Exterior Insulation and Vapor Barrier for Wall Upgrades
I have to reside vinyl-sided house as house wrap is patchy on the sheathing. I also have to get a new roof, due to missing drip edge. top-flared style fascia wrap and shingles cut to short so some water seems to have infiltrated an exterior wall in at least one place, and excessive water has gotten under siding. Insurance company is requiring re-siding, house wrap, and re-roofing. I’ve limited finances and limited time to fix, so I’ll stick with vinyl siding. My local government doesn’t inspect or answer any questions for renovations or building codes as opposed to new builds, but I do know they’ve adopted IRC building codes. I know in climate zone 4a, Class I vapor barriers/retarders under drywall, such as poly vapor “barrier” are not in compliant with recent IRC, and as I understand for most, if not all, zones, poly vapor barrier should not be used on inside under drywall if foam insulation will be placed on exterior as it can trap moisture inside that cannot dry to either side. Considering I moved in at the beginning of 2003, with house mostly constructed in 2002, I do not know if IRC was followed then but could not find vapor retarder info in a 2003 version of IRC. I also know if using a Class II vapor retarder, such as Kraft based insulation, there are certain insulation requirements per the 2021 IRC if using continuous foam on exterior, and I assume for a good reason. However, I have no idea what insulation I have, or if I have a poly vapor barrier or Class II vapor retarder, such as kraft paper-faced insulation, and I am trying to avoid destructive investigation to find out. I’ve read there’re many older homes in Canada with poly vapor “barrier” on inside and continuous foam on outside that seem to be without rot problems even if not advised to do this currently, but that is a colder climate. My main questions are: 1) Considering financial and time constraints, and difficulty finding a local contractors who would be willing to do something new for them, for homeowners who have or might have poly vapor “barrier” in a mixed-humid zone 4a climate does that mean they should just skip insulation rather than add exterior continuous foam – assuming there is not a nearly as affordable option that a local contractor would consider installing under vinyl siding? 2) My house was built in later half of 2002 and beginning of 2003 in MD by a builder who built over 100 homes in my neighborhood and also built in other MD neighborhoods – meaning he might have been following standard practices – so would it be likely or unlikely a Class I poly vapor retarder/barrier was used under the drywall? 3) If there is any chance I have poly vapor barrier inside should I avoid using exterior foam insulation? If this is the case, I’ll likely pay tens of thousands of dollars to reside and put on house wrap and just skip insulation, which seems a shame. 4) Was Class II Kraft-based insulation which is a vapor retarder used commonly in MD by builders in 2002 and 2003 then, and if I have this do I also need to avoid foam exterior insulation because I do not know how much insulation I have? IRC 2021 states if using foam plastic insulating sheathing in conjunction with a Class II vapor retarder (e.g., Kraft-faced fiberglass insulation) there are certain continuous insulation requirements, for instance R-value > or = to 3 over 2×4 wall. Without knocking a hole in my wall I do not what if any vapor retarder I have under drywall, nor do I know what insulation is in my wall or if it is continuous or not. 5) When considering building codes, does exterior foam insulation under vinyl siding satisfy requirement for house wrap? 6) If only some foams do count as house wrap, do you know which exterior foam insulation counts or how to find out? 7) Regardless, is it best to also use house wrap? My windows are “innie” windows I believe, so if so, is it best to put house wrap against the sheathing, foam on top and then vinyl siding? I’m a beginner, but unfortunately under a time crunch to prevent cancellation of insurance policy, so any advice is appreciated.
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Chances are pretty good that you'll have kraft faced fiberglass batts in the walls, as that is far and away the most common way to insulate a residential exterior wall assembly. Depending on age, you'd have either R11 or R13 batts (most likely), assuming a 2x4 framed wall.
House wrap is really there to act as a WRB, a Water Resistant Barrier, to help keep rain out of the structure. You can accomplish the same thing with taped rigid foam, polyiso probably working the best here if you use a foil facer, but you can get poly faced EPS that would work fine too. Go with what the tables recommend for your climate zone in terms of how much R value you need. I would try to find some reclaimed or factory second polyiso to save some money. Tape the seams, seal the perimeter with a bead of sealant (I like polyurethane sealants for this), and then the exterior rigid foam will act as an air barrier too which is a big plus.
"Innie" windows are aligned with the interior side of the wall, so they look inset from the exterior. "Outie" windows are the other way around, aligned with the exterior wall and having a big sill on the interior side. Which way you go is mostly a matter of personal preference. Both perform pretty well when properly flashed. I prefer outies myself. In your case, you'll be building up the exterior, so you'll have innies, otherwise you'd have to move the windows out, adding a lot of expense to your reside project.
Martin's article about rigid foam thickness is linked here:
https://www.efficiencyvermont.com/Media/Default/bbd/2019/docs/presentations/efficiency-vermont-bbd-rethinking-the-rules-on-minimum-foam-thickness.pdf
In your CZ4, You need R2.5 for 2x4 walls, or R3.75 for 2x6 walls. That works out to 1/2" polyiso or 3/4" EPS, or 3/4" polyiso or 1" EPS for 2x6 walls. I personally would go a little thicker than that, probably 1"+ of polyiso for 2x4 walls, or 1.5"+ for 2x6 walls. 1" and under isn't going to really complicate your exterior details very much, so adding rigid foam isn't going to add much complexity to your project.
Bill
Bill
Thank you for responding. I'm a true beginner here so when you wrote "poly faced EPS" did you mean the EPS is covered in a thin layer of polysio or did you mean a thin layer of polyethylene and is ply faced EPS foil faced? Can you explain why foil faced would be best for my area, and why would recommend polysio over XPS or EPS for CZ4? I seem to recall a warning against foil faced but maybe it wasn't for my CZ4, and I recall reading the R-value drops if the foam temperatures drop to 50 degrees or cooler, so I was concerned about that and thought maybe EPS or XPS would be better, although maybe there are other advantages to polysio that outweigh the drop in R-value at colder temperatures. After you explained innie vs outie windows, I realize I actually have an outie, if looking from the outside of the house as I have an interior window sill. I think regardless you are saying I will wind up with an innie - is that correct? Do any furring strips need to be placed on the foam considering vinyl siding will be used?
Faced EPS usually has a clear poly facer. A good example of this is the R-max EPS product that Home Depot carries. The facer is there to make the EPS sheet more physically durable, and thus easier to handle. The facer also makes it a vapor barrier. EPS itself, if unfaced, is a vapor retarder, and is the most vapor open of the commonly available rigid foams.
Polyiso is usually either foil faced or is faced with a sort of fiberglass kraft paper. The foil facer makes it a vapor barrier, the kraft facer is vapor open, making the entire sheet act like a vapor retarder. In some situations, one or the other is a better option.
I'm not a big fan of using XPS for other than underground applications for various reasons.
I think foil faced polyiso is your best option here because the foil facer is easy to tape (I like to use Nashua 324A foil tape for this, but regular flashing tape works fine too). The "loses R value in the cold" argument if often exaggerated. It is the mean (average) temperature through the foam that influences that, not the absolute outdoor temperature, so the "below 50*F" isn't really accurate. When you get down below freezing temperatures you can see some reduction, but it's still better than the other materials all the rest of the time, and even when derated for cold weather performance, polyiso still performs similarly to EPS, if not a bit better.
In the warmer climate zones, polyiso's one negative is less of an issue, and it's not even that much of an issue in the more northern climate zones. Much of what you read about on GBA is for CZ5,6, and 7, the "frozen North". Cz4 isn't as severe, which makes things a little easier. More of your season will be warmer (relative to us frozen Northerners), so you don't have to worry about performance at extreme low temps as much as we do. That's where the "more R per inch" helps you, since it makes the trim details easier, and you don't have to deal with as long of fasteners for the siding.
Bill
Thank you so much for going into the detail.
If this is a budget job adding the exterior foam will drive your costs way up. As 100% of the old flashing will be in the wrong locations and the foam make everything harder and take longer.
"Water seems to have infiltrated an exterior wall in at least one place, and excessive water has gotten under siding. "
To my ear this sounds like the original flashing was install poorly and is the root cause of all your problems. For flashing to work it has to be in layers in the same way shingles on a roof are install staring at the bottom with the next layer on top of the last directing the water back to the surface. Getting the new layer of house wrap behind the sides and bottoms of the existing windows and doors is unlikely at best. I am afraid the contractor will slather everything with caulking and in a few years, it will be leaking all over the place and you will be redoing it again.
Pick your contractor very carefully. Get him to explain to you how he will flash around the windows and doors. No one ask this type of question the fact that you are showing interest will make him up his game.
Walta
Thanks. I'll make sure to ask about the flashing around windows and doors. Even on a side of my house where there were no windows or doors there was excessive water behind the siding which I believe was due to lack of drip edge. However, besides flashing around windows and doors, and drip edge at the roof, is there any other type of flashing that would help prevent water getting behind siding? Should J-channel be at the top and bottom of the siding, and does that just function as a trim?
The drip edge under the shingles is a nice touch but not a necessary and assuming you have more than an inch of roof overhanging the wall and would be unrelated to water getting behind your siding.
If you have a look at this photo, you will see how the window is installed like shingles on a roof.
They start with a pan under the window sill to catch and direct water to the outside.
Then the window goes in on top of the pan and house wrap. The sides of the window are tape to the wall. Over that tape is the tape across the top of the window. Over that tape is cap flashing. Over that goes the house wrap. Over that goes the siding. Every joint will force any water back to the outside.
In your case you can not be sure anything was done correctly and how far will you dissemble the house to be sure?
https://www.finehomebuilding.com/list/window-and-door-flashing-guide
Walta
In certain areas there is not much if any roof overhang, and a roofer told me my fascia wrap is not the usual style. He said it is an older style, and flared out at the top so there is about a 1 inch gap between top edge of fascia wrap and top edge of fascia board, and the shingles were not left long enough to span the gap, so water running off the roof might be falling into the gap. Drip edge is required by code, so he said he would either have to remove the fascia wrap to replace it with a straight style fascia wrap or use a tool to saw off the top 1" of fascia wrap to get a drip edge that would span the gap. The top of the fascia wrap is at same level as top of fascia board, so I think that was also part of the problem , as it makes sense the drip edge should continue a very short distance from the roof but it should continue the same downward slope. I think I read something by GAF that stated the drip edge should not overhang the roof by more than 1/4th inch, but that would drop water right into the gap between flared fascia wrap and fascia board, and to extend beyond the gap would require the drip edge to overhang at least 1 inch per this roofer. If the overhang of the drip edge were to be level or tilted up for any reason I read the water that would defeat the purpose and is even worse as it leads to water running back along the drip edge under the shingles - and I think that is another reason why he said the top inch would have to be sawed down, other than to get rid of the gap. Have you encountered flared drop edges, and are their drip-edge solutions that would work in a case like this? My understanding is it is an old-style that was common years ago before drip edge was required by code. I might have no choice but to replace fascia board.