Energy upgrades and utility programs are all the rage, and rightfully so.  With energy prices soaring and global warming on the rise it only makes sense to try to make our homes more energy efficient.  Not only is it good for the environment but its good for the wallet as well!  Most programs in our area rely heavily on dense pack cellulose insulation as their go-to insulation material.  It’s inexpensive, ecologically friendly and effective- when installed correctly.  Even in new construction, cellulose is widely used. But beware- the recommendations given by utility companies are not always the best or safest for your home.

Firstly, what is cellulose? Cellulose is recycled newspaper that is treated with boric acid. Boric acid acts as a pest repellent and a fire retardant.  Unlike blown or batted fiberglass, mice and other rodents will most likely not nest in the cellulose due to the addition of this product. Because cellulose is made of paper it has 2 downfalls: 1- it will allow air to pass through it and 2- it is very likely to absorb moisture.

Airflow through cellulose is normally curtailed by the use of sheet rock and air sealing of penetrations.  The exterior walls and ceilings of homes act as a natural air barrier allowing the cellulose to do its job as an insulator. Walls are nearly impossible to build without penetrations- it would be difficult to live without light switches and outlets!  However, ceiling penetrations are completely optional and should be avoided at all costs.  Air sealing penetrations is difficult and time consuming.  Minimizing penetrations is the most energy efficient way to build.  The addition of track lighting and other light sources in cathedral ceilings is a much more effective method.

Moisture is the single largest source of building failures.  Keeping the house tight and dry is the number 1 goal of any good designer or builder.  Because cellulose will allow moisture to pass through it is imperative that a vapor retarder is used. Warm, moist air rises inside a home and vapor drive and air pressure along with that air make vapor retarders in sloped and flat ceilings a necessity.  In new construction a vapor retarder is required over any air permeable insulation and is required by code. In a retrofit (insulating an already existing and occupied home) properly installing a vapor retarder is nearly impossible in walls but because moisture is not as much of an issue in walls the insulation benefits greatly outweigh the possible problems.  This benefit/problem ratio is not the same for sloped and flat ceilings however!  Moisture is a much greater concern in these higher area where moisture tends to accumulate and where pressure is the greatest.  If a vapor retarder is not used, moisture will pass through the cellulose and in cold weather will condense on the roof sheathing, causing mold and eventually rot.  Here is an example of this:

 

 

 

Moreover, dense packing a sloped or cathedral ceiling can cut off existing ventilation to kneewall and attic spaces which violates building code. The only way to effectively dense pack these slopes is to install a minimum of 3 inches of rigid foam board insulation on the exterior of the roof (this will prevent condensation by increasing the temperature of the roof sheathing) or spraying 3 inches of closed cell spray foam on the underside of the roof deck before dense packing.  This second option is more invasive as it would require the sheet rock to be removed in order to access the slopes.  In new construction this is a widely used and easy insulation method.

 

 

 

 

 

 

 

 

 

 

In conclusion, dense packing existing exterior walls in an effective method to increase your homes efficiency but cathedral slopes and flat roofs should NOT be dense packed without first installing a proper vapor retarder.  Ask an insulation professional, building science professional or HERS rater before deciding on an insulation material and method.  What you do now will greatly effect the performance, comfort and safety of your home for years to come.

For more detailed information and for other great building science articles please visit https://buildingscience.com/documents/insights/bsi-043-dont-be-dense.  Photo Credits: Buildingscience.com