Am I at risk of double Vapor Barrier? Mixed-climate region, Gyplap sheathing

[parabola]

Hello all,

First post. In my lurking on this site I've found a very knowledgeable user base with sound advice. I hope someone might be able to shed some light on a recent concern of mine.

I am doing an extensive reno of my house in Charlotte NC. Aside from doing an addition, I demo'd all interior plaster in order to get to the studs so I could rearrange a majority of the floor plan. I am to the point where I am air sealing, insulating, and dry walling.

House specs:
- Charlotte NC (mixed climate???)
- 1940 brick bungalow
- Gyplap exterior sheathing behind the brick of the original parts of the house

I am concerned that I may have introduced a double vapor barrier situation with my air sealing and insulation. I cannot find any details specs on Gyplap sheathing so of course my ignorance on the material is really making me second guess my approach before continuing.

In one room in the original portion of the house, I used a combination of caulk and Great Stuff spray foam to air seal gaps in the top plates, bottom plate to floor union, around window framing, headers, between the studs and Gyplap, and between any loose fitting unions of the Gyplap...all done from the interior side of the wall. I then went in behind that with kraft faced R13 batts, vapor barrier towards living space naturally, and did a superb job with the installation so that the facing is continuous across the stud faces. Because of the thickness of the original plaster and jamb depth of these windows, I had to hang 2 layers of 1/2" drywall to set me up for the casings.

Does any of the above raise concern with you guys? Again, because I do not know much anything about Gyplap, I am not aware of its perms, whether it is considered a retarder, barrier, etc, or if you treat it like any other subsiding/sheathing. I would hate to find out that I shot myself in the foot when thinking I was doing something as best case. To add to the confusion, there is some debate over the use of vapor barriers in the mixed climate of Charlotte NC. Either way if I created a double vapor barrier that will lead to problems, I would rather know that now before I do the same treatments to the other rooms this weekend.

Any thoughts, experiences, info is greatly appreciated and will certainly be put to good use!

Thanks for your time and help on this one!
-matt

[Martin Holladay]

Matt,
I'm not sure what "Gyplap" is -- is it gypsum sheathing?

It doesn't sound like your walls will trap any moisture; they should be able to dry in both directions. However, the walls aren't particularly well insulated.

[parabola]

Hi Martin,

Thanks for the reply. Yes it is tongue and groove black paper backed gypsum sheathing. I just looked at a piece of it and the product actually reads GYP-LAP "the fireproof sheathing".

I agree that my walls could be better insulated. I really wanted to go Icynene but with the small amount of original wall space and the phrased approach of doing a room at a time while living here, getting an outfit in here to spray would be a hassle. Well, in retrospect, my situation screams diy kits. shoot. I did the best I could with the materials I had by concentrating on air sealing and proper batt installation technique. For the added on portion of the house, I will insulate differently, but they are still 2x4 walls. With access to the exterior though, I could supplement with rigid foam under the siding, or even build another 2x4 wall on the inside if I wanted to lose a little interior space.

Do you still feel the same knowing more about the exterior sheathing? Also can you think of anything i am missing with my insulation approach (other than the use of inferior materials, ha)?

Thanks again; much appreciated!
-matt

[worthy]

Gyplap doesn't even come close to being a vapour barrier. You need a perm of .1 to rate as a vapour barrier while Gyplap is speced at 23.3( see page 29)

This must be some old product, as CGC announced they were discontinuing production in 2004 referencing that this was in line with the move towards the use of more suitable products in construction.

I used Gyplap in 1998 strictly to save money vs. OSB. There was so much waste--excess boards turned to mush on the ground before they could be returned--that there was no saving at all. And since the supplier didn't provide any guidance, I didn't seal the joints. Fortunately, I did cover it with Tyvek and brick paper.

Perhaps you are using Gyplap Treated Core.

[parabola]

Worthy,

Thanks for the reply! That is good news/info. Not so much that the material is flimsy at best, but that it will not trap moisture, haha.

This is definitely the old iteration of the product, circa 1922 I believe. The house was built in 1940 and we are talking the original exterior sheathing here.

Basically what I have is a small story and a half house, brick front, brick gables at each side. I have done a new addition on the back that is the full width of the house. All brick portions are the original parts of the house where is where the GYP-LAP is and where I am currently doing my air sealing and insulation.

Once that portion of the house is done, I can partition it off temporarily and move my life support rigs (make-shift kitchen, bed, clothes racks, drum set (ha), etc into this area. I will then concentrate on finishing off the envelope and insulation system for the addition. Piece meal indeed and I know the entire building envelope is not consistent but it is the best I can do without repeat insulation contractor visits, demo'ing brick, or encroaching on the interior floor space of the original parts of the house for thicker walls/insulation potential. My planned approach for the addition with 2x4 walls is again best case for my budget and construction methods chosen... air sealing, batt insulation (facing dependent upon exterior insulation), 1 to 2 layers of 1” XPS fastened to sheathing, tyvek, furring strips to make drainage plane, then Hardie plank siding / MiraTec trim. I know this approach isn't the most high performance one out there, but hopefully it will yield a space that is comfortable enough to make me happy for my efforts. Anything has to be better than it was, or slapped up stick construction in general from the "pre-green" decades of residential construction.

Again, not looking to win any awards or attain a certain performance number...I am concerned with relative comfort, health, and above all, it being done right. Done right in my interpretation is building in a way that the house does not self destruct in terms of moisture, etc.

Any potential hang-ups with my approach for this application? Thanks so much for reading and sharing your expertise.

Cheers,
-Matt

[worthy]

Yes, you're in a mixed humid climate. This Building Science link on building in specific climates may be of help. There are at least four examples of homes built in your climate.

[parabola]

Awesome link, Worthy. Thank you. The page referenced some great detail docs that were very helpful.

The notes and questions below are quite lengthy so I wanted to extend thanks to anyone that reads. Please do not think I am someone that has not done there homework and is looking for a guru to simply hand them an answer. Truth is, I have hashed over this stuff for weeks now and now that it is crunch time to button the exterior of the project up, I just want to make sure I am doing the right thing. Again, many thanks for any help and advice on the matter!!!!

Building Sciences actually had a few recommended designs for mixed humid climates. The kicker is they are all distinctly different, so much in fact that it is hard to get a feel for the fundamentals in order to apply them to my project.

Here's a quick run down of the mixed humid climate designs by project city:

Atlanta- 2x6 framing, OSB with vapor barrier [retarder rather], spacer mesh, fiber cement siding

Charlotte- 2x6 framing, 1” XPS sheathing w/ sealed joints, air gap, wood siding

Louisville- 2x6 framing, XPS sheathing w. sealed joints, vinyl/aluminum siding

The above designs feature 2x6 walls whereas mine are 2x4. I know 2x6 provides the extra depth for more insulation to give a better thermal break; unfortunately, it is not feasible to go back and reframe in 2x6 lumber.

I did a search on recommended insulation/envelop designs for 2x4 walls in mixed-humid climates and the only hit I got was this link to the US Dept of Energy's Energy Efficiency and Renewable Energy. The site even references the Building Sciences link you provided, although they consider the BuiSci recommendations a more "advanced framing" approach due to the use of 2x6 wall framing.

https://www.eecbg.energy.gov/buildin...xed_humid.html

The DoEnergy states that external walls with 2x4 framing are adequate in the mixed-humid climate. As part of their sound design, these walls should include the following features:

*Exterior sheathing, preferably insulating sheathing with joints taped to provide a water and air barrier. Use insulating sheathing that does not have a film facing.

*R-13 friction-fit unfaced fiberglass insulation, Kraft-faced fiberglass insulation, OR blown-in cellulose insulation.

*Rim joists that have kraft-faced R-19 friction-fit batt insulation cut to fit.

*Foam-sealed or caulked top-plate and exterior wall penetrations.

*Sealed gypsum board to control air leakage through the walls, especially in penetrations to garages and porches, and where the walls meet the ceiling.


With my 2x4 wall framing, I am more inclined to follow the above design featured on the Energy gov site, however I get confused when they recommend to use one of the sheathing options but do not explicitly state whether you can use XPS applied to wood and OSB sheathing. I do not mind using only the ½” ply on the east wall and 7/16” OSB elsewhere (more evidence of a phased approach to the addition, geez) exterior sheathing already installed; it would be cheap and easy, but I would like more R values afforded by 1" or so XPS to complement the R13 kraft-faced fiberglass batts in the walls. I definitely do not wish to compromise the house’s ability to properly transmit moisture for the sake of more insulation though. It should be noted that wall cavity insulation in the newly built addition off the back of the house has not been chosen/installed yet and can vary. I would like to concentrate on the exterior in the short run while the weather is so nice right now.

One thing that is consistent among all the building resources is that a system in the Mixed-Humid Climate areas should be built to enable the transfer of moisture both inside and outside of the structure. They continue with recommending the vapor barrier be in the middle of the assembly from a thermal perspective. I interpret this statement literally in terms of vapor barrier vs vapor retarder. With the different permeability of all the various materials, the position of those materials in the wall system, and the different iterations of the systems throughout the entire mixed-humid climate range, this is getting so dang confusing.

According to Building Science’s document “Insulations, Sheathings, and Vapor Retarders”, all of the materials used/to be used in my project are semi permeable. These include kraft-faced R13 fiberglass batts and any 1” or less XPS rigid foam used. The permeable materials are the ½”-1” thick gypsum board on the interior. The wild cards are whatever Latex paint will be used and the GYP-LAP exterior asphalt paper faced sheathing in the original brick parts of the house.

Even after all of this research, a few questions remain for the 2x4 new construction portion of my project:

1) Is there a reason why rain screens/ drainage planes were not featured in the building design recommendations by Dept of Energy for 2x4 wall construction? It would be easy for me to create one for my hardie plank siding and miraTec trim.
2) Not to challenge all the case studies and research done by these institutions, but could XPS be applied to the OSB/plywood sheathing for more insulation without adverse effects in my climate/area? The two materials are vapor retarders so would they cause detrimental problems in my system if combined?
3) If tyvek is considered permeable, what is the definitive recommended placement of it in the system? I’ve noted instances where people say exterior of the XPS, between XPS and OSB, and even omit it if XPS is used.

I also still have some confusion in regards to my approach with sealing and insulating the original portions of the house with 2x4 framing, Gyp-lap sheathing, R13 Kraft faced fiberglass, and 2 layers of ½” gypsum board. Based on what I read about the materials, I assume the wall system should be able to transfer moisture in both directions, although drying to the inside will be a little more impeded. Am I correct in this assessment?

My apologies to all for the long read. I certainly appreciate your time and any feedback or information!

Cheers,
matt

[worthy]

Really, the general requirements of building in mixed humid aren't that much different than building in many cold climates--except for the low temps. Heating and cooling seasons, and lots of moisture year round.

The combination of one inch XPS as sheathing or on top of OSB or plywood sheathing in addition to high density fg batts (R15) already exceeds the recommended insulation levels for your area. Building in a cold climate, my preferred method is exactly that but with 2x6 for greater RValue. In light of all the application problems that arise with housewrap, you could well consider dropping it. Or put it between the sheathing and XPS, another Building Science alternative. There is no single best place for the housewrap. (I don't use it at all anymore in conjunction with XPS.) But do use t&g XPS and tape the joints. Behind masonry veneer, I use #30 felt. Your local building inspectors may have different requirements.

Any veneer wall, EIFS or siding finished home should have a drainage plane.

*Rim joists that have kraft-faced R-19 friction-fit batt insulation cut to fit.

There was a thread on this at JLC. This recommendation is way out of date. Fibrous insulation there is useless and only serves to trap water and encourage mould growth. Fit with XPS, EPS (thicker), polyiso, or sprayed-in foam.

BTW, 1/2" ply sheathing is overkill. I've used XPS as sheathing after strengthening the framing.

Remember whatever you decide on that seems sound to you in terms of building science still has to pass muster with local building authorities.

[parabola]

Worthy,

My apologies for the delayed reply; things have been crazy lately. Ever had a strap break and lost a load of 12' drywall in the middle of an intersection at 9PM with no one riding shotgun? I would NOT recommend it for leisurely activity!!!

Once Again, great info...many thanks.

I thought the R19 batts in against the rim joists was suspect. Agree with you that cut down XPS spray foamed in place is the ticket.

Also, skipping the WRB would simplify things a bit so I may consider that after all. I also like the idea of my SAF adhering to bare rough opening framing and over the XPS. Either way, my ½” ply/osb sheathing has already been installed. To that, I will apply unfaced XPS, tape the seams, create a rain screen, and go to town with trim and siding.

Actually the exterior system with 1/2" sheetgoods sheathing, 1" XPS, 1/2" or so drainage plane, and 5/4" miraTec trim will put me a minimum of 3" past the CMU foundation. With a discrepancy like this, I could go exterior applied rigid foam and stucco or other thin veneer on top of that for the foundation. We’re only talking roughly 40” height so a rigid foam sheet would get decent coverage. I'm planning on retrofitting a conditioned crawlspace and partial basement on the project so this might just work out for the better anyway. Perhaps I could even extend the wall xps down over the cmu's, lath the foundation portion, stucco/veneer, then z-flash at the drainage plane behind the band trim over the stucco of the foundation and maybe some drip flash applied to the back side of the band trim if the gap is still significant. Think this is asking for more trouble/probs than it's worth?

There are a few minute particulars to work out like thickness of drain furring strips (thinking 1/2" currently so the siding/trim do not extend past the cmu foundation too terribly much as discussed above), whether to use WRB at butt seams of the XPS vs. just tape, flashing detail on the band trim, and drip cap detail above window on the rigid foam in light of no WRB at that layer of the system to overlap.

I'm getting there! Thanks again for getting me on the right track!
-matt

[worthy]

I could go exterior applied rigid foam and stucco or other thin veneer on top of that for the foundation

Regardless of Code provisions allowing it, I don't like the idea of providing a right of way for hidden highways for carpenter ants and termites