap my head around it and communicate to various product specialists and my team about how we should properly seal the walls. Our project is a bit more complex because it combines traditional and unconventional materials and several different vapor profiles. By tapping the eco-building community, I was introduced to a product specialist at Rothoblass. Although he was not used to talking directly to the home owner–the nitty gritty of vapor systems are usually worked out by the architect and engineers at the outset–he was very generous with his time in explaining the functionality and working out the right mix of products to for our special combination of Blox + stick frame construction.
Here’s a helpful podcast episode from Build Your House Yourself University explaining what a rainscreen is.
From the outset we planned to use our windows as ventilation. This may sound obvious, but it’s apparently very old fashioned logic. Our plaster / mason guy told us his engineer buddies insist on installing HRV or other mechanical ventilation systems to ensure that there actually is a fresh air exchange, because “they didn’t trust people to open their windows”. Now, this is just one anecdote, but read a few reddit and building science threads and you’ll start seeing that people are getting pretty obsessed about sealing that building envelope. Graeme once mused that, “if it was possible to design a house without doors or windows, someone probably would.” I’ve been convinced that while it’s important to let the house breath and outside moisture will get in, you should try to control where that happens, i.e. the open window rather than a crack in between the wall and window’s rough opening.
This is where fancy sealing tapes and glues or spray foam comes into play. I’ve been talking to a number of product specialists to determine what the right mix of products is for our house. I think it’s a highly personal decision, which is what makes it so hard. It has to be so specific to your materials and climate and goals so that there’s no such thing as “the right way” or “one size fits all”.
How we insulated the roofs
The Studio was meant to have this raw, exposed rafter & decking look on the inside. But, this detail got lost in translation from the architect and the Engineers hadn’t drawn that in their plans. And since I didn’t really examine the Engineer plans too closely (mistake!), we didn’t discover this omission until we were trying to work out the vapor barrier + insulation plan. I wish we had called a meeting much earlier with the Engineers + Architect + Builder entitled something like “Roof, Vapor and Insulate” so each group making decisions and drawings that impacted the final performance and look of this system could have been more in sync. Each group had made assumptions about what and how materials would be used and it took us awhile to all get on the same page. We did lose some time (and sleep) and spent more $$ having specifications reworked in order to ensure all the materials would work together. In order to achieve the exposed rafter look on the inside, we needed our engineers to draw a solution that permitted us to stack insulation OUTside, on top of the roof decking, to achieve our insulation target of R30. We finally sourced a PolyISO board thick enough and the engineers reworked some plans so that the roof decking would support the extra weight and keep the PolyISO in place.
The recommended value for roof insulation in our climate is R38, but given that our walls are ~ R60, our team felt that hitting R30 would suffice. So, here’s what we did for our roof insulation:
- M House: 4-in PolyISO above roof decking (R17), 6-in Celbar loose fill in rafter bays (R21)
- Studio: 6-in of PolyISO above roof decking (R28), empty below
- Casita: radiant barrier above roof decking, 9-in of Celbar loose fill at roof line (R32)
It was amazing to feel the temperature difference once the insulation was put in place. June, in Texas…so yeah, it felt great to have the roof insulated.
For more on what we did on interior insulation, read this post.