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Zone 3 re-siding - exterior insulation options

Hi everyone - my name is Loren. I’ve learned a ton about energy improvements for an older home from this website – thank you for creating such a valuable resource!

I’m seeking guidance and advice on alternatives for adding insulation and improving air/vapor sealing to my home while some walls are being re-sided. Some details:

• 1964 construction, Climate zone 3 (Atlanta), 2 stories (2,330 SF), 600 SF basement w/adjacent
crawlspace. Graded “leaky” in energy assessment done 5 years ago (CFM50 = 5050 N = 17.2
ACH natural .71)
• 2x4 stud walls – drywall, fiberglass batts in cavities, Celotex blackboard sheathing
• Front and side walls of 2nd floor are cedar siding, rest of exterior cladding is brick.
• Current windows are all original wood, single pane
• Ventilated attic (ridge vent), no attic ductwork
• Recently replaced HVAC – variable-speed heat pump/condensing furnace. Even with the featured
humidity control of the variable speed compressor the unit still went into "overcooling mode to lower
humidity levels this past summer.
• Wife and I both have some respiratory sensitivities – improvements in air quality from envelope
sealing are valued.

Current project underway:
• Extend 2nd floor front balcony (and roofline) rom 3’ to 7’.
• Replace siding with Hardie fiber cement board
• Update 48” French door in one BR, replace double windows in other BR off of balcony w/matching
French door, install larger window (bathroom) between the French doors.

Unplanned add-ons as project evolved:
• New standing seam metal roof
• Replacement windows (4) on 2nd floor end walls

Tentative plan for improving the walls (formulated from advice here):
• Remove Celotex sheathing
• Replace old FG batts in wall voids with mineral wool batts
• Re-sheath with OSB (tape seams)
• Apply WRB
• Flash windows
• Install 1” of foil-faced Polyiso foam to exterior walls (tape seams)
• Install fiber cement siding

I’m not set on any material or approach on the walls – and I’m willing to invest more for incremental performance where it makes sens from a cost/benefit standpoint. I also want to ensure the siding contractor my GC is considering will handle the install properly – so, I need to explain what we want accomplished in terms a professional tradesperson understands.

What are thoughts and opinions on a plan for insulating the 2nd floor walls?

Thanks - and I truly appreciate your help!


PS - After the current project I see myself using a “cut and cobble” approach to sealing the rest of the walls in the house (behind brick exterior) over time as rooms get rehabbed and re-windowed (https://www.energyvanguard.com/blog/how-to-fix-a-leaky-underinsulated-ex...). And I still need to seal up the attic.

Asked by Salaryman
Posted Apr 9, 2018 12:57 PM ET


7 Answers

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Caution against using the OSB for your air seal: http://www.greenbuildingadvisor.com/blogs/dept/musings/osb-airtight

One option you may consider is a product called Zip-R sheathing that includes varying thicknesses of polyiso insulation. Just tape the seams and you'll have the air seal, WRB and continuous insulation covered.

Also, since you mentioned crawlspace and basement, have those given you any indications of moisture related issues in the past? Since you are tightening the system above you'll want to make sure that your home wasn't drying out the crawlspace through the leaky old construction.

Answered by Tyler LeClear Vachta
Posted Apr 9, 2018 1:21 PM ET


Tyler, thanks for your idea - I wasn't aware that insulated Zip sheathing (properly taped) would, in one product, accomplish everything I hoped to get from the combination of new sheathing, Polysio, and WRB (air barrier, vapor barrier, R-factor). I'm guessing that it may simplify installation for siding and windows/doors as well. Is it comparable in cost to using the component pieces (OSB, WRB, Polyiso)?

Answered by Salaryman
Posted Apr 9, 2018 2:10 PM ET


Re: "...make sure that your home wasn't drying out the crawlspace through the leaky old construction."

In my home the basement area (both semi-finished and crawlspace) exhibits higher levels of humidity that the floors above it. In summers I've had to run a dehumidifier there to prevent mold and mildew growing on items.

Answered by Salaryman
Posted Apr 9, 2018 2:14 PM ET


The ZIP detailing is pretty straightforward, but on the materials side of the equation it will run higher than the individual components. Ask your builder if they are familiar with the system.

As to the crawl-space and basement it would be a good idea to audit your current system and reference some of Martin's advice on http://www.greenbuildingadvisor.com/blogs/dept/musings/building-unvented.... If your current crawlspace and basement is a significant source of moisture sealing up the above grade exterior could exacerbate the humidity and related air quality issues.

Answered by Tyler LeClear Vachta
Posted Apr 9, 2018 3:49 PM ET


Tyler, thanks again for the advice (hope you can still "see" this thread...I don't see where in my free account I can get to it, other than browsing through the list of submitted questions).

I reached out to two local distributors of Huber products - neither had pricing information on the Zip-R sheathing (not surprisingly they have none in stock) and are contacting Huber about the R-3 and R-6 products for me. Apparently Zip system sheathing gets used a fair amount locally (Altanta) but not the insulated Zip-R. I hope to find a subcontractor that has worked with Zip-R, but that might prove a challenge. Hopefully the installation detail you refer to is similar for both products.

Thanks also for the heads-up on the role the crawlspace may be playing in humidity control - I'm monitoring temp/humidity levels in it and brushing up in the sealing methods described by the knowledgible folks here. Looks like that might be my next project!

Best Regards,
Loren Weaver

Answered by Salaryman
Posted Apr 15, 2018 11:37 AM ET


Unless you're going for PassiveHouse type air-tightness using OSB as the primary air barrier will be fine. If going for Passivehouse it may (or may not) need further treatment.

Celotex has more than twice the R-value of OSB. Leaving it in place and adding a taped layer foil-faced polyiso detailed as an air barrier would be less work, cheaper and higher performance than the outlined plan. The OSB is somewhat more structural than Celotex (assuming the Celotex was used as structural sheathing, which isn't always the case.

ZIP-R is more expensive than adding exterior polyiso, and doesn't have the thermal benefit of a radiant-barrier facing the air gap to the siding.

In GA a vented crawlspace brings more moisture into the house than it purges. A ground vapor retarder and a code-min R5 or more of continuous insulation on the crawlspace walls would be the "right" way to go.

Answered by Dana Dorsett
Posted Apr 15, 2018 2:23 PM ET


On a project I was involved with we could not get below 1.0 ACH50 with OSB sheathing, with otherwise impeccable details. With ZIP sheathing we routinely got below 0.3 ACH50. As a result, I do not recommend conventional OSB if you want to be sure to get below about 1.5 to 2.0 ACH50.

Answered by Michael Maines
Posted Apr 15, 2018 6:17 PM ET

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