Replies

1. GBA Editor
   Martin Holladay | Jul 23, 2018 05:06am | #1

   User-7117614,
   First of all, can you tell us your name? (I'm Martin.)

   Q. "I have read articles here from a few years ago about adding dense-packed cellulose blown into the wall cavities from the outside. Our walls are relatively thin (studs are 2x4s), so I am worried that we will be unable tor each the R-value required to not reach the dewpoint inside the walls."

   A. I'm not sure what you're talking about. The surface where homeowners worry about condensation is generally the exterior sheathing. Adding insulation between the studs makes the sheathing colder in winter, and therefore more susceptible to condensation, not less. The thicker the insulation, the colder the sheathing, and therefore (theoretically) the greater the condensation risk.

   All of that said, people insulate 2x4 walls all the time. Done correctly, this insulation work will increase your comfort and lower your energy bills without putting your sheathing at risk.

   Dense-packed cellulose not only insulates the stud bays; it also greatly reduces air leakage. This reduction in air leakage is what makes this insulation approach safe for your walls (since the moisture responsible for condensation problems generally piggybacks on exfiltrating air).

   If you want a higher R-value than can be achieved in a 2x4 wall, you can always make your walls thicker. This is generally done by adding exterior rigid foam or interior rigid foam, although there are other ways to thicken walls to raise the wall's R-value.

   Q. "The rafter space consists of 2x4s and there are soffit and ridge vents throughout. ... It does not seem like there is enough depth to the rafter space to even think about an unvented roof, right?"

   A. If you want to insulate a roof assembly with 2x4 rafters, here's what you need to do:

   1. Consult an engineer to see if your rafters are structurally adequate. (My guess is they aren't.) An engineer may tell you that you'll need new rafters -- perhaps 2x6s, 2x8s, or 2x10s.

   2. Once your engineering is complete, you'll need to evaluate the best insulation approach. Options include exterior rigid foam, flash-and-batt, or various vented options (some of which may require even thicker rafters than recommended by the engineer). All of these options are described in the following article: "How to Build an Insulated Cathedral Ceiling."

2. 
   user-7117614 | Jul 23, 2018 07:26am | #2

   Thanks a lot, Martin. My name is Chris.

   Unfortunately, thicker walls and/or rafter bays are not in the budget at this time. It does sound like you are saying to me that dense-packed cellulose, even in these thin walls is not likely to cause moisture problems.

   Thanks again.

3. Expert Member
   Dana Dorsett | Jul 23, 2018 09:02am | #3

   In zone 5A NY a cellulose insulated 2x4 wall would still need an interior side vapor retarder tighter than latex paint for dew point control at the sheathing.

   Some houses that vintage don't have sheathing- the siding is nailed directly onto the studs. These walls should not be simply filled with insulation without first gutting the interior walls and putting an exteriors side air barrier in each stud bay with at least 1/4" of air space between the air barrier and siding. If that step is skipped, a second layer of BACK VENTILATED siding needs to be placed over the original.

   If there is plank sheathing with tar paper or rosin paper between the sheathing and siding it can usually be safely dense packed.

   The most critical detail to get right in all of this is window flashing, and may homes of that era have none. Bulk water incursions into empty framing still dries reasonably quickly, fast enough to avoid mold/rot problems, but once it's insulated that will no longer be the case.

4. 
   user-7117614 | Jul 23, 2018 09:47am | #4

   Thanks, Dana. Much appreciated!