Certain systems do require installation by licensed professionals. Students do not run electrical wire, plumbing, or HVAC ducts, but they do gain experiential knowledge of these systems by working on the house alongside the licensed professionals who do. A few specialty trades are also handled by seasoned professionals: for example, drywall installation and finishing is completed by local tradesmen as it can take years to perfect or even become proficient at this craft, spray foam insulation is applied professionally, and some sheet metal flashing and trim details are crafted by a local artisan.