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Adding ISO board under siding??

Behardy | Posted in General Questions on

I have a 160 year old ballon framed farm house in NE Pennsylvania. I want to add insulation to the exterior walls. The current wall profile is 1/2″ sheet rock, 1.5″ foil faced ISO board, 1.5″ verticle rough cut planks, wood lap siding. Most of the siding is original and is very weathered but has no rot issues whatsoever. Can I add ISO board under the siding or will I run into moisture problems by having 2 vapor barriers??



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  1. GBA Editor
    Martin Holladay | | #1

    I assume that when you talk about "ISO board," you are talking about polyisocyanurate (rigid foam insulation).

    I don't recommend that you install exterior rigid foam. Instead, you could install exterior mineral wool, which is vapor-permeable. More information here:

    Installing Mineral Wool Insulation Over Exterior Wall Sheathing

    Installing Roxul Mineral Wool on Exterior Walls

    Wrapping an Older House with Rock Wool Insulation

    Mineral Wool Boardstock Insulation Gains Ground

  2. Expert Member
    Dana Dorsett | | #2

    With foil faced iso on the interior you could still use up to ~2" of unfaced Type-I or Type-II EPS under the siding without creating a moisture trap. It's not nearly as vapor-permeable as rock wool, but it's still well over 1 perm- it's semi-permeable, and plenty of drying capacity for anything but chronic & gross bulk-water leakage from mis-lapped or absent flashing, etc. (And it's a lot cheaper than rigid rock wool in most markets.) Even some exterior latex paints are more vapor-tight than 2" of Type-I EPS.

  3. Behardy | | #3

    Thanks for the info. I have never seen the rigid rock wool before. EPS? is that the white stuff that Styrofoam coffee cups are made of?

  4. Behardy | | #4

    Thanks for those links Martin. The "wrapping an older house with rock wool" was very interesting. The metal siding was a very neat design concept and maintenance free. Do you know what size the rigid rock wool panels come in? I'm sure they are not cheap either.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "Do you know what size the rigid rock wool panels come in?"

    A. Check out the information in the articles I linked to.
    "ComfortBoard IS, the residential product, has a density of 8 pounds per cubic foot (pcf) and is available in four thicknesses: 1 1/4 inch, 1 1/2 inch, 2 inches, and 3 inches. ... Although Roxul literature shows ComfortBoard IS being available in three sizes — 24" x 48", 36" x 48", and 48" x 96" — it is most commonly stocked in the smaller sizes."
    "we selected 4-inch Cavity Rock DD for our inner layer and 2-inch Cavity Rock MD for our outer layer. ... The 2 ft. by 5 ft. rock wool panels ..."
    The 2 inches of Roxul we installed will add R-8.4 to our walls. ... Each 2’ x 4’ piece of Comfortboard (installed with the 4-foot dimension running horizontally) had one nail per stud."
    "Most of the BSC tests were performed on two thicknesses (1 1/4 inch and 3 inches) of Roxul ComfortBoard IS. ... Available panel sizes: 3 ft. x 4 ft. and 4 ft. x 6 ft."

    Q. "I'm sure they are not cheap."

    A. Alex Wilson wrote, "The contractor pricing for ComfortBoard IS came to $0.64 per board foot, compared to $0.48 per board foot for standard polyiso, $0.75 for fire-rated polyiso (Thermax), and $1.07 for XPS. While pricing will doubtless differ in other regions and for different quantities, the fact that ComfortBoard is in the same ballpark as these other materials is great. Even after correcting for the lower insulating value (you need more thickness of ComfortBoard to achieve R-10 than with the foam plastics), Comfortboard IS locally was more affordable than XPS: roughly $1.59 per square foot at R-10 for ComfortBoard vs. $2.14/sf @ R-10 for XPS."

  6. Expert Member
    Dana Dorsett | | #6

    "EPS? is that the white stuff that Styrofoam coffee cups are made of?"

    Styrofoam is a trademark for Dow's XPS, which is NOT the stuff coffee cups are made of.

    EPS (the stuff with apparent macroscopic bead structures visible) is the stuff of coffee cups and cheap coolers. It's the same polymer as XPS (polystyrene), but blown and processed in a very different manner, using pentane as the blowing agent, at less than 1/100 the global warming potential of the HFC blowing agents used for XPS. It comes in a number of standard densities, with slightly differing R/inch, vapor permeance, and compressive strength characteristics. At any density it is far more vapor permeable than XPS. Type-II (1.5lbs per cubic foot nominal density) is probably the most commonly used and appropriate version for insulating sheathing, and it runs ~3 perms @ 1" thickness (R4.2) , ~1.5 perms @ 2" (R8.4), 1 perm @ 3" (R12.6) , etc. By contrast 1.5lb density XPS is about 1.2 perms @ 1" (R5).

    Unlike XPS, EPS doesn't rely on the blowing agent for thermal performance, and has a stable R-value over decades, whereas 1.5lb XPS drops to about R4.5/inch in the first ~50 years (having done the majority of its global warming damage from loss of blowing agent over that period), and would have about the same R/inch as Type-II EPS in 100 years.

    EPS is usually substantially cheaper per R than XPS and slightly cheaper per R than polyiso. Unlike polyiso, EPS increases rather than decreases with falling temperatures- Type-II EPS is rated at R4.2/inch when tested under ASTM C 518 conditions with a 75F average temp through the material (the FTC mandated test temp for labeling), but rises to about R4.5/inch @ 40F average temp through the material (a commonly tested spec for EPS, relevant in exterior sheathing applications.) By contrast, 1.5lb polyiso runs about R6 to R6.5 @ 75F, but drops to the high 5s at 40F average temp, mid-to-high 5s @ 25F average temp.

    EPS isn't fireproof the way rigid rock wool is, like XPS will melt and spread under fire conditions (unlike polyiso), and has a lower kindling temp than polyiso (but the same kindling temp as XPS). But unlike rock wool panels it's readily available at competitive pricing from multiple vendors, and is usually the price/performance winner on raw thermal performance in heating dominated climates, and sufficiently vapor permeable for your stackup, which is why it's worth considering.

    Don't get me wrong- I like rigid rock wool in this application too, but it'll be something like 75-100% more expensive than Type-II EPS per R/foot and you may have to order it well ahead of time through distributors- it's never found on the shelves at the local box-stores or lumber yard type outlets. If you use Alex's pricing per board-foot that Martin mentions, rock wool at 64 cents/ per board foot @ R4.2/inch is about 15 cents per R-foot, compared to 48cent/board-foot polyiso at R6/inch is 8 cents per R-foot. Type-II EPS will sometimes come in as low as 7.5 cents/R-foot, and rarely more than 10 cents/R-foot.

  7. Behardy | | #7

    Thanks Dana I appreciate the information. Its nice to have options. What ever I choose to use I was planning on using great stuff to foam the gaps between the planks. Is it neccessay to put tyvek on the outside prior to adding the insulation board??

  8. GBA Editor
    Martin Holladay | | #8
  9. Behardy | | #9

    Thanks Martin I will.

  10. bvillebound | | #10

    Dear Ben: Another important note on polyicocyanurate foam board -- it is moderately hydrophilic, which means that it will absorb and store water when it gets humid or wet. Not a good idea for an exterior application! XPS and EPS foam board are hydrophobic and will not absorb water.

    We used XPS foam board to cover the exterior of our latest project, a 1950 era home along the shore in Massachusetts. A photo is attached, showing 2" of XPS on the outside of the house, with PT strapping for the siding and trim. We used long, corrosion resistant "washer / truss head" screws (available from Home Depot and probably Lowes) to attach the foam board and strapping to the sheathing.

    The warning above about ozone damage etc. from the XPS blowing agent does not apply to the pink DOW Formular board sold at Home Depot; they changed the blowing agent in 2009 to eliminate this problem. DOW also warranties R performance over 20 years.

    One more note: air sealing is obviously important. We used the 'Windows' version of Great Stuff to seal all of the XPS edges, gaps and holes. This foam remains flexible, which should help to avoid cracks as the assembly expands and contracts. It is also tough and does not degrade due to water and weather, unlike water-based foam products. To be doubly sure, we then coated all of the panel seams and the joints around windows and doors with a wide and thick layer of DAP 230 sealant. A 3" paint roller works well for panel seams -- just empty a few tubes into a paint tray and roll it on.

    We also installed new housewrap under the XPS foam board. This was easier and made the most sense.

    I hope this is helpful.


  11. GBA Editor
    Martin Holladay | | #11

    You wrote, "The warning above about ozone damage etc. from the XPS blowing agent does not apply to the pink DOW Formular board sold at Home Depot; they changed the blowing agent in 2009 to eliminate this problem."

    You are confusing two issues: damage to the ozone layer and global warming potential. Sadly, your cheerful conclusions about the blowing agents used by Dow to manufacture XPS are mistaken. All brands of XPS sold in the U.S. -- those rated at R-5 per inch -- use blowing agents with a very high global warming potential. While European manufacturers of XPS use a more benign blowing agent, the resulting foam has a lower R-value (R-4 per inch). This European product is not available in the U.S., to the best of my knowledge.

  12. Expert Member
    Dana Dorsett | | #12

    That's right- European XPS is blown with CO2, at exactly 1x the global warming potential of CO2 (go figure! :-) ).

    All XPS manufactured in North America is blown with a combination of HFCs, the predominant component of which is HFC134a (the same stuff used as automotive air conditioning refrigerant), with a global warming potential of about 1400x that of CO2. Yes, HFC134a is ozone-safe (and legal to use per the Montreal protocol), but it's a killer greenhouse gas, and is being rapidly phased out for almost all applications in Europe. The pink stuff is still blown with HFC134a as the primary blowing agent, even though they cut the amount of HFC134a by 30-40% a few years ago. (The exact composition of the co-agents they replaced the 30-40% fraction with is proprietary.)

    For automotive air conditioning some manufacturers are going to HFO1234yf (at about 4x CO2), but it's flammable (less flammable than gasoline vapor however). Others are using CO2 as the auto-AC refrigerant, despite the much higher cost of compressor types required. The closely related HFO1234ze, (GWP of about 6x CO2 ) is a potential replacement for HFC134a for XPS, but SFAIK no foam vendors are currently using it. It's a moving target, but it's still moving pretty slowly:

    The CO2-blown XPS has identical R-values as EPS blown at the same density, eg: 1.5lb XPS runs about R4.2 per inch, the same as density & performance of Type-II EPS. The water vapor permeance of CO2-blown XPS is a bit tighter than EPS though, comparable to HFC-blown XPS.

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