Birmingham, Alabama: Historic house
Hi Folks! Here’s the rundown:
– 1920’s home
– Zone 3 (Birmingham, AL)
– ALL WALLS ARE DOWN – Exterior wood siding directly on studs, with no vapor barrier/underlayment or insulation in wall cavity.
– I plan on replacing the all INTERIOR lath and plaster walls with 1″ thick southern pine shiplap.
My question is this: what would the best method be for my climate to insulate the walls. One of my biggest fears of using fiberglass batt insulation is essentially choking the air flow to the house and creating a mold friendly environment. Would EPS board within the wall cavity be the best fit? If so, how thick and what type of EPS board would be best (Type 1 + Type 2) I am.
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Replies
User-7404160,
First of all, can you tell us your name? (I'm Martin.)
I urge you to read this article: "Insulating Walls in an Old House With No Sheathing."
You absolutely want to stop air flow through your walls. You can't install insulation without a plan to stop air leaks. Every wall needs an air barrier -- it's a code requirement and common sense.
Sorry about that! My name is Brian.
I do not have an option of taking down the siding to put in a vapor barrier. So that being said, what is the next best option to insulate a non sheathed wall from the inside out in a humid climate.
Brian,
I understand that you don't intend to remove the siding. That's why I suggested that you follow the advice in this article: "Insulating Walls in an Old House With No Sheathing." Did you read it?
You don't need to install a vapor barrier. Building codes don't require a vapor barrier, and there is no compelling reason to install one. I think you are confusing vapor barriers with water-resistive barriers (WRBs) like housewrap. (Most WRBs aren't vapor barriers -- they let vapor pass right through.)
The fact that your wall lacks a WRB is, indeed, an issue, which is why I suggested that you follow one of the methods recommended in the article.
You definitely WANT to choke air flow into the house. Air-transported moisture to the interior finish wall in an air conditioned building is a real problem in your climate, a far bigger problem than water vapor diffusion.
Install 1" wide strips of half-inch foam-board on the sides of each stud as a spacer to keep a gap to the siding, and install 1" foil faced foam board against the spacers, filling in the rest of the stud bay with compressed batts (rock wool prefered) or blown fiber (cellulose preferred).
The ship lap interior needs to be air tight too, which will probably require broad-sheet layer under the ship lap. It would be fine to use housewrap in that location.
If I use a foil face product for filling the stud bay, won't that create a great area for condensation to collect?
That is why I initially brought up using an unfaced EPS foam board that is somewhat permeable.
>"If I use a foil face product for filling the stud bay, won't that create a great area for condensation to collect?"
I wasn't recommending FILLING the stud bay with foil faced foam (which is expensive), but only using it as a a weather resistant exterior side air barrier for fiber insulation.
Condensation only collects on surfaces that are at a temperature colder than the dew point of the proximate air. When is that even going to happen? (A: Almost never, and not long enough to collect appreciable amounts of moisture when it does happen.)
The dew point of the entrained air in fiber-insulated part of the stud bay will track that of the air conditioned interior air, and the interior side foil facer is WARMER than that interior air in summer, and MUCH warmer than the dew point of the conditioned air when the air conditioning is running. The dew point of the outdoor air in summer is often even higher than the dry bulb temperature of the indoor air.
If the cavity fill were vapor permeable moisture from outdoor air would collect in the interior wall coverings, creating potential mold/rot conditions. By having reasonably well sealed foil faced foam that outdoor moisture never reached the finished interior side. A big air leak could defeat the vapor barrier foil, but water vapor diffusion is effectively blocked except for the small amount of vapor diffusion through the stud framing itself.
In winter the outdoor air is dry, often drier than the indoor air. But an inch of polyiso foam is more than sufficient R-value to keep the average temp at the interior side foil facer well above typical indoor dew points during Alabama winters even if there were 5" of fiber insulation between the foam board and the conditioned space.
The exterior foil's temperature will track the temperature of the outdoors, and will always dry to the outdoors as long as there is a bit of air gap between the foil and the siding.
Martin, I have read the article and am a little confused about if EPS or XPS, faced or unfaced would be a better fit for my region?
If I were to use an XPS board for the stud bay fill, would you suggest any certain thickness for my climate?
Would you suggest using a some kind of canned spray foam on every overlap of the sheathing before putting the rigid foam on?
XPS is a slow motion environmental disaster that you don't need.
You're in US climate zone 3A- where summertime outdoor dew points are higher than the indoor air temperatures, so you want a vapor retarder or vapor barrier somewhere on the exterior side of the assembly. With a plywood or OSB sheathing building plywood or OSB serves as that vapor retarder. (Both are about 1 US perm or less when their moisture content is low.)
A foil facer is a vapor barrier, and when adjacent to a 0.5-1" air gap adds about R1 of thermal performance. But fiberglass or asphalted paper faced roofing polyiso could be used too, and would have a higher R/inch than XPS or EPS.
You don't need or particularly want to fill the stud bay with cut'n'cobbled foam. You only need enough to provide an exterior side air barrier for any (much cheaper & greener) fiber insulation.
You also need an interior side air barrier. If the interior side sheathing is going to be pine ship lap, there is no way to make it reliably air tight with caulk or can-foam, since the normal seasonal dimensional changes of the planks with humidity and temperature will inevitably cause the seal to fail. A layer of gypsum board detailed as an air barrier under the ship-lap can work, as can a "smart" membrane type vapor retarder such as Intello Plus or Certainteed MemBrain detailed as an air barrier.
Brian,
Any type of rigid foam will work, although green builders avoid the use of XPS for environmental reasons. For more information on this issue, see "Choosing Rigid Foam."
It doesn't really matter whether or not it has foil facing. What matters is that (a) there is an airspace between the rigid foam and your siding, and (b) the rigid foam is installed in an airtight manner.
Once the rigid foam is installed, you can fill the rest of the stud bay with fiberglass batts, mineral wool batts, or blown-in insulation.
@Dana Dorsett,
Thank you for that insight, it is very helpful. If I am understanding you correctly, you believe that a 1" thick foil faced rigid foam board with wood sheathing spacers would do best for the Alabama climate? Would you suggest dual faced foil rigid foam or single faced?
Any other suggestions?
Your thoughts from outside in:
- Exterior Shiplap boards
- 1 inch air gap
- 1 inch foil faced rigid foam (Do I go single faced or dual?)
- Certainteed smart membrane
- 1" Interior pine shiplap boards
In my original response #4 I recommended (and still do) using half-inch thick spacers (x 1" wide) for a half-inch air gap, and 1" foil faced foam board, filling the rest of the cavity with fiber insulation, followed by an interior side air barrier & interior finish sheathing. That would make the stackup:
- Exterior Shiplap boards
- half-inch air gap
- 1 inch foil faced rigid POLYISO foam (double faced is customary) can-foamed to the studs for air tightness, seams taped with aluminum tape
- _x_" fiber insulation (whatever the remaining depth is)
- broad sheet membrane air barrier (semi or variably permeable
- 1" Interior pine shiplap boards
Assuming the studs are full dimension 2x4s, after the exterior air gap and foam board it would have 2.5" of space left for fiber insulation. Using 3" sound abatement batts compressed to 2.5" would deliver ~R10-R11. With 1" foil faced polyiso the foam would be about R6, and the air gap another R1 for R7, for about R17-R18 cavity insulation value total. With more than 1/3 of the total R on the exterior of the fiber insulation there would be effectively zero hours at the foam/fiber boundary where the interior facer is over an entire winter, and no accumulation even if there were minor air leakage to the interior conditioned space.
This is great information! Thank you so much. I will definitely give an update and pictures once its completed!
Hi Dana and Martin, I started taking down the walls of the house and found that I do have 1"x6" boards between my studs and cedar siding of this house.
Given that your advice was for a home with no sheathing, would you still recommend above?
Are the studs standard milled lumber 2x4s, or are the full dimension 2x4?
Is there tarpaper or rosin paper or some other layer between the siding and plank sheathing?
Is there any window flashing?
With no flashing or tar paper it's safer to still build in a half-inch drain gap/capillary break with spacer strips and use foil faced foam board as the air barrier.
If there is tar paper & flashing it's usually safe to fill the cavity with blown cellulose, which can wick and safely manage incidental moisture (but not inundation).
If the studs are on standard depth and spacing batts can work just fine if care is taken to trim carefully for no voids & gaps, even if they need to be compressed a bit to accommodate a foam board air barrier. (An R13 compressed to 2.5" still performs at R10, and the foam board has R-value.) With odd & uneven stud spacing batts often need more custom trimming than they're worth, tipping the balance toward blown fiber or open cell spray foam.
These are standard 2x4 and There is no tar or paper in it.
You would still suggest a 1” polyiso foam board with the foil facing the exterior?
What are your thoughts on going 1/2” polyiso instead of 1”And 3.5” of non faced pink stuff? I wasn’t sure if there was a benefit of going 1” over the .5” polyiso rigid foam
>"What are your thoughts on going 1/2” polyiso instead of 1”And 3.5” of non faced pink stuff? I wasn’t sure if there was a benefit of going 1” over the .5” polyiso rigid foam"
Half-inch polyiso would likely prove too flexible for compressing 3.5" batts down to 2.5", but it would be fine with a supporting strip of 1/2" depth spacer of the same type you're using adjacent to the studs midway between the studs.
It's fine to use kraft faced pink stuff rather than unfaced, if that's any cheaper or easier to install. (Seems "contractor roll" kraft faced is usually cheaper than unfaced.) The kraft facer is not a vapor barrier and can't create a moisture trap- it's a "smart" vapor retarder that becomes fairly vapor-open when the humidity in the cavity rises to the levels that would support mold growth.
At 2.5" the R13 batts perform at R10, the half-inch polyiso would deliver R3, and a shiny foil facer next to a half-inch air gap is good for about R1, so you'd be looking at R14-ish value. That wouldn't hit the current IRC code min (=R20), but is way better than nothing. With 3/4" polyiso edge strips on all of the framing the 3" compressed batt would be good for R11.5, for a center cavity R of about R15.5, but with the R5-ish thermal break on the framing it would probably meet code on a U-factor basis. With 1" edge strips and R15 batts (~R19 total at center cavity) it would definitely make it.
Edge strips can add a lot of labor to the assembly though. Using R15 batts and compressing it to 2.5" would yield R11 for the fiber, R14 for the foam, R1 for the foil facing the gap for R15 total at center cavity, and doesn't eat into interior space or complicate the wallboard & window trim, etc.
Going with the original method .5 air gap, 1” polyiso, 2.5” pink insulation, is probably the best option.
Be sure to use a blower door to verify air sealing.