Eave/chimney transition
Hi all. I Have a detail question. I’ve installed vent baffles in every joist/rafter bay of my house. I’m installing certainteed Membrain to transition from wall to about 4 or so feet in on the ceiling on all perimeter walls to block air leakage through the wall and ceiling joint and to help my blow in insulation in the corners of the ceiling that will be inaccessible once drywall is up due to low slope hips. Before I do this to the family room, I was planning on spraying masonry sealant on the bricks and mortar and then just picture framing the top plate, roof decking, and ceiling joist gaps with canned foam. Is this what I should do or should I allow air flow to dry out possible moisture accumulation here? Thanks in advance. Picture 2 shows the brick chimney I mentioned and the portion of brick above the top plate. Maybe important to note the chimney is on the outside of the wall.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Christian , it could obviously be me. I have read the description several times and don’t quite understand what you are proposing.
How does the Certainteed membrane help
“block air leakage through the wall and ceiling joint”?
Maybe doing some rough simple hand drawn sketches, taking photos and uploading would help explain what you plan.
Again, it could be just me, in which case I apologize.
Thanks for responding. I had originally attached 2 photos to describe what I’m talking about. Are they not showing up? I have a really low sloped roof and once the ceiling drywall is up I won’t be able to reach the corners of the house under the hip rafters to insulate or to seal any air gaps. I thought a big source of air leakage was the joint where the wall meets the ceiling. I’ll be able to foam or caulk the interior walls but on the exterior walls where it gets too tight for me to crawl over to it I thought if I glued some of the membrane and extended it from the top of the wall and along the ceiling a few feet like in the picture it would seal that joint. I also need it there so I can cut a hole in the bottom, blow in insulation in the corners of the house, and then tape it back up.
The chimney question is about what to do at that location between top plate and roof decking. All other rafter bays have vent baffles to allow air into the attic from the soffit. There are about 5 rafter bays that are blocked off by a brick chimney that goes from the ground/foundation outside, though the soffit, then through the roof. I was wondering if I should spray foam those bays like they do at the basement floor joist/ sill plate area, or if I should leave it more open to dry since the brick will be going though many temperature cycles, is porous, and maybe would condensate? I think what I want to do is spray a good amount of brick sealer on those spaces, and then use canned foam to just picture frame a beam of foam and seal all the air gaps in that area. Insulation will be blown in there making contact with the brick but at least it won’t be sealed in as if I had spray foamed it.
Hello Christian,
Item #1 - I had originally attached 2 photos to describe what I’m talking about. Are they not showing up?
Comment 1: Apologies but they were not helpful originally. More helpful with the additional explanation.
Item #2 - "I’m installing certainteed Membrain to transition from wall to about 4 or so feet in on the ceiling on all perimeter walls to block air leakage through the wall and ceiling joint (...)" "I’ll be able to foam or caulk the interior walls but on the exterior walls where it gets too tight for me to crawl over to it I thought if I glued some of the membrane and extended it from the top of the wall and along the ceiling a few feet like in the picture it would seal that joint. I also need it there so I can cut a hole in the bottom, blow in insulation in the corners of the house, and then tape it back up.
Comment 2: Having the Membrain in place to install insulation sounds reasonable. But be careful how you attach the Membrain to the ceiling joists. If you put insulation before drywall on top of the Membrain, you will most likely get insulation that finds its way between the bottom of the ceiling joist and the Membrain that is also attached to the bottom of the ceiling joist. That will very likely cause problems when you install your drywall (unless I am missing something.)
In terms of air sealing the "air leakage through the wall and ceiling joint", I don't fully understand how extending the Membrain 4 feet will help with that. I saw another of your questions where Bill was explaining how to seal the top plate (and windows and bottom plate.) That series of exchanges made lots of sense to me. I am just not clear how you would use the extra 4 feet to improve air sealing. Can you share more details?
Item #3 - picture framing the top plate, roof decking, and ceiling joist gaps with canned foam.
Comment 3: I don't think that a canned spray foam is going to seal anything when it is applied on a vertical or horizontal surface or even a corner. These foams are meant to be applied in gaps. Even in gaps, using them as a sealant does not always give great results. If ever you do this just the same, I suggest that you use a can that is identified as fire block. Fire block spray foam is not meant to be applied on the entire surface of a joist/rafter bay and may not be fire resistant enough to meet your code. Fire block foam would be better than just using the regular stuff. Myself I would seal with.... a good quality sealant. I would buy a good quality fire resistant sealant meant for outdoors. I would apply it around the picture frame as you suggested. See link for examples:
https://www.3mcanada.ca/3M/en_CA/p/c/building-materials/fire-protection/sealants-foams-putties-mortars/
Item #4 - I was wondering if I should spray foam those bays like they do at the basement floor joist/ sill plate area, or if I should leave it more open to dry since the brick will be going though many temperature cycles, is porous, and maybe would condensate?
Comment 4: If the chimney is in good shape right now, don't try to fix what is not broken (DMWI). Things need to dry. If they can't dry, they deteriorate. I would not seal the brick myself for the same reason. In theory, the hot air going through the chimney would help dry the chimney to the inside (of the chimney) but I don't know how often the chimney is used so hard to say. If it blocks off 5 bays that are usually 24" wide, that is one wide chimney servicing multiple rooms or mulitple fireplaces. If it is servicing multiple pieces of equipment, it will not be fully used every single day and will go through cold/hot cycles.
I would be using a few batts of mineral wool insulation right up against the chimney, not sprayfoam, not fibreglass, not cellulose. That should be easy to install while the ceiling is open. Why mineral wool? "Because it is made of stone, an inorganic material, it does not act as a food source for spores or mold. As such, stone wool insulation will not contribute to the growth of mold, mildew, bacteria or rot."
https://www.rockwool.com/north-america/advice-and-inspiration/blog/designing-with-mold-and-mildew-resistant-insulation/
Christian, here is some reading about how to choose sealants and how to seal different types of gaps. Good article.
https://www.greenbuildingadvisor.com/article/construction-sealants
In the comments, someone has added a link to this document from Owens Corning.
https://www.dow.com/documents/en-us/app-tech-guide/62/62-1112-dow-corning-americas-technical-manual.pdf?iframe=true
Thanks again. Regarding the certainteed sealing the air leak. I’ve seen methods of air sealing the ceiling from the attic where they go around and can foam or spray foam or even caulk areas where there are common leaks. I have plastic boots that will completely seal my recessed lighting and will address other protrusions in a manner similar to this (canned foam around a boot or box of some kind).
Most research I’ve done says other big sources of air leakage is around the top plates of walls (either interior or exterior) where the vertical wall drywall meets the horizontal ceiling drywall. Most seem to spray a bead of foam or sealant in that area to block an air from passing through. I assumed that leaks would develop in these areas after wood framing expanding and contracting throughout the seasons and eventually any paint or mud or tape on the interior side would develop small air leaks. Maybe the source of the air is actually from the cavities in the wall? Either way I can address most walls in this manner from the attic but when it comes to the exterior walls and especially the corners, there are huge hip rafters and the low slope of the roof that prevent me from having access to that area to seal it. I thought glueing the 4-5 foot wide certainteed at the top plate to the wall certainteed and extending it from the top plate into the house along the ceiling would block any air that could leak in those areas on the exterior walls. Am I wrong in thinking this area needs addressed or additional air sealing in this manner other than the drywall corner tap and mud and paint or whatever?
I also need it there for the insulation which luckily you approved of haha. I put an excessive amount of staples in the joists where the insulation could get between the membrane and wood as you mentioned because I had the same concern but I may also just fasten a 2x4 to press against the joist and Membrain to make sure nothing gets in between.
I agree that the wall top plate can be a source of air leaks through the wall assembly. So I fully agree with your efforts to keep the air out completely or diverting into the vented attic with the extra 4' of Membrain.
One challenge is the way the house was framed. High performance home often have a small wall between the the top plate and the ceiling joists (lower part of the truss). This improves the ability to air seal where the wall meets the ceiling and gives place for a full layer of insulation right to the outside wall. With older framing techniques (still used in many houses today) is normal to sit the trusses right on the top plate. It is more challenging to air seal - but it must be done to the best of our ability. You end up tappering the insulation at the edges of the roof and there is more air that can affect the temperature of the insulation around the edges of the roof.
You must have been reading lots of material but I found this older GBA article that appears to have some really good information if you are interested.
https://www.greenbuildingadvisor.com/green-basics/air-barriers
You mentioned "...I can address most walls in this manner from the attic..."
I would suggest that you do as much as possible before installing the drywall. Working from the attic is definitely not easy.
Hello Christian, I thought you might want to read this Q&A from today. One comment explains a strategy to have multiple air barriers on the ceiling. Drywall is a common air barrier but having 2 air barriers can help reduce air infiltration. That would mean extending the Membrain over the entire ceiling. Food for thought.