Editor’s note: This post is one of a series by Eric Whetzel about the design and construction of his house in Palatine, Illinois, a suburb of Chicago. The first blog in his series was called An Introduction to a New Passive House Project; a list of Eric’s previous posts appears below. For more details, see Eric’s blog, Kimchi & Kraut.
Our structure was designed with a “cold roof,” or ventilated roof assembly. By having continuous ventilation in our north and south soffits, with a ridge vent on the top of our roof, outdoor air can freely enter the soffits and exit out the ridge vent. The benefits of this set-up are explained in this article by the Building Science Corporation, plus the first two articles you see listed in the “Related Articles” sidebar below.
We plan on using Cor-A-Vent in the soffits.
In order to make this kind of roof assembly work, ventilation baffles (also called ventilation chutes) are necessary, especially if the attic is going to have a significant amount of insulation, in particular blown-in insulation, that could potentially move around and block off the soffit ventilation from the attic, thereby short circuiting air flow from the soffits through the roof’s ridge vent.
When it was time to install the ventilation baffles, I assumed I could just go to one of the big box stores and (thankfully for a change) just buy something off the shelf. It didn’t work out that way. Home Depot had Durovent (a foam-based ventilation baffle) and an AccuVent baffle (black plastic). Both were a disappointment.
I didn’t buy the Durovent — when I saw it on the shelf and handled it in the store it looked cheap and unimpressive. It was hard to imagine it holding up under the pressure of any significant amount of blown-in insulation pressing against it.
The AccuVent product that Home Depot carried only worked in a straight line (no curve to wrap over the back of the Zip sheathing at the top of our wall assembly), which would be ideal for a cathedral ceiling application. After looking around online, I found another AccuVent product described in a company video that looked like it would work (see Image #2, below.)
Seeing the video made me think it would be an easy installation, but once I had the product and tried to install one, the realization hit that they would be a pain to air seal properly, and again, I had concerns about how it would do over the long term with blown-in insulation pressing up against it.
Making ventilation chutes on site
When I realized the AccuVent wasn’t right for our project, it was a moment of, “Uh-oh, now what the hell do I do?”
I assumed there must be a sturdier plastic baffle, but I never found one. Instead, I came across this article by Martin Holladay. As usual, old reliable — GBA — had already addressed the issue.
It was nice to have a solution, but I also knew it would be time-consuming and back-breaking (also neck-straining) — the only thing worse than working with sheet goods is working with sheet goods above your head on a ladder. Nevertheless, I would sleep better knowing it was panels of OSB rubbing up against two feet of blown-in cellulose insulation rather than sheets of flimsy plastic. Long-term solutions do wonders for peace of mind.
I used 2×4 blocks as a base for each ventilation chute — first, screwing the blocks to the roof trusses, then after putting the OSB into place, screwing through the OSB and into the bottom of each block. The image at the top of this post shows the first chute after I had put it into place and sealed the perimeter with OSI Quad Max sealant.
There weren’t always sizable gaps where the OSB chute met the top of the Zip wall sheathing, but they did occur. When there was a gap, caused by a bend in the OSB chute or unevenness in the top of the wall sheathing, it looked like Image #3, below. These gaps could be fixed with narrow strips of OSB and OSI sealant, in the end looking like Image #4.
I showed up on one rainy morning to continue installing the chutes. Image #5 shows the dramatic difference in before and after — light spilling into the interior through the soffits without chutes, and with chutes installed and sealed on the right. Image #6 shows the top of the wall from the outside with an air gap that will allow air to flow from the soffit into the attic, and then out at the ridge vents.
Once the chutes were installed, I was finally ready to put the Intello membrane on the ceiling, which thankfully I didn’t have to install by myself (Image #7). I’ll describe that process in my next post here at GBA.
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