Replies

1. GBA Editor
   Kiley Jacques | Feb 16, 2022 11:53am | #1

   Hi Thomas,
   Am I right to think you are treating the OSB as the air barrier?

2. 
   bigapplerunner | Feb 16, 2022 12:07pm | #2

   Hi Kiley,

   Yes, exactly. Not sure it makes sense on the inside of the studs, however, as I should tackle the air leaks as close as possible to the exterior wall. I will have one layer of OSB flush against the old siding, taped off with quality tape, which may be sufficient? If so, I can fur out the existing studs to deepen the cavity for more insulation once the stud bays are air sealed.

3. 
   gstan | Feb 16, 2022 12:20pm | #3

   OK, here's the deal - if you put rock wool on both sides at the same thickness you end up with the OSB at the exact center thermally - the OSB then becomes both an air barrier and a moisture barrier (think condensing surface). Now, let's talk about the dew point - most houses maintain a humidity level (in cold weather) between 20 and 50 percent (generally, the colder it is outside the lower the humidity is inside) so for purposes of discussion we will assume it's zero out and your humidity is 35% - take a look at a dew point calculator (there's dozens of them on the web) and you will see that if you are holding the inside temperature at 70 the dew point is 41. The next question is "what is the temperature of the condensing surface?" and the answer is "exactly halfway between the outside and inside temperatures - or 35 degrees. Now you know that it will get wet inside the wall!
   So, how do we fix this? The solution is to add more insulation to the outside of condensing surface or to reduce the amount if insulation on the warm side of the condensing surface - here is the easy way! Place a layer of Foil faced Polyiso insulation on top of (or underneath) the OSB (tape the seams and tape it to both the floor and the ceiling) - the foil face then becomes the condensing surface - even 1 inch Polyiso will raise the temp of the condensing surface above 41. here's how you calculate it:
   The total R rating of the wall will be the sum of the two stud walls (full of Rock Wool) plus the Polyiso or about 11+11+6 = 28. Then find the percentage of the R rating of the insulation on the warm side of the condensing surface or 11/28=.39 - now multiply this times the difference between the inside and outside temperatures or .39x70=27.5 then subtract this from the inside temperature or 70-27.5=42.5. So, you will now be above the dew point when it's zero outside and you won't get condensation in the walls. What if temperature goes lower than zero? Then you need a thicker layer of Polyiso or less insulation in the inside stud wall or a lower humidity level in the house. You are the only person who can decide what temperature and humidity you are going to maintain in your house - the weather records will give you a good idea of the lowest temperatures you're likely to see in your area - if it were me, I would go for two inches of Polyiso with the Rock Wool on both sides - always better to have a little more insulation than not quite enough! GOOD LUCK!

4. 
   bigapplerunner | Feb 16, 2022 01:24pm | #4

   Forrest,

   Thank you very much for this. I had to read it a couple of times but it makes total sense.

   If we take the polyiso approach, I don't see a reason to have the layer of additional OSB, do you agree? The taped polyiso becomes my air barrier. Layers are then sheathing > rockwool > polyiso with foil facing inwards > new rockwool.

   Also, if I understand correctly, this approach will also allow us to add foam boards on the exterior when we redo the siding and windows? Assume that would have to be unfaced EPS foam or so.

   Thanks again!

1. 
   gstan | Feb 16, 2022 01:48pm | #5

   Sounds like you've got the basic principles under control - yes, I agree that the OSB probably is unnecessary - yes, if you are going to add foam on the exterior it needs to be fairly permeable but if you use thicker Polyiso your R rating may be more than high enough without adding - Oh! Polyiso usually comes with foil on both sides (which has no effect on moisture control on the cold side).

5. Expert Member
   PETER Engle | Feb 16, 2022 02:30pm | #6

   Big Apple,

   You should read this article: https://www.greenbuildingadvisor.com/article/insulating-walls-in-an-old-house-with-no-sheathing

   I'd skip the layer of OSB and use an inch of permeable EPS foam against the inside of the siding, then fill the remainder of the cavity with Rockwool. If you plan to redo the siding and windows in the near future, I would consider skipping an air gap entirely. The back of the siding is rough enough to create some gap and allow some drying and drainage. That way, there's nothing to fill when you come back from the exterior and install rigid exterior foam and sheathing.

   While there is some moisture risk to this assembly, it is low. It gets even better once the exterior insulation is added. Forrest's posts above assume design conditions of 35% interior RH and 0F outdoor temp. These conditions are unlikely in an old house. Even with aggressive air sealing and lots of indoor moisture sources, it would be difficult to hold a house at that humidity without serious additional mechanical humidification and you shouldn't do that. If you allow the indoor humidity to float down to its natural levels, it will probably range around 15%-20% in the coldest weather. This is somewhat uncomfortable, but normal in old houses. At 20% interior RH, the dewpoint is 25F, far below the temperature of the inner face of the interior sheathing. Even if you artificially raise the interior RH enough and you manage to get some condensation on the sheathing, it will dry out when the conditions change. Remember that the design conditions only exist for a few hours each year. I have yet to see any condensation issues with a mid-wall air/vapor retarder in CZ 5, and I've looked inside lots of walls.

   FWIW, you can get even better performance by replacing the layer of OSB between the walls with a smart membrane. Might be a little cheaper and easier.

1. 
   bigapplerunner | Feb 16, 2022 04:01pm | #7

   Thanks, Peter. Much appreciate the detailed response!

   Between both of your responses, I now understand the impact of relative humidity and dew point, which is tremendously helpful. Our thermostat measures RH and to your point, it has ranged between 15-20% the last 2 months, so the dew point is low enough. I had read the article you referenced but I'm going to read again.

   Just to confirm that this approach would work: old siding > 1" EPS > 3.5" rockwool > Certainteed Membrain > new stud wall with more 3.5" rockwool? The Membrain being there essentially as an extra precaution. Ideally, it's in the middle of the wall so I don't have to deal with outlet issues.

   Separately, I was going to stagger the new stud wall as to break the thermal bridge. Shouldn't impact the moisture situation but do let me know if you think there's something wrong with that approach.

1. 
   Montypbbc | Mar 10, 2022 08:26pm | #8

   Looks like this got pretty dialed in and I think the smart vapor retarder is a good idea and best bet for a clean shot at air sealing.

   I haven't worked with that many tapes but it seems unlikely to me that even the high end ones would adhere well to your (most likely) rough sawn studs. Maybe an acoustical sealant would serve well to help air seal the eps between the studs. If your timeline to redo the exterior siding with a continuous layer of insulation/air barrier is within a few years you might also be able to go with an inexpensive caulk.

1. 
   bigapplerunner | Mar 10, 2022 09:15pm | #9

   Thanks. ZIP tape works very very well. Definitely recommend. Just the 1" EPS taped off with ZIP made a significant difference. The smart vapor barrier as well as making the drywall seams airtight should help with air sealing.

1. 
   Montypbbc | Mar 11, 2022 07:35am | #10

   That's great, only ever hear good things about Huber. Thanks for sharing, glad it's going well!