Go Back  DoItYourself.com Community Forums > Interior Improvement Center > Insulation, Radiant and Vapor Barriers
Reload this Page >

Air sealing while remodeling room, moisture/condensation concerns

Air sealing while remodeling room, moisture/condensation concerns


  #1  
Old 10-29-19, 08:15 AM
C
Member
Thread Starter
Join Date: Dec 2011
Location: USA
Posts: 367
Received 30 Upvotes on 28 Posts
Air sealing while remodeling room, moisture/condensation concerns

I'm in the process of remodeling a ground floor room with 2 exterior walls and while I've got the drywall down, I'd like to air seal the walls prior to installing batt insulation. I'm located in Long Island, NY, which is at the northern end of climate zone 4 (mixed-humid). From the outside to the inside, the wall assembly is brick, exterior gypsum (apparently this was popular in the 50's and 60's in this part of the country), 2x4 framing, insulation, drywall. The brick veneer only covers the first floor; 2nd floor exterior cladding is some sort of tiling.

For some reason, which I presume is related to how the second floor exterior was built, the exterior gypsum doesn't reach the top of the 2x4 framing on the ground floor (pic below). The exterior gypsum stops ~5" shy of the top plates (some of this gap is covered by lumber framing as seen in the pic).


So this is a massive air gap, as air readily penetrates the brick veneer and then can flow right into the stud bays via the openings in the top of the stud bays. I'm thinking of sealing this up by cutting foam board pieces ~6" tall and the width of each bay (less 1/4" on each side), tacking it to cover the gap and sealing it in place using canned spray foam (will also help seal around the wiring).

As for the rest of the wall assembly, the plan is to use Rockwool batts, and to break the thermal bridging of the studs, I'll either (1) attach 1/2" rigid foam board sheets to the interior of the framing, or (2) cut strips of 1/2" rigid foam board to cover just the framing members (studs, bottom plates, top plates). And then 5/8" drywall.

Do you see any problems with this plan in terms of moisture/condensation issues? Clearly water vapor is going to travel through the brick and exterior gypsum. I was initially thinking 1/2" rigid foam sheets between the framing and drywall, but it was suggested to me this might cause condensation issues inside the wall assembly in the summer. I guess the idea is that any water vapor that gets into the stud bays will then tend to condense on the exterior side of the relatively impermeable foam boards. If I use 1/2" rigid foam only on the framing members, then that water vapor has a better path to drying to the inside.

The brick veneer does have weep holes at the bottom, and the roof overhang over the second floor has vented soffits, so I assume there is a continuous path for air flow behind the exterior cladding to help dry out that cavity. So I'm thinking sealing up those gaps at the top of the stud bays isn't going to create a moisture problem.

Thoughts?
 
  #2  
Old 10-29-19, 09:05 AM
Marq1's Avatar
Member
Join Date: Sep 2016
Location: USA MI
Posts: 9,745
Received 1,210 Upvotes on 1,098 Posts
If you installed foam or batt insulation with craft paper they are both considered a class 2 vapor retarder so performance would be same.

Yes you should air seal the top, your description is similar as doing the rim joist but should caulk around the perimeter of the foam inserts.

Thermal break on the studs, have never seen that in new construction but willlet others comment!
 
cartman voted this post useful.
  #3  
Old 10-29-19, 09:39 AM
C
Member
Thread Starter
Join Date: Dec 2011
Location: USA
Posts: 367
Received 30 Upvotes on 28 Posts
Caulk around the foam inserts as opposed to using canned spray foam? Or did you mean just seal around it, with either product? I was thinking spray foam since I don't have to be very precise about sizing/cutting the rigid foam inserts, as I would with caulk.

New construction doesn't need thermal break of studs on the interior because they tend to have it on the exterior (i.e., if you use foam board outside the framing, you've already insulated the wood framing, so the studs aren't acting as a thermal bridge to the interior).
 
  #4  
Old 10-29-19, 04:08 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Posts: 27,245
Received 1,959 Upvotes on 1,756 Posts
The board in your photo looks like the ledger board for your exterior soffit.

I would probably put the foam insulation between the studs all the way top to bottom, not just a small patch. Seal the edges with gun foam. You can add a thermal break to the interior side of the stud if you want but you are going to have to extend all your electrical boxes... Add extension jambs to any windows and doors to account for the added thickness of the foam and 5/8" drywall. You will also need longer nails for your baseboard since they will go through 1 1/8" of "nothing" before the nail gets to the stud. An alternative idea might be to add a 1/2" plywood backer across the bottom of the studs instead of foam.
 
cartman voted this post useful.
  #5  
Old 10-29-19, 06:47 PM
C
Member
Thread Starter
Join Date: Dec 2011
Location: USA
Posts: 367
Received 30 Upvotes on 28 Posts
Do you mean foam board as the only stud bay insulation?

I was thinking the patches of foam board to seal that air gap in each bay, then Rockwool batts. The problem with foam board as insulation is (1) it takes a lot of time to cut all that foam, (2) the thickest foam at the big box stores is 2", which would leave 1-1/2 of stud exposed, so that reduces the R-value of the wall. To solve (2), I could add additional 1" and 1/2" foam boards (or 1-1/2" if they sell that), but that's a whole lot of cutting. Cutting the small patches, sealing the edges with canned foam and then placing batts is a lot quicker (less expensive too).

Not sure I understand the plywood backer - do you mean just a narrow strip the height of the baseboard for purposes of more easily nailing in the baseboard? I could do that, though that area would serve as a thermal bridge to the drywall. Most of the studs are sistered, so there's a lot of wood, and if I use screws, I think I can reliably hit the studs even with the 1/2" of foam board (nails and a hammer, not so much, as I suck with a hammer).

So you don't see any condensation concerns if I air seal those gaps at the tops of the stud bays? Strange that they built it this way (I was initially wondering if it was by design to NOT air seal), but I guess they needed to cut that exterior gypsum shorter in order to place what you're calling the ledger board.
 
  #6  
Old 10-29-19, 06:49 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Posts: 27,245
Received 1,959 Upvotes on 1,756 Posts
No, I was commenting on your current plan. If you put one layer of foam in the wall against the sheathing and a strip of foam on the face of each stud, you still have 3 1/2" for your rock wool. I'm saying instead of just putting a small patch of foam over your gaps, do the entire length and width of each stud bay. It adds r value and air seals, making your insulation more effective. Insulation needs to be in a dead air space in order to be effective.

yes, on the narrow strip of plywood behind the base. In lieu of the foam thermal break.

Air sealing will not cause problems, it will prevent them.
 
cartman voted this post useful.
  #7  
Old 10-29-19, 07:02 PM
C
Member
Thread Starter
Join Date: Dec 2011
Location: USA
Posts: 367
Received 30 Upvotes on 28 Posts
Got it. So if I put 1/2" foam in each bay, then Rockwool, I've got a vapor barrier in the middle of the wall assembly. Trying to understand how that would play out.

In the summer, warm moist air will diffuse through the exterior gypsum (and right through the gaps at top of stud bays) and hit the foam board. I guess the Rockwool keeps the foam board (especially the exterior facing side) from being cold enough to be a condensing surface?

In the winter, warm moist air will diffuse though the drywall and Rockwool and reach the cool foam board. Wouldn't it condense on the interior facing side and collect water between the foam board and Rockwool?
 
  #8  
Old 10-29-19, 07:06 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Posts: 27,245
Received 1,959 Upvotes on 1,756 Posts
It is not any different from the millions of homes in all parts of the country that have foam sheathing on the outside of the studs. As long as you air seal all the edges and do not put any type of vapor barrier on the inside it is not a problem. You don't have to reinvent the wheel here.

If you wanted to change anything to improve on it, it would be the thickness of the foam. Thicker is better. 3/4" would be a reasonable thickness, and better than 1/2. If you go with 1", your baseboard backer could be 1/2" foam plus 1/2" plywood.
 
cartman voted this post useful.
  #9  
Old 10-30-19, 02:10 AM
Marq1's Avatar
Member
Join Date: Sep 2016
Location: USA MI
Posts: 9,745
Received 1,210 Upvotes on 1,098 Posts
If you installed foam or batt insulation with craft paper they are both considered a class 2 vapor retarder so performance would be same
The moisture does not get trapped with a vapor retarder unlike a vapor barrier!
 
cartman voted this post useful.
  #10  
Old 10-31-19, 05:29 PM
C
Member
Thread Starter
Join Date: Dec 2011
Location: USA
Posts: 367
Received 30 Upvotes on 28 Posts
I was first looking at faced foam boards, which are much less permeable, hence my concern about sticking it inside the wall assembly (which seems to be different than using the same material exterior of the framing).

But how about this unfaced XPS board?
https://www.lowes.com/pd/Kingspan-In...ion/1000235443

1/2" unfaced XPS has a perm rating greater than 1, making it a Class III vapor retarder.

So going with @XSleeper's suggestion, I could cover the entire stud bay with 1/2" unfaced XPS board, then batt insulation, then strips of foam board on the face of the studs to break the thermal bridge, then drywall.

Would that work without causing condensation issues since it seems this would provide a path to drying both in the winter (when inside warm moist air diffuses through the drywall into the wall assembly - that moisture can pass all the way out to the outside without being trapped) and in the summer (when outside warm moist air diffuses into the stud bay - that moisture can pass all the way to the room without being trapped). Am I thinking about this correctly?
 
  #11  
Old 10-31-19, 09:00 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Posts: 27,245
Received 1,959 Upvotes on 1,756 Posts
sticking it inside the wall assembly (which seems to be different than using the same material exterior of the framing).
Sure.. there is 1/2" of difference between putting it outside the studs as it normally is when used as sheathing... than if it's between the studs. That is the only difference... which is pretty meaningless. But if you seal the perimeter it will be better air sealed than it would be otherwise. This assumes you furr out the inside of the studs by the same depth as the foam insulation you put between the studs or else you would compress your batt insulation which is never desirable. So there is no difference.

Some siding guys will add it over the top of the existing sheathing on the outside of the home. Again, not much difference where it's at. The r value and the perm rating remain the same no matter where it's at. You just don't want to create a sandwich where moisture has no way to escape by putting two vapor barriers on opposite sides of a wall.

Another option would be putting the foam inside the wall and just use it immediately behind the drywall. But that would not air seal your batt insulation from outside air

I would defer to whatever Bud suggests you do. .
If he doesn't come along shortly, send him a message.
 
  #12  
Old 11-01-19, 12:34 PM
C
Member
Thread Starter
Join Date: Dec 2011
Location: USA
Posts: 367
Received 30 Upvotes on 28 Posts
OK, so I was doing a bit of research on vapor drive and came away with the following.

1. air leaks carry a lot more moisture into a wall cavity than vapor diffusion through building materials. So air sealing should be a priority.

2. as @XSleeper points out, when dealing with relatively vapor impermeable materials, the key is not to sandwich part of the wall assembly between these vapor barriers.

3. in my location - Long Island, NY; northern edge of climate zone 4 - inward vapor drive in the summer is a bigger issue than outward vapor drive in the winter.

4. So if I use @XSleeper's suggestion of lining the stud bays with foam board (let's say 1/2" thick) and sealing around the edges with canned spray foam, I'll have an air sealed cavity, which lets the batt insulation perform better (1/2" foam also applied to interior faces of framing to limit thermal bridging, so 3.5" depth maintained for the Rockwool batts). It also limits moisture intrusion into the cavity by preventing moisture laden air from simply flowing in.

5. So any foam board lining the stud bay cavities will deal with air leakage. So then we move on to vapor diffusion through the wall assembly.

6. Foam board is available unfaced (such as this unfaced XPS) or faced (such as this faced XPS). Not sure why the Lowes link works, but HD link doesn't; but the Home Depot link is to store sku# 409923, Foamular 1/2" rigid foam board insulation sheathing.

The 1/2" unfaced XPS board should have a perm rating ~2.2 (Class III vapor retarder) while the 1/2" faced XPS board should have a perm rating ~0.2. (Class II vapor retarder, bordering on being a vapor barrier, but not quite)

7. Vapor will readily pass through the brick veneer and exterior gypsum sheathing.

8. So it seems to me that restricting vapor passage into the stud bays is a good thing, so long as moisture doesn't get trapped in the wall. So if I go with faced foam board, that will cut down on inward vapor drive during the summer, and any vapor that does get inside can easily dry out to the interior (Rockwool, then drywall). And there wouldn't be any condensation because even if the Rockwool or drywall are below dew point (from A/C inside house), they would absorb the moisture rather than condensing it. Is that correct?

9. In the winter, the vapor drive is outward. And I guess it's not a huge amount in this climate zone (maybe a bigger problem in zone 5 and 6), so whatever vapor gets into the wall assembly can dry out through the faced foam board since it's a Class II retarder and not quite a vapor barrier. What about condensation? The warm inside air entering the wall assembly will pass through the Rockwool and encounter the cool 1/2" foam board - I suppose it will condense here? Or is the vapor drive in the winter not significant enough for condensation to be an issue here?
 
  #13  
Old 11-01-19, 02:47 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Posts: 27,245
Received 1,959 Upvotes on 1,756 Posts
The only areas that really need to worry about any sort of vapor drive in the summer are the deep South states along the coast.

The biggest worry in almost all other climates is the temperature differential during winter months when it's 0F outside and you want it 70F inside. That's why air sealing the exterior side of the wall is key. Air leakage is the primary means by which condensation will occur.

Your wall doesn't experience such drastic temperature swings in the summer.
 
  #14  
Old 11-02-19, 07:03 AM
C
Member
Thread Starter
Join Date: Dec 2011
Location: USA
Posts: 367
Received 30 Upvotes on 28 Posts
This article makes the point that summertime interior vapor drive is a bigger threat than wintertime exterior vapor drive, and the importance of having a path to dry to the interior, especially if the cladding gets wet and holds water. In my case, brick veneer over exterior gypsum, so for sure, water vapor will want to diffuse inward in the summer.

https://www.finehomebuilding.com/201...moisture-walls

They do make the point you've made before about air sealing, when they point out that wintertime condensation issues are mostly driven by air leakage, not vapor diffusion.
 
  #15  
Old 11-02-19, 07:31 AM
C
Member
Thread Starter
Join Date: Dec 2011
Location: USA
Posts: 367
Received 30 Upvotes on 28 Posts
So between 1/2" faced XPS and 1/2" unfaced XPS to line the stud bay cavities, I'm thinking faced is a better choice. Both will air seal, but the faced will reduce vapor diffusion.

The reason I'm going 1/2" instead of 1" foam board (for stud bay cavities and stud faces) is because I've got baseboard heat along one of the two walls and 1/2" foam on the faces of the studs will get the drywall pretty close to the pipe; 1" even more so.
 

Last edited by cartman; 11-02-19 at 08:31 AM.
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: