Insulation, Radiant and Vapor Barriers - need advice on adding rigid foam to my exterior walls.

I’m trying to make final decisions about how to reside my house. I’ve done a lot of research but I’m hoping to get some final advice before it becomes permanent. There is no way to blow anything into the existing wall cavities. I am looking to add two or three inches of either XPS or ISO rigid insulation to the existing wall sheathing (ripping off the two layers of existing siding) and finish it with cement fiber siding. My original research led me to believe that XPS would be more forgiving of moisture problems and more environmentally friendly but I’ve seen some things recently that make me think that I should just go ahead and use an ISO like Dow’s super TuffR board. I have made ample use of HVAC-calc 4.0. This program does not have an option for rigid foam only, but my predictions are that my load with 2” of XPS would be about 8,000 BTU of heat loss for the walls, 2” of ISO would be about 6500 BTU and 3” of ISO would be about 4500”. Currently with no insulation the load is estimated to be about 20,000 BTU, so anything I do is a huge improvement. Obviously from those numbers this is a small house. The house is roughly 20’ x 30’, although the first floor has enclosed and heated porches in front and rear.

At this point I am leaning towards 2” of ISO and use furring strips to create a “rain screen” wall system. Three inch just seems like fasteners would be getting too long and awkward, not to mention the loss of another inch of overhang and added depth to all the windows. 2” is also readily available although some places have recommended two layers of 1” and stagger the seams. So I guess 1” staggered over 2” could be an option too but where do you stop?

So the first question is which insulation and how thick. Assuming the majority agrees with 2” of ISO what exactly would be the assembly process? House wrap or no? What type of fasteners? My other concern is how to safely attach that heavy siding off the side of 2” or 3” of foam board. Cement siding was the “family” compromise between vinyl or cedar, however vinyl would obviously is light and has it’s own drainage system so the furring strips and “rain screen” type construction wouldn’t be necessary.

Think that covers the first round of ideas. Thank you for any insight or suggestions about how this will all work out.

I can't answer your question about the rigid foam but whats the reason for not being able to add blown-in cellulose to your existing walls?

I can't answer your question about the rigid foam but whats the reason for not being able to add blown-in cellulose to your existing walls?

technically there are certain areas that I can, but the majority of the long walls I can't. North wall has a porch downstairs that I will rebuild and do just that. upstairs will get blown cellulose (it would already be done if my pro hadn't disappeared) and 1/2 or 1" board just to cover it all over as there are a few places that you won't be able to get anything blown in. The east and west walls of the original structure appear to have started life as a post and beam structure. Lath and plaster were applied directly to the exterior sheathing. Then 2x4s were nailed flat side to that plaster and another layer of lath and plaster was applied to that. So not much gap and what gap there is would be very hard to get to. To do it right I'd have to gut the entire house and frame it over. So this is my only logical solution that I can come up with, short of adding a new frame to the outside of the house just to hold insulation. Attic floor was air sealed and blown in last January. It's a shallow space but it brought the 2nd floor temp up about eight degrees. Also had interior partition walls dense packed. Great noise reduction between the bedrooms but I still have a lot of plaster to repair.

I will say that if I could get something in that gap I would do that AND do the 2" of rigid.

Thinking purely about furring out for the siding, here's what I'd do:

Carefully map out where all your studs are. Make yourself a drawing that shows the exact location of every one. Sheath the house completely with one layer of 1 1/2" square edge ISO Styrofoam or XPS Thermax. Then chalk the exact location of all your studs. Think about where you need to nail once your trim is up... you don't want to find out your siding has nothing to nail to after the trim is installed! If you plan on trimming the corners with 5/4 x 4, then you need at least 5 1/2" of solid wood on the corners- you'd screw a 2x6 to the corners. If you are putting 5/4 x 4 around the windows, you'd want 2x6's around the windows so that the trim AND your siding will have something to nail to.

Once your tricky areas like that have all been framed with 2x6 or what have you, you would then start filling in the rest of the wall with rips of Styrofoam or Thermax, then screw on a 2x2, and repeat. Your 2x2's would get screwed to your existing studs with 4" screws. (GRK's 4" Torx would work great). If you need to stick one layer of foam to another, you'd use foam adhesive such as PL300, or maybe you can find some 3 1/2" long roofing nails that would reach the wood sheathing.

Once all the foam and 2x2's are up, you could tape over every 2x2 to cover all the seams with Styrofoam's 2 7/8" wide Weathermate construction tape. This will reduce air infiltration between layers and by covering the 2x2's it will make the sheathing completely weather resistive. If new windows will be installed over the top of your new sheathing, be sure you use membrane tape, such as Tyvek Straightflash or Grace Vycor Plus, or Protecto Wrap to seal the nailing flanges to your 2x6's and foam sheathing. Even with all that foam, a breathable building paper might be a good idea to protect the 2x2's and 2x6's from any water infiltration. IMO, the membranes and tape also stick better to building paper. But that's up to you.

Both ISO Styrofoam and XPS Thermax come in 1 1/2 thicknesses which will work well with 2x2 furring. And your fiber cement and trim should be nailed into 1 1/2", not into 3/4 furring, IMO.

They say XPS offgasses as it's cells degrade and lose their nitrogen, and that after a few years it's R-value is equal to ISO anyway. One benefit to using the Thermax is the reflective foil that can face toward the home which supposedly helps reflect radiant energy back toward its source.

You sound like you have a good plan. Hopefully these suggestion makes sense. If you're not adding new windows, you'll have to figure out what this added 3" of sheathing will do to your existing window and door trim. And hopefully you have some soffit or overhang on all 4 sides of the house for your sheathing to butt up into. Adding 3" of sheathing to a home with no overhang would create some problems along the roof edges. You also might want to install a solid 2x2 all the way around the bottom edge and top edge of the home, so as to provide solid nailing for your initial starter piece, and the final piece. You'll likely want to cover the bottom edge of all that sheathing with some custom flashing, bent on a metal break using painted aluminum trim coil of the color of your choice. Having a 2x2 all the way around the bottom will give that flashing something good to nail to. Good luck!

.......solid nailing for your initial starter piece, and the final piece. You'll likely want to cover the bottom edge of all that sheathing with some custom flashing, bent on a metal break using painted aluminum trim coil of the color of your choice. Having a 2x2 all the way around the bottom will give that flashing something good to nail to. Good luck!

THANK you for taking the time to type all that out. I am a little confused by a couple of things. I guess primarily the thickness of the foam and using 2x2s for furring strips. Do I understand that you are suggesting 2x2s primarily to have something with real meat for the siding to attach to? I was planning on screwing Through the furring strips all the way to the sheathing. So I wasn't too worried about that part. And that's also why I was happy to quit at 2", to make sure I wouldn't have to have a 5" screw to embed all the way through the sheathing.

2x4s are pretty erratically spaced but the existing sheathing is old 1x12 rough stock run from foundation to peak. A couple of the boards are OVER 25' LONG. Apparently when the 1.5 story was raised to a 2 story there was a saw mill down the road. You can see where they just lifted the roof about four feet and added knee walls.

Back on topic. Thanks for all the detail on tape, screws etc. That's exactly what I need.

I believe that ISO is the higher R value with the degrading issue that you speak of. My understanding of how reflective barriers works is that it is completely useless unless you have a 1" gap around the barrier for it to work properly. In this case I'd be better off adding 1" of insulation rather than worrying about the air gap.

Edit - overhang is adequate. windows are for another thread once we get this one "nailed" down.

Glad to try and help! If I understand you correctly, you want to install 2" of foam, plus a 3/4 or 1" furring strip, for a total of 2 3/4 to 3". Between the furring there would be a rainscreen / air space behind the siding.

I'm suggesting you install 1 1/2" of foam and 1 1/2" wide furring and 1 1/2" foam (to fill the void between 2x2's and add more r-value) for a total of 3" with no rainscreen behind the siding.

It sounds like basically the same thickness to me while getting an extra 1" of insulation value. I'm suggesting the back of the siding will lay flat against both the 2x2 furring and the insulation that is between the 2x2 furring.

You made the comment about the air gap- Air spaces/rain screens have no value IMO when using fiber cement- rain screen details were designed with wood siding in mind. (equalize the climate on both sides of the wood to promote drying and reduce the differential in temperature and humidity which helps paint jobs last longer by stabilizing the wood) Air spaces have practically no insulating value of their own if they are not DEAD air spaces. I have never heard of the need for an air space behind a radiant barrier. (to my knowledge, Dow does not recommend any air space behind any of their reflective products) If you had an air space behind or between your insulation, your r-value would be compromised by the introduction of (cold) air infiltration. Not trying to contradict you... just trying to explain myself a little more clearly. :)

If you're using fiber cement siding, I don't recommend that you use screws to attach it at ALL... especially not ones that have to go through 2" of foam to get to the sheathing. I'd be ready to jump off the nearest bridge if I had to install an entire house of siding with 4" long screws. ;) In addition to it being unbelievably slow compared to using a siding nailer, the cantelievered weight of the fiber cement could certainly be a problem if you are trying to hang it on a furring strip that is over 2" of foam, the weight of the siding could cause the screws to sag, since foam is not like solid wood, it would not provide any type of support for the screws. I suggested the 2x2's and 4" screws mainly because once the 2x2's are tightly screwed down to the first layer of 1 1/2" foam, the 2x2's should be quite secure and should not sag- a bit of foam adhesive would assure that they stayed put. Then you would use the usual 2 - 2 3/8" galv. ring shank nails to attach the fiber cement. If you use 2x2 furring, it would be just like siding any other house. You could blind nail everything (if using hardiplank) or if using hardipanel, your normal small headed nails will look a lot better and disappear completely when painted as compared to a big headed screw that can't be countersunk completely flush and will always be visible even after being painted.

I know what you mean about the old 1x12 rough sawn sheathing. It's pretty beefy. If you forget what I said about screwing the furring into the studs and wanted to space everything out on 16" centers, you'd probably be fine. It would be a alot easier than trying to line up everything over your existing studs.

OK - I'll have to draw this out a little bit to get a real clear picture of how it would work but I get it now. I knew the advantages of the rain screen wall were pretty minimal with cement siding compared to wood but I still felt the drainage extra drainage plane would be worth to make sure there was no trapped moisture. With your layout it would still be possible to have the space but just keep it very minimal. 1/4" maybe.

I also had thought about something solid on the bottom to help support all the weight. even bolting angle iron to the concrete to give a ledge to set the furring strips on. Even with your layout it could help but probably wouldn't be necessary.

I was seriously considering screws anyway just so that if I completely fowl it up the wall could be disassembled for service. I will be doing this on one wall to start with before I rip any other walls apart. But it's the hardest wall, has the services on it.

I'd also be happy to bring in a pro to help but I haven't found one that is willing to help on such a small project that is also willing to think outside the box a little bit.

All of this would be simplified with vinyl but this is an old house and having seen a house sided with the certainteed beaded cedar fiber cement I do like the looks of the fake wood with real paint over straight vinyl. The 50 year warranty is interesting too.

I'm quite sure reflective barriers need an air gap to be effective. If I have a chance I'll look it up again. I have also read in two places that the air gap behind the siding in a rain screen wall setup does increase the effective insulation because it allows for a fairly immobile air barrier next to the insulation. I can see how that detail could be easily argued both ways.

talked to competent product specialists at Johns Mansville and Certainteed. JM is comfortable with however I chose to install it. No house wrap required, or even taping if the seams are staggered, just silicone the perimeter to make sure air doesn't migrate in around the panel. They are not concerned with mold in this type of installation, but it's not their house is it? They agreed that any benefit of the reflective barrier would require a good size air gap to make it work. A rain screen type gap is not required but "anytime you give a way for water to get out of there quickly it's a good thing".

Certaineen suggest XPS with house wrap to allow the house to "breath". On this point I've done enough research that I'm pretty comfortable with the foil faced ISO. They are more concerned with the weight. Warranty is void if the fasteners go through more than 1" of foam. Nails are required to have 1.25" of penetration but screws only 3/4".

That's all for now.