External insulation vs. internal insulation
Hello all,
I have a house built in 1974 that has a mansard roof. I am replacing the cedar shake on the mansard. I am wondering whether it makes sense to dense pack the wall cavity inside the mansard or to add external insulation. Here’s the thing: the wall cavity is about two feet deep at the base and tapers up until it is the depth of about six inches at the roof line. This cavity separates the outside wall of the mansard with the living area, with its own vertical wall on a separate set of studs that meet the studs of the mansard wall about 5-8 feet below the roof (the ceiling heights are about 16 feet.)
My original plan is to dense pack the wall cavity, even if I can’t get any insulation at the very top because there already exists insulation.
However, I am wondering if it makes sense to instead add external insulation to the plywood that sheathes the mansard.
Regardless, I am adding a waterproof, vapor open barrier around the external sheating before adding furring strips and attaching the cedar shake to the strips.
The only question is internal dense pack cellulose (which is cheaper) vs. external insulation. Thank you!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
With a dense packed unvent roof you'll need exterior insulation to keep the average wintertime temp at roof deck above the average dew point of the interior air. How much exterior insulation that takes is climate dependent- where are you?
JS,
First of all, can you tell us your name? (I'm Martin.)
Here is a link to an article that explains the ratio rules behind the principle introduced by Dana Dorsett: "Combining Exterior Rigid Foam With Fluffy Insulation."
Remember that for the purposes of this ratio calculation, the assembly you are talking about is a roof, not a wall.
I had not thought of this before, but a mansard roof essentially creates a knee wall space. And you are pulling that knee wall space inside the building. Whether or not you go with the exterior insulation approach or not, you still need to be sure you have a continuous air barrier as part of your new insulation system.
Peter