Home Improvement--General/exterior walls sweating


winter SW side of house
winter SW side of hous  
I live in a northern climate and have a 2 story 2x6 standard frame construction home with fibreglass insulation, plastic vapour barrier on inside and 7/16 sheathing, house wrap and vinyl siding on the outside.  While fixing a problem with a faulty downspout, I discovered rotted sheathing apparently centred and spreading from the mid point of the house around the rim joist and header of the second story floor.  I noticed these areas were insulated but had no vapour barrier.  assuming this was the problem I removed all siding and replace rotten sheathing after removing the header and rim joist in sections, installing 1.5" rigid insulation and sealing it so I had a vapour seal in this area.  I then wrapped the house in 1.5" rigid styro.  The house is now ready to re-side but on the SW facing and west facing wall I started to notice moisture running down the outside of the newly installed styro and inside the new house wrap.  I'm at my wits end. forced air furnace and pellet stove for heat.   Interior RH is max 35%. 10 ft high ceilings.  well ventilated attic with continuous soffit and ridge vent

After all that work you've done, I can imagine that you're at wit's end.

Ok, lets look at a few different elements.  The moisture you are seeing is most likely caused by exterior elements, not interior.  The West and Southwest portions of the home see the sun in the afternoon and late afternoon.  So, condensation in these location is not surprising.  Those sides of the home stay cool during the majority of the day.  Because of this, they are the likely areas where condensation occurs from the natural moisture in the air.  This explains the west side as the eaves serve to shade that wall even longer; keeping it cooler.

If this was the result of interior conditions (missing insulation or lack of vapor barrier) - you would see condensation on the interior of the home, not the exterior.  Think of a glass of ice tea out in the summer heat.  Condensation occurs on the "warm side" of the glass; not the "cool side" where the ice drink is.  Same for your home - condensation would be occurring on the warm side (inside) your home.

From the photograph it's hard to tell - but something else to check.  Are there any exhaust fans in the areas where you see condensation?  Dryer vent - bathroom exhaust fan - etc?  These would pump quite a bit of condensation into the air immediately around them and can quickly lead to the condensation you observed.

Lastly -- is there a porch on the front of this home?  What you described sounds more like air entering a porch roof and traveling between the floors of the home.  Moving air passes through insulation so the fact that these areas were insulated would not stop the air from passing through.  This outdoor cold air passing through the floors would keep the band joists cooler; leading to condensation; leading to the rotting you observed.

If there is a porch roof on the front of the house (the side hidden in the picture), you should air seal where the porch roof and framing members between 1st and 2nd floor meet.

I hope this was helpful!  Best of luck with this frustrating issue.  

Home Improvement--General

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Anthony Procaccini


Residential Construction questions - specifically diagnosing an issue or concern in the home. My main area of expertise is Home Energy Efficiency and reducing your utility bills or increasing the comfort of your home.


I was a home builder for over 5 years. I have been doing energy auditing and analysis for the past 4 years.

Building Performance Institute, United States Green Building Council - NC Chapter

My blog - bsiofnc.blogspot.com

I am a Building Performance Institute Certified Building Analyst, Certified Manufactured Home Specialist, Certified Envelope Professional, and a Certified Quality Control Inspector

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