8 Factors in Building Safe Double Stud Wall Cavities With Cellulose

On the Passive House in the Real World discussion board…

Expert instructor Mike Duclos discusses key factors to building double stud wall cavities (packed with Cellulose) safely

Student 1:

Hi Everyone, I’m excited to get started! I’m interested in looking into wall assembly details to compare and contrast the benefits of each. 

Mike Duclos:

In our area (MA) it is possible that double stud walls filled with dense packed cellulose can be done safely, given some key factors be kept under close control. Among them:

1. Plywood rather than OSB sheathing: The more vapor-open the sheathing, the more forgiving of higher Wood Moisture Equivalent (WME). WME is an indication of how ‘wet’ the material has become.
2. Airtight layer (e.g. membrane over sheathing): Use a sheathing that is vapor-open to the outside. This can also serve as a drainage plane.
3. Vapor-open wall assembly to the inside: (E.g. latex paint, not oil based, as this is too vapor-closed for good inward drying) Try a 4 Back ventilated rain screen. This is good for the siding and good for vapor migration from the cavity, through the sheathing, to the outside.
4. Solid control over winter RH (relative humidity): Prioritize quality point source extraction ventilation design and execution and sufficient hygienic air exchanging.
5. Airtight assemblies: Minimize air transported moisture (there should be no more than 1.0 ACH 50 for typical homes!)
6. High quality Dense Packing of cellulose: This will eliminate voids and the possibility of settling.
7. Use a rigid six-sided container for that DP cellulose 
8. No contact of cellulose: Cellulose in the walls should not make contact with cold surfaces on which condensation can easily occur and accumulate moisture over a winter. This can happen on the bottom plate area where there is concrete between the double stud walls.

This is not exhaustive- there are other factors to consider. Still, it’s important to pay attention to all of the above, as all are requirements for a safe double stud wall cavity packed with cellulose. Poor performance on only one of those items can cause problems, so taking good care of all of them is important. 

I prefer to install cellulose behind netting so I can verify the density by pushing on it with my hand. When it resists the way an extra firm mattress would, the cellulose is at about 3.5 to 4 lbs of material per cubic foot. At this density, it will not settle.

However, if you leave the netting to retain the cellulose over a long period of time, it will stretch and the cellulose can move down under the force of gravity, aided by slight building movements caused by the wind and vibrations in the house (think high speed spin cycles of a clothes washer, etc.) So some rigid material (often gyp) should be used, and it should be in contact with the netting to prevent stretching.

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About instructor Mike Duclos– Principal, DEAP Energy Group, LLC

Mike Duclos is a principal and founder of The DEAP Energy Group, LLC, a consultancy providing a wide variety of Deep Energy Retrofit, Zero Net Energy and Passive House related consulting services. Mike was an energy consultant on the Transformations, Inc. Zero Energy Challenge entry, and has worked on a variety of Zero Net Energy, DER and Passive House projects, including two National Grid DER projects which qualified for the ACI Thousand Homes Challenge, Option B, and a feasibility study of a retrofit to the Passive House new home performance standard. Mike is a HERS Rater with Mass. New Homes With ENERGY STAR program, a Building Science Certified Infrared Thermographer, a Certified Passive House Consultant who certified the second Passive House in Massachusetts, holds a BS in Electrical Engineering from UMass Lowell, and two patents.

Continue learning about Passive House:

• Free Lecture: Passive vs. Conventional Floor Planning
• Passive House in the Real World
• Passive House Design
• Building Energy Analytics: Measuring and Understanding Building Energy Use