Air/vapor barrier on flat side of LVL
Finally framing! We are confronting the confounded floor sill/rim detail at intermediate floor connection in terms of air sealing. Our original detail had called for Mento to cover the entire Rim joist, but our framer was concerne, with floor hosts on hangers over Mento. Our builder was concerned about the hundreds of nail holes. So we decided to install TJIs and air seal around them. My question is whether the face of an LVL rim is sufficient for air and vapor sealing, or whether a vapor barrier should be applied to the inside face of the LVL. Otherwise we could just seal the top plate of the first floor wall and the bottom plate of the second floor wall and call it a day. Is there a reason that the air stealing should be continuous over an LVL Rim joist?
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Replies
Ethan,
Your questions are a little confusing. One of your questions concerns "whether a vapor barrier should be applied to the inside face of the LVL." That one's easy: there is no requirement for an interior vapor barrier in Zone 5 -- merely a requirement (for most but not all walls) for an interior vapor retarder, which is a less stringent layer than a vapor barrier. As far as I know, the requirement for an interior vapor retarder does not apply to rim joists.
That said, it would be helpful to know how you will be insulating the LVL rim joist. Will it be insulated on the exterior? On the interior? What type of insulation will you be using?
Air sealing is a separate issue from insulation and vapor retarder questions. Of course your air barrier needs to be continuous -- so the seam where the bottom of the LVL meets the top plate, and the seam where the top of the LVL meets the subfloor above, needs to be sealed against air leakage.
Martin, I am sorry for the unclear nature of my question... I was excited about the new ease of use of GBA on mobile, so I posted from a jobsite using my phone. I think that lead to unclear question...
1. I could sheepishly edit my question.. but I meant to be referring to an air barrier not a vapor barrier ... we will be using Intello smart air/vapor barrier on the interior of our 2x6 framing..
2. The LVL Rim joist will be insulated on the exterior with 6" rockwool and 2"+ Rockwool on interior.
3. As you can see in the picture, we are planning to tape (or calk in some locations) the top plate/LVL connection. We can then use caulk at top of plate between plate and 2nd floor decking.
4. What I don't know is if the LVL needs the air barrier on the interior face (installed between each joist and taped around joist seams... or does the LVL itself act as an air barrier? I know this area of the joist bay is often filled with spray foam, but we are not using spray foam...
Ethan,
I think part of the confusion comes from the location of the rim joist. It looks for the photo as though it is flush with the interior face of the top plates. So the I-joists will be supported by hangers? Whats on the exterior of the LVL? Where is the air-barrier located? At the interior of the wall or the sheathing?
Ethan,
Malcolm's questions are good ones.
But to answer your question:
Q. "Does the LVL need an air barrier on the interior face (installed between each joist and taped around joist seams), or does the LVL itself act as an air barrier?"
A. The LVL is an air barrier. Just make sure that your air barrier is continuous. If you tape the LVL to the top plate of your wall, that means that your top plate becomes part of your air barrier -- so any interruptions in the top plate need to be sealed. Moreover, if your exterior sheathing is part of your air barrier, then any seams between the top plate and the exterior sheathing need to be sealed as well.
Malcolm and Martin
My rim joist is set to the interior of 2x6 framing to give 2" on exterior for a band of rockwool (see attached). Yes, the I Joists will be supported by hangers. I think the contractor now plans to tape all around each I joist (blocking web to create a square surface). This may be overkill, but can't see that it will hurt.
Martin, I don't understand what you mean by "Moreover, if your exterior sheathing is part of your air barrier, then any seams between the top plate and the exterior sheathing need to be sealed as well."
Exterior sheathing will be plywood, but is not intended as air barrier. Exterior sheathing will have Mento and taped/flashed window surrounds acting as a WRB.
Ethan,
Not all builders create an air barrier at the sheathing layer, and if you decided not to, that's OK. But you need to know where your air barrier is if you want to have any hope of making it continuous.
If you are using a housewrap (Solitex Ment0) as your exterior air barrier, that will work. Just make sure that the perimeter of the Solitex Mento has an airtight connection to your ceiling air barrier and your foundation air barrier. Transitions are the tricky areas.
If you always had the intention of using Solitex Mento (housewrap) as your air barrier, and if the housewrap is continuous from one floor to the next, I'm not sure why the question of whether the LVL is an air barrier even arose in the first place.
Martin
My drawing shows the intello to be continuous from one floor to the next but that was wishful design thinking, and the carpenters cancelled that plan. The ultimate solution was to tape around each I joist after blocking out the webs. No if it is true that the LVL is an air barrier than it seems that we would only have to tape the air barrier to the bottom the LVL and then again to the top of the LVL making sure that the sills are sealed and part of the air barrier.
Ethan,
I also had an interior air barrier in my design and we worked around the rim joist issue you have run into by placing a 2’ wide strip of membrane starting on the inside of the top of the stud wall and up and over the top plate. That left an approximate 16” wide piece of membrane hanging over to the outside. Once the floor above was framed, we wrapped that piece of membrane up the rim joist and stapled it to the floor before starting work on the walls. This membrane will then be taped to the membrane placed on the walls to create a continuous air barrier.
Jon
Thanks for your reply. But if Martin is correct, maybe this wrapping you suggest is unnecessary because the rim joist itself (double LVL) is an air barrier itself so air barrier?
Ethan
Ethan
Ethan,
I thought that you had an exterior air barrier (using housewrap).
Jonathan had an interior air barrier.
Which approach are we talking about?
I have an interior air barrier (intello). I have an exterior WRB (Mento).
Ethan,
It is not the rim board in my case, or in your case the LVL that is the issue, it is the connections between the rim board/LVL and the plates.
Why can’t you use the Mento as the air barrier?
Here is a picture to show what we did. I had Siga sign off on this detail before proceeding.
Thanks, Jon. As you can see from the photo I posted above, in which we taped the top plate to the LVL, I think we have the air control layer under control, assuming the LVL and 5.5" of plywood on the flat count as part of a continuous air barrier. I'm concerned that the Intello won't be continuous so our smart vapor control will have a gap at the rim. To be honest I don't know if this is a problem.
Martin... after reading back through your responses, I started asking myself... why not make the sheathing/SolitexMento my primary air barrier? I think I had it in my mind that my interior vapor retarder COULD also be the air barrier, so it SHOULD be... but our building form is very simple and at this point I am not clear why I couldn't make the exterior sheathing+Mento my primary air barrier, with the Intello on the interior acting as a secondary air barrier. I basically have a rectangle with a shed roof and flange windows, so this could be the primary air barrier as long as it is OK to tape it to the underside of the roof sheathing and then have the air barrier shift to the top of the roof sheathing (I don't imagine we would have to wrap the eave...
Ethan,
Indeed, most high-performance homes establish the primary air barrier at the exterior sheathing. How you make the transition from the wall air barrier to your ceiling air barrier depends on where your ceiling insulation is located.
Martin, I'm assuming by your use of the word "primary", that there can be a "secondary" air barrier at the interior wall that serves more as a vapor control layer keeping vapor out of dense pack cellulose.
(Ceiling insulation is dense pack cellulose in cathedral TJI rafters.)
Ethan,
Don't confuse air barriers with vapor retarders.
I'll rephrase your question: "Can a wall have both a primary and a secondary air barrier?" The answer is yes. For more information on this issue, see "One Air Barrier or Two?"
If you need vapor control on the interior side of your wall, that layer would be called a vapor retarder, not an air barrier. Some materials can act as both an air barrier and a vapor retarder, but most do not. For example, vapor retarder paint or kraft facing are commonly used as vapor retarders, but they are not air barriers.