Insulation in eaves that are part of ceiling and no overhang
Michael318
| Posted in Energy, Heating & Insulation on
Hello, I recently discovered this forum and have been enjoying reading the posts. I’m deciding between a few options for a ceiling insulation plan in a bathroom I’m remodeling. I’m not going to achieve R38, but I want to make an improvement for R value, air barrier, and moisture control. I don’t have photos at the moment, so I’ll describe in detail.
I’m remodeling a second floor bathroom in a two story colonial house built in the late 40s/early 50s in Pennsylvania. The house is cinderblock/stucco on the back and sides and stone in the front. 7/12 roof pitch with no overhangs. About 18″ of the second floor ceiling, in the front and back of the house, is sloped because the ceiling is above the top plate of the exterior walls. I haven’t found much online about this configuration or have seen it in any diagrams. It’s sort of like a partial cathedral ceiling.
The bathroom is 5′ x 7-1/2′ in a corner of the house (i.e. two exterior walls). 7-1/2′ high ceiling. I gutted the bathroom walls and ceiling and want to redo the insulation. This post only concerns the ceiling. The rafters and ceiling joists are 2×6. The attic above is unconditioned and vented with two gable vents. There appears to be no soffit or ridge vent.
Their is a window just under the sloped part of the ceiling. The top edge of the old window casing was the point at which the sloped part of the ceiling met the wall. It’s a vinyl replacement window so I have some room to add wood around the inside of the window frame and move the casing tighter around the window. But I can only lower the top casing 1-3/4″ max, maybe 2″.
I’m not concerned with the depth of the ceiling joists because there is an attic above and I can put another layer of insulation as thick as I want running perpendicular over the joists. The sloped part is the tricky part. The old insulation that I removed was a thin layer of mineral wool with a black paper facing (appears to be original to the house) and pink fiberglass on top of that. The fiberglass must have been stuffed down into this area from the attic if not original. Likely compressing it. I suspect that it held some moisture against the roof decking which I want to avoid.
Plan A is to nail 18″ long 2x2s to the bottom edges of the rafters that make up the 18″ of sloped ceiling. Then, install RockWool 2×6 R23 insultation. This will keep the insulation 1-1/2″ away from the roof decking. I don’t believe there is a soffit vent, but if one was added in the future it would provide an air space for venting. Also, I’m not an insulation expert, but I feel like the insulation shouldn’t touch the roof decking. The old fiberglass insulation was stuck to the roof decking in some spots. Some of the roof planks are in bad shape and may need to be replaced in a few years when the roof is due for a new one. A small section in this area was already replaced the last time the roof was done.
Plan B is to not add any depth to the rafters and cut two layers of 2″ polyiso rigid foam board to fit into the 2×6 rafter bays. R26. This also leaves 1-1/2″ of an air space between the roof decking and insulation. However, the consensus of other posts I’ve read is that people don’t like fitting foam board in the bays. Additionally, I could add a layer of 1″ foam board across the rafters with 1×3 strapping for something extra. R32 total. It’s a small bathroom. 5’x7-1/2′. I would cut one 2 inch 4’x8′ sheet up into 14 inch x 2′ long pieces that would extend from the top of the wall to the beginning of the attic. I’ll only need one or two sheets of foam board so I’m not worried about labor.
Plan C is similar to Plan B, but use Rockwool 2×4 R15 insulation in the 2×6 sloped part of the ceiling to leave an air space and add a continuous layer of 1″ polyiso rigid foam board strapped across the rafters. Then, drywall. Also, add 3/4″ or 1/2″ foam board between the strapping. R24 total
The window and also the desired mounting height of the shower curtain rod are limiting me from adding thicker insulation in this area. Am I right that the insulation shouldn’t touch the roof planks even if there isn’t a soffit vent?
Replies
I like your plan C
You can think about skipping the strapping under the foam, and use thicker foam board, and glue the foam to the rafters (with a couple screws to hold in place), then apply the drywall, using adhesive and screws long enough to get to the rafters.
If you get some 3 inch drywall screws, you can put up 1.5 inch foam, use half inch drywall and still have an inch of threads into the rafters.
A horizontal 2x4 screwed into the wall can help hold the drywall in place while you get the screws in and let the adhesive set.
If you are working by yourself, you can put up a helper in the ceiling to hold the top of the drywall. A short length of 2x4 with a hole to clear a screw, and the long screw through the 2x4, into the ceiling joist, but loose enough to let it rotate can be quite a help. With a little planning, you can lift the drywall, set on the wall end, and rotate the little helper to keep the drywall from falling down. Then you can focus on screw application. I would want to cut and dry fit everything before getting the adhesive out.
Thanks, Uncle Mike! I like it. I still have to look at cost and what materials I'll have leftover from other areas, but it's a small room so I'm willing to spend a little more to be happy with the result.