Just completed an audit and manual j for a home in downtown Chicago this week.  Home is less than 1 year. 5 stories include basement.  Block walls with brick over.  Closed cell entire wall bays.  2 ACs of 3.5 tons each. 1 in basement for 3 zones and one on 2nd floor with 2 zones.  HRV with each system.  All Carrier Infinity equip.  Man j calls for 30,704 Btu for total AC. Blower Door was at 2.5 ACH50.  They have experienced mold in the window cavities & builder is in process of taking out drywall casements and re-air sealing.  W/ BD running, we saw small amts of air.  The mold had already been removed so we did not see the mold. The house is 6836 sf with some ceilings at 10 some at 9.  1.3 mil $ home. Home owner is asking me what to do and when I point to the AC being over sized, they will not hear of it.  Owner is doctor and wants it cooler and Rh of 50+.   

I am struggling with the best approach to avoid mold issues again.  I have advised that ventilation is the key but they have the HRVs.  They say they rarely run bath exhaust fans. The contractor did a great job of tight foaming but there was a small amount of air.

Looking for questions or suggestions.

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Jerry, Your second line may be the key. "Brick over block" I think you may be experiencing vapor pressure drive thru the wall assembly. The oversize AC acts as a giant vacuum drawing the Heat and moisture to it thru the walls. Mold forming on the jambs of windows indicates lateral flow from the outside to interior. Check construction documents for vapor barrier also under floor. Drop the AC size immediately to right size or that doctor could get a real life encounter with "Legionaires Disease."

Note: closed cell foam location is inside of block between stud bays, is this correct because that should have sealed the walls. Unless application at window openings was blocked by king stud and trimmers.

I'm curious if the humidity problems are winter or summer.  Is the mold forming on the back of the drywall or on the inside?  With 2 HRVs, I wouldn't expect they have high humidity in the winter.  But they might not be using or maintaining them properly.

ACH 2.5 isn't that tight.  Spray foam isn't perfect as we can see in the photo.  Also, the jambs extend outside of the foam.  So, if there is reverse vapour drive (as Dave Taylor suggests) the vapour barrier is less than perfect in more ways than one.

I would suggest that the contractor use two component foam between the jamb and block - if there's room and if they can inject it without blowing out the drywall. 

AC sizing and design is always a compromise.  The homeowner in this case is stuck on numbers.  

What did the original HVAC design contractor say about their Manual J and D?  Why so oversized from their point?

Pull the ASHRAE Environmental Design Criteria for this area.  Sit down with the homeowner and the contractor and share these Design Criteria, both heating and cooling,  So him the huge number of environmental considerations for Cooling compared to the heating,  Ask him to choose which ones your Manual J should consider and their weight,  Then which ones to ignore.  Recalculate.  It will still be oversized.   I run into lots of Aeronautical Engineers in this area.  They dwell in the details. So is this homeowner,  Like them he doesn't have much idea of the design criteria, he is talking about the result.  Comfort is what he wants, Comfort is not numbers like 50% or 55% RH.  You know that, so let him figure out what we already know,

Oversized A/C systems wont pull out enough moisture from the conditioned air. I would check to make sure the systems are set to the right fan speed. Make sure the filters are clean and appropriate for the filter rack. No merv 12 in a 1inch filter cavity.  Or slow them down a bit to increase the air exchange over the coil (not so much to freeze, aim for 400cfm per ton of cooling btu). This is a common comfort issue I see. I would suggest a whole house dehumidifier and humidifier if they won't change out the system. I would also suggest humidity sensor switches in the bathrooms that way they dont have to think about turning on bath fan switches. The occupants most likely create the moisture issue. How is the Attic space or ceiling insulated/air sealed?? 5 stories is a perfect recipe for stack effect bringing moist outdoor air in the bottom floors and allowing it to escape through the attic space. You can always run a reference hose for a manometer to the top floor and stand on the bottom floor to see if there is a pressure difference indicating a stack issue. If so seal bottom first then top. Sounds like the owner needs some education on some common comfort misconceptions. 

New build may not have been "dry" before finished and occupied. Residual moisture from materials may have been "stranded" leading to localized high moisture areas causing problems.

Need to add dehumidifier for cooling season if homeowner wants tight and precise control of the relative humidity. Chicago does experience periods of high outdoor dew points throughout the summer which are not coincident with peak sensible cooling loads. High dew points can linger for days while the diurnal solar sensible load cycles from night to day. Even a "right" sized AC is oversized when the sensible cooling load drops while the latent load remains high. A dehumidifier will handle the latent load independently of the sensible load. Recommend a ducted whole house dehumidifier.

Mold in windows could be a wintertime issue in Chicago if indoor dew point is high. Increasing ventilation and slight depressurization (if no open combustion appliances) in the winter can help control indoor moisture and reduce condensation potential.

Jerry, you should look at Buildingscience.com" High Mass Walls" 105 paper for more documentation for your owner.

Not an efficient solution but he would benefit from a whole home dehumidifier ducted into the system.

Do 1 on each system.

The heat it rejects as it dehumidifies will be an added load but the net effect is controlling humidity.

Sizing it for pints is yet another question


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