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Adding ISO board under siding??

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??



Asked by Ben Hardy
Posted Wed, 07/10/2013 - 10:54


9 Answers

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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

Answered by Martin Holladay, GBA Advisor
Posted Wed, 07/10/2013 - 13:26

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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.

Answered by Dana Dorsett
Posted Wed, 07/10/2013 - 14:56

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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?

Answered by Ben Hardy
Posted Wed, 07/10/2013 - 22:20

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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.

Answered by Ben Hardy
Posted Wed, 07/10/2013 - 22:36

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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."

Answered by Martin Holladay, GBA Advisor
Posted Thu, 07/11/2013 - 04:24

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"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.

Answered by Dana Dorsett
Posted Thu, 07/11/2013 - 10:54

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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??

Answered by Ben Hardy
Posted Thu, 07/11/2013 - 13:06

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You should read these articles:

How to Install Rigid Foam Sheathing

Where Does the Housewrap Go?

Answered by Martin Holladay, GBA Advisor
Posted Thu, 07/11/2013 - 13:38

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Thanks Martin I will.

Answered by Ben Hardy
Posted Thu, 07/11/2013 - 13:50

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