Seeking Expert Advice on Moisture Management for Historic Home with New Insulation
Hello GBA community,
I’m seeking advice on managing moisture in a historic duplex built approximately 125 to 175 years ago. I recently participated in the Connecticut Home Energy Solutions℠ program, which included a comprehensive home energy assessment and recommendations for energy efficiency improvements.
During the visit, a blower door test was conducted (results below), and weatherization services such as air sealing and weatherstripping around doors were performed. Additionally, the contractors recommended dense-packed cellulose insulation for the walls and loose-fill cellulose for the attic. While I’m eager to improve the duplex’s energy efficiency, I have some concerns about moisture management, given the building’s age and construction.
Home Details:
- No vapor barrier on either the interior or exterior walls.
- No central cooling or dehumidification system; tenants may or may not use window air conditioners.
- Historically, the natural draft of the house has helped keep the wall cavities dry.
Blower Door Test Results:
- Before Air Sealing:
- Flow Rate: 4339 CFM @ -50 Pa
- After Air Sealing:
- Flow Rate: 1201 CFM @ -50 Pa
Concerns and Questions: My primary concern is maintaining proper moisture management both before and after installing the dense-packed cellulose insulation, as no interior or exterior sheathing or siding will be removed; this is a drill-and-fill wall cavity insulation method, which will alter the natural airflow dynamics. I’m hoping to get some expert advice on the following points:
- Assessment: What methods should be used to evaluate existing moisture levels and identify potential moisture sources in the walls before insulation?
- Mitigation: What steps can be taken to address any moisture issues discovered during the assessment without removing the siding or interior walls?
- Preventive Measures: Are there any treatments or strategies to ensure the cellulose insulation remains dry and effective over time? For example:
- Vapor Retarder Paint: Is it worth applying vapor retarder paint to the interior walls to slow down moisture migration into the wall cavities?
- Borate Treatment: Is all cellulose treated with a borate solution to inhibit mold and mildew growth?
- Wall Cavity Drying: How can we address the drying of the walls post-insulation, considering the house has historically relied on natural drafts?
I appreciate any insights or recommendations you can provide to help ensure my home remains energy-efficient and structurally sound. Thank you for your assistance!
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Replies
The biggest concern you should have is exterior moisture in your wall assembly. It would be helpful to know what your wall assembly is, and also what kind of wrb is currently on your walls, if any. Air sealing the interior is more important than vapor barriers and a couple a coats of paint is more than adequate as a vapor retarder. Air leaks will introduce significantly more moisture to your assembly than diffusion.
Cross-section from Exterior to Interior:
Lap Vinyl Siding
1/4" Foam Insulation
Wood Planks or Boards (Horizontal Sheathing)
Wall Studs (2x4 spaced 16” on center) Balloon framed with Empty Cavity
Drywall or 1980's Style Board and Bead Panels
Gable Roof with 1' overhang
"Natural drafts" aren't what's keeping the walls in good shape, it's the lack of insulation that allows heat to flow through unimpeded, carrying moisture out with it. Air sealing is a good way to offset the introduction of insulation, which will slow down the drying action. I would monitor the indoor relative humidity and keep it below 50% in winter.
Hi Michael,
Thank you so much for responding to my post! I appreciate your insights.
Thank you for clarifying that natural drafts weren't the main factor keeping the walls in good shape; instead, the lack of insulation allowed heat to flow. Your explanation about how air sealing helps offset insulation's introduction makes perfect sense now.
By the way, I wanted to express my direct appreciation for your book. A chapter on making 100+-year-old houses "Pretty Good" would benefit people like me, working on improving historic homes while maintaining their integrity.
Based on my blower door test scores before and after air sealing (4339 CFM @ -50 Pa before, 1201 CFM @ -50 Pa after) and the lack of any vapor membrane in the wall sections, do you think it’s safe to proceed with the cellulose insulation?
Here is the proposed work:
• Attic Insulation:
o Pre: R0, Post: R49 Open Blown Cellulose
• Kneewall Floor Insulation:
o Pre: R0, Post: R49 Open Blown Cellulose
• Exterior Wall Insulation:
o Pre: R0, Post: R13 Dense Pack Cellulose
• Attic Slope Insulation:
o Pre: R0, Post: R13 Dense Pack Cellulose
• Vent bath fan through gable wall:
o Bath fan kit/hose
• Kneewall Insulation:
o Pre: R0, Post: R13 Fiber Glass Batts
• Maintain Proper Ventilation:
o Soffit Baffles
Thanks again for your valuable advice!
i hope you don't have any knob and tube wiring in the framing cavities. in this case you could not insulate without re-wiring.
Fortunately, one of the previous owners replaced all the knobs and tubes.
I think you are mostly fine. Generally that fanfold is somewhat permeable (perforated ones especially so) plus the vinyl siding should allow for drying to the exterior.
I wouldn't worry about the plaster, something old has enough layers of oil paint on it to act as a vapor barrier.
The usuall problem spot in these older houses is your rough openings. There is usually no flashing details around these. Small water leaks with uninsulated walls there is generally not an issue since there is a lot of air and heat flow to dry. Once insulated, these leaks can saturate the cellulose and cause issues. Without opening things up, it is impossible to say if this will be a problem.
Since you have overhangs, most of the problems will be on the lower windows, as budget and time allows, I would look at replacing these and flashing the openings properly to the fanfold. In the mean time would be good to make sure your exterior sills all slope away from the windows and the sills properly caulked to the windows. Same for the top of any decorative trim above the windows.
Just to reinforce and clarify what several others have said, there are two separate and distinct lines issues with modernizing an older house:
1. It was really only in the past couple of decades that we figured out how to make siding reasonably watertight. Older houses often leak, particularly around windows and doors. When houses are poorly insulated and air sealed there is enough drying action that the leaking isn't a problem. Sealing and insulating a house can cause minor leaking to become an issue. The solution is to address the leakage, which usually means reinstalling the affected doors and windows.
2. In any house in a heating climate, if the dew point of the interior air is higher than the outside temperature condensation can occur if interior air comes in contact with a cold surface. Generally, insulation and air sealing makes this less likely, but it also means there is less drying if it does happen.
The most important step is keeping the interior air away from cold surfaces, which means making sure the insulation and air sealing are continuous. Water molecules are smaller than air molecules, so a lot of substances that are good air barriers allow water vapor to pass -- in particular drywall, latex paint and plywood. In new construction you'd put some sort of vapor retarder inside the drywall. I agree with Akos that with cellulose insulation in zone 5, the accumulated layers of paint are probably sufficient.