The Story behind a Newton Accessory Apartment: Insulation

It’s time to insulate the Newton accessory apartment that we’ve been writing about. We know … we’re probably the only ones that get excited about insulation, but choosing the right materials for the job and using the best installation techniques are not only what will keep this family warm in the winter and cool in the summer, it’s the key to keeping their energy bills down.

We always put a lot of thought into energy efficiency – that’s what we pride ourselves on here at steveworks and is what makes us different from other design build contractors. We’re lucky that our clients are also environmentally aware and actually requested that no foam insulation be used anywhere in the project – we were thrilled to hear that!

The best choice for this project is cellulose insulation which is made of shredded newsprint or cotton that is densely packed into the walls and floors. To accommodate the correct amount of cellulose, we had to create 6” thick walls rather than the typical 4”. We’ve blogged previously about reducing the carbon footprint of remodeling projects by using this type of insulation.

Once thing to note with accessory apartments: fire separation between units MUST be included. What does that mean for our project? We used fire-rated drywall and doors between the units that have a slower burn rate. In other words, if the worst happened and a fire were to start in either unit, it would take longer for the fire to penetrate the walls/doors. There are also special electrical junction boxes that are used for fire safety and wisely required to meet code.

We’ll be working on the HVAC system next, including a heat pump system. Read about that here.

Mesh netting is used on the walls and ceiling in preparation to hold the cellulose insulation.
Before we put wallboard over the netting we’ll also add a “smart vapor barrier.” The barrier is “open” during the summer, allowing moisture to pass back into the room, and is “closed ” during the winter so moisture won’t get into the wall cavities and condense against the cold exterior sheathing. The system is designed to prevent moisture and mold.
Cellulose insulation in an attic space.
This plan shows the location of the fire walls (solid red) and ceiling (hatched areas). Code also requires separation between garage and living areas which is standard for all homes, not just those with accessory units.