Insulating old house with single-wall construction using polyiso rigid foam
I removed the lathe and plaster from an old house to remove/replace the electrical and plumbing. The house was constructed using cedar siding nailed directly to the studs, so there is no sheathing or house wrap. On the one hand this allowed the house to breath, but over the years some large gaps formed between the rows of siding. Where that was the case, I caulked the gap from the inside, but the number of rows needing caulking was probably less than 5% of the total so I believe the house will continue to breathe. (I did this to block any liquid water, i.e., rain from entering the wall cavity)
The next thing I want to do is add insulation. I do not want to use fiberglass batting or loose fill since some moisture will still get into the wall cavity and possibly condense. I have access to enough 2″ rigid polyiso foam to fill the cavities. This foam has a black fiberglass paper on both sides. I don’t have any data on its permeability, so I would like to determine if and where I should use a vapor barrier.
My plan is to cut the foam to fit in each cavity and to leave a 1/2″ gap between the foam board and the exterior siding. I am planning to use spray foam around the perimeter of the foam board as needed to seal it and keep it in place.
I am in Oregon where the winters are cool and wet. Typical winter weather is 40-50 F ( 5-10 C) with frequent drizzle to light rain. The summers are mild with sun and the daily high in July rarely exceeding 95 F (35 C) and cooling off at night to the low-mid 60s (15 C).
My questions:
– Are there any general concerns about using polyiso in wall cavities of exterior walls if sealed with expanding foam?
– Will it be necessary to install a vapor barrier, and if so should it be on the interior or exterior side of the wall cavity. Bear in mind that I cannot remove the siding at this time.
– From my study I have learned that moisture doesn’t adversely affect the thermal performance of polyiso. Does it degrade with age? The reason I ask is because I have a stock of polyiso sheets in reasonable condition that are 10-15 years old.
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Replies
As long as there is the 1/2" gap between the siding and polyiso you should be fine. With 2" of foam on the exterior side you can fill any remaining space on the conditioned-space side of the cavity with fiber insulation without condensation risk. (As little as R2.5 of foam on the exterior of a 2x4 wall is sufficient dew point control for R13 fiberglass, no vapor barrier needed.)
Polyiso of different ages has different degradation properties based on the particulars of the blowing agents used. Some of the pre-Montreal Protocol stuff from the 1980s will still test at R7/inch even after 30 years or more of service. Almost any polyiso of any era will still be delivering at least R4+ per inch. Most polyiso blown with hydrocarbons have temperature derating issues, but not at temperatures normally seen in the coastal OR zone.
Most fiber faced 2lb roofing polyiso like yours would be labeled R5.7 per inch (R11+ for 2"), but at your outdoor temperatures it's performance INCREASES during the colder season.
No matter what the actual R-value is, when installed between framing the whole-wall performance will be about the same. Very high R/inch is robbed of it's potential performance by the R1/inch framing wood that constitutes ~25% of the exterior wall area in a typical 2x4 framed house. See:
https://www.finehomebuilding.com/membership/pdf/184243/021269086NRGnerd.pdf
Craig,
First of all, you should read this article: "Insulating Walls in an Old House With No Sheathing."
Q. "I would like to determine if and where I should use a vapor barrier."
A. There is no requirement that you include a vapor barrier in your wall assembly. However, you may need an interior vapor retarder -- a less stringent type of layer than a vapor barrier.
The answer to your question depends on your climate zone. Oregon has two zones: Zone 4C to the west, and Zone 5B to the east. To determine your zone, consult this map: Climate Zone map.
In Zone 4C, building codes do not require an interior vapor retarder.
In Zone 5B, building code generally require an interior vapor retarder like kraft facing, vapor-retarder paint, or a smart retarder like MemBrain. An exception is made, however, for walls with exterior rigid foam -- which (arguably) your wall assembly will have. Frankly, it makes little difference from a building science perspective whether your wall has a vapor retarder, as long as you have well-sealed rigid foam toward the exterior side of the assembly. Either kraft facing, vapor retarder paint, or MemBrain will be harmless if installed, but can be omitted.
Q. "Are there any general concerns about using polyiso in wall cavities of exterior walls if sealed with expanding foam?"
A. Not really. Air sealing is good. If you live in Zone 5, thicker exterior foam is a little safer than thin exterior foam. In Zone 4, any thickness of exterior rigid foam will work. For more information on this issue, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."
Q. "Will it be necessary to install a vapor barrier, and if so should it be on the interior or exterior side of the wall cavity. Bear in mind that I cannot remove the siding at this time."
A. The vapor retarder, if installed, goes on the interior side of the assembly.
Q. "From my study I have learned that moisture doesn't adversely affect the thermal performance of polyiso. Does it degrade with age?"
A. Not appreciably. Go ahead and use the polyiso you have on hand.
The typical weather is described as " ...winter weather is 40-50 F ( 5-10 C) with frequent drizzle to light rain ", which indicates that it is definitely the zone 4C, not 5B part of Oregon. (East of the Cascades it's substantially colder and much drier than that.)
The foam itself is a vapor retarder (probably Class II, even without the facers), and at 2" is thicker than needed for dew point control on R20 fiber insulation even in zone 6. So even if the walls get padded out on the interior to accomodate some fiber insulation, in this stack there should be NO interior side vapor barrier or other vapor retarders. Latex paint is fine, foil or vinyl wallpapers may increase moisture risks.
An IRC 2018 code-min wall in zone 4C is 2x4/R13 + R5 continuous insulation, or R20 cavity fill. With the 2" roofing foam you're looking at about R11.5-ish, and with the half-inch gap you'll have 1" of depth behind the stud edges. If you can give up 1.75" of interior space code-min performance could be achived by adding 1" foam Bonfiglioni strips to the framing which cuts the framing losses in half, increasing the cavity depth to 2.25". See:
https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf
Installing compressed fiberblass R13s or compressed 3" sound control batts into that 2.25" space would add more than the R9 needed at enter-cavity to bring it up to R20 and with the R6 thermal break of the framing strips it would beat code min performance with some margin.
The additional depth would also allow installing the electric wiring completely to the interior side of your cut'n'cobbled cavity foam.
Dana and Martin,
Yes, the house is on the west side of the Cascades, in zone 4C. Thank you both for the detailed replies and links to additional information. I will read through them and let you know if I have additional questions.
Craig, Dana, Martin -
I'm facing the same situation and was planning on following a similar approach although I would like to use 3" of polyiso (2" + 1") in the stud bays (seal with foam). The "Insulating Walls in an Old House With No Sheathing" article suggest using at least 3/4" of air gap... Craig was planning to have a 1/2" air gap, is that okay?
Thanks in advance!