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Insulating and sealing an old house

Folks,

I am renovating a 1920's summer cottage. It is in poor condition, but needs (deserves) to be preserved and updated. The structure has survived because it can dry both ways, interior/exterior. Wall system is to be upgraded and insulated. Attached is a proposed typical wall section and a interior photo The 1/2" Zip is being placed over the 3/4" board sheathing for bracing/racking (no bracing exists). Interior finish will vary: 3/4"pine beaded board and 5/8" drywall. Air sealing is at the Zip layer
Questions:
-Can the XPS be applied directly to the existing sheathing and then 1/2" Zip? (eliminating the OSB) Would this provided the needed racking resistance?
-Is this a safe system, condensation/dew point ?

Presently the place is being lifted for a new foundation....more later?

A side note: this cottage was built by John Boothman Sr. His work is featured in a new book: The Hand of the Small-Town Builder: Summer Houses in Northern New England, 1876-1930, by Tad Pfeffer. A good read and worth a look, Google it.

Thank you,
Chris Hawkins

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Asked by Chris Hawkins
Posted Jun 11, 2014 12:20 PM ET
Edited Jun 11, 2014 1:25 PM ET

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3 Answers

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1.

Chris,
Q. "Can the XPS be applied directly to the existing sheathing and then 1/2-inch Zip? (eliminating the OSB) Would this provided the needed racking resistance?"

A. No. If you expect to use the Zip sheathing as bracing, it needs to be nailed directly to the existing sheathing and fastened through the sheathing to the studs according to a fastening schedule provided by your engineer. If you suspect that the house has some racking resistance from existing components, allowing you to deviate from normal practice, you'll have to get that hunch confirmed by an engineer.

Q. "Is this a safe system [from a standpoint of] condensation/dew point?"

A. Yes. You should be aware that polyisocanurate is considered to be a more environmentally friendly type of rigid foam than XPS, which is manufactured with blowing agents that have a high global warming potential.

Answered by Martin Holladay, GBA Advisor
Posted Jun 11, 2014 1:20 PM ET

2.

Thanks Martin,
Which sheet good, 1/2" Zip or 1/2" OSB, provides the best for racking resistance. I am inclined to put the OSB against the existing 3/4" board sheathing and the Zip over the rigid foam. XPS is called out because of polyiso's degraded thermal performance in cold weather.

Answered by Chris Hawkins
Posted Jun 11, 2014 1:39 PM ET
Edited Jun 11, 2014 1:40 PM ET.

3.

Chris,
Zip Sheathing is simply a brand of OSB. Like other brands of OSB, Zip sheathing provides adequate bracing when properly nailed. Either product will work for bracing.

Answered by Martin Holladay, GBA Advisor
Posted Jun 11, 2014 2:14 PM ET

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