This article is only available to GBA Prime Members Start Free Trial Already a member? Log in Designers and builders who do their homework before construction begins have few problems. Unfortunately, some projects happen backwards: the design and construction are well under way before the homework begins. That type of project can be problematic.At GBA, we see examples of the latter group all the time. Designers, builders, or homeowners who are in the middle of a construction project will post basic questions on our Q&A page. “I’m looking at the rafters and trying to decide how we should insulate the roof,” they write, or “We’re trying to figure out the best place to put the HRV.”Answering these questions is part of my job; however, I don’t look forward to another ten years of similar questions. I’d rather be unemployed.To help reach that goal — putting me out of the Q&A business — I’m providing a list of ten common mistakes. Let’s banish these blunders. Mistake #1: Forgot to install raised-heel trusses Raised-heel trusses should provide enough vertical space above the top plates of the exterior walls to allow for the installation of a generous depth of cellulose or fiberglass insulation, plus about 2 1/2 inches for a ventilation baffle and an air space.For a code-minimum home in Alabama, raised-heel trusses might need only 13 inches of vertical clearance at the top plates, while a pretty good house in northern Maine might need 19 inches of vertical clearance. First, figure out how much insulation you want to install, and then let the truss company know your needs when you place your truss order. Mistake #2: The insulation contractor did a bad job of installing fiberglass batts This age-old problem is still with us, as Carl Seville’s many blogs on the… Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.