Insulation retrofit for a 1920s wood-framed home in a mixed climate–Help!
After a lot of research, we continue to go ’round and around’ attempting to select the best strategies for envelope improvements for our home. It is a wood framed structure built in the 1920’s and we are located in NE Kansas (zone 4 non marine). We live in a rural area that is quite exposed to wind on a regular basis all year, so adding an air barrier is a must.
The walls: 2 x 4 framing @ 16″ o.c. with 1 x 4 solid wood sheathing and wood exterior siding. The stud cavity was insulated with loose cellulose that has settled to the bottom 2/3-1/2 of the wall, there is a plaster interior finish.
For the walls, we’d like to remove the siding and apply rigid insulation to the exterior of the sheathing. We are wondering what weather barrier is recommended for this type of application, what rain screen clips might work best to attach wood siding back over the insulation, and whether we should go ahead and fill the top of the wall cavity with more loose cellulose.
The roof: 2 x 6 framing @ 16″ o.c. with 1 x 4 spaced shearing, 1/2″ OSB, and asphalt shingles. We have a partially finished attic that stops short of the low eaves of the roof. So, the attic is open to the second floor and a vaulted area that we created on the first floor by removing the drywall from the ceiling/floor framing to access the additional space between the ceiling/floor and the underside of the roof. When we did that work, we placed R-13 fiberglass insulation between the roof joists and applied a foil faced (both sides) EPS rigid insulation to the interior side of the roof rafters. We then covered it with 1/2 drywall and applied a plaster veneer finish. The second floor space has loose cellulose in the joist cavity and over a small vented attic portion at the top of the gable.
We would like to add exterior insulate over the entire roof, and replace the shingles with a standing seam metal roof. At the vaulted roof condition that is open to first floor, we are concerned about trapping water vapor or moisture in the stud cavity if we go ahead and install rigid insulation on top of the roof sheathing, given that we already have foil faced EPS on the interior face of the roof joists. metal roof. We are also wondering if we should replace the loose cellulose in the vented attic and between joists on the second level with something else.
So, If you are still reading this post, we are deeply grateful to anyone willing to lend their recommendations and expertise! Thank you!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Katherine,
Q. "For the walls, we'd like to remove the siding and apply rigid insulation to the exterior of the sheathing. We are wondering what weather barrier is recommended for this type of application, what rain screen clips might work best to attach wood siding back over the insulation, and whether we should go ahead and fill the top of the wall cavity with more loose cellulose."
A. Every wall needs a water-resistive barrier (WRB). For most installations of exterior foam, builders choose to use plastic housewrap (for example, Tyvek or Typar) as a WRB, but it is also possible to use rigid foam as a WRB if you want to go that route. Here are links to three articles on the topic:
All About Water-Resistive Barriers
Using Rigid Foam As a Water-Resistive Barrier
Where Does the Housewrap Go?
When it comes to rainscreen details, the most common way to establish a rainscreen gap is with vertical furring strips (usually 1x4s), not with clips. For more information on this topic, see All About Rainscreens.
Yes, you should fill the insulation voids with cellulose. Ideally, this cellulose insulation should be installed by a contractor who is familiar with the dense-pack method. For more information, see How to Install Cellulose Insulation.
Q. "At the vaulted roof condition that is open to first floor, we are concerned about trapping water vapor or moisture in the stud cavity if we go ahead and install rigid insulation on top of the roof sheathing, given that we already have foil faced EPS on the interior face of the roof joists."
A. As long as you install enough rigid foam insulation above the roof sheathing to keep the sheathing about the dew point during the winter -- in your Climate Zone, that means R-15 of rigid foam -- and as long as the sheathing is dry on the day you install the rigid foam, you should be fine. Here is a link to an article that explains what you need to know: How to Install Rigid Foam On Top of Roof Sheathing.
Q. "Should replace the loose cellulose in the vented attic and between joists on the second level with something else?"
A. If you are installing at least R-15 of rigid foam on the exterior side of your roof sheathing, you are committing yourself to transforming the vented attics into unvented conditioned attics. That means that you need to install enough insulation on the interior side of the roof sheathing in these areas to achieve a total R-value (exterior insulation plus interior insulation) of at least R-49. When you do that work, seal up the vents.
To be clear, with R15 exterior foam on the roof you are limited to a total R of about R50. If you put only R15 on the roof and put R50 on the interior (total R65), the roof deck will run colder, and run an increased risk of moisture problems at the roof deck. Rather than relying on the prescriptive exterior R alone, look at the ratio of the IRC specified code min (R49) and the prescriptive exterior insulation (R15) for guidance. For zones 4A & 4B that's about 35% of the total R on the exterior. At higher total R values you have to maintain that minimum ratio to have effective dew-point control at the roof deck.
For example: If you have 2x10 or 2x12 rafters it's possible to install R38HD or R38 batts under the roof deck. With only R15 above the roof deck you'd have a total R of (R15+ R38=) R53, but a ratio of only 15/53= 28%, which might be OK at the warm edge of zone 4A, but it would be pushing your luck at the cold edge. Going with a minimum of R16.5 above the roof deck (4" of Type=II EPS is enough) would be necessary to maintain the 30% minimum ratio for dew point control.