Screen Ceilings

I was asked this week to add cellulose insulation to the attic floor of a building that was built in the early 1990’s.  It is a block wall structure about 6000 sf with a truss roof.  The architect specified trusses with R-38 batt insulation in the truss.  He has several mechanical appliances in the attic space.  He specified a dry sprinkler system and a drop tile ceiling grid. 

The building has massive ice damming problems and high heating bills ($ 7000 for oil heat in the 2004-2005 heating season).  Why? 

The ceiling is a screen.  The attic is vented.  That creates a low pressure system in the attic space.  The drop tile ceiling grid allows air to travel from the heated space into the attic.  The attic ventilation accelerates the lift of heated air.  Ice dams and high heat bills are the result.  If I add cellulose over the batts, I may help the building retain more heat, but not much more. AND, I run the risk of making the roof deck colder. -cold enough to begin forming frost in December or January, and accumulating it until March.  Then we get a sunny day and it rains inside for a few hours.  (not good for business)

 Please, always specify a ceiling that is going to stop air from traveling from the heated spaces into the attic.  Fiberglass does not stop airflow.  Sheetrock, polyethylene that is taped and caulked at all seams, rigid foam or spray foam, are good air barriers, but fiberglass is not, and never was supposed to be.  This is a landmine you should avoid.  It is most commonly found in commercial applications like church halls, convenience stores, banks, and post offices.  I have seen buildings built with insulated concrete forms for the exterior walls, and a screen ceiling.  What a waste of money.  This type of ceiling should be specified with a warning that utilizing this spec will be hazardous to your financial health for as long as you are in the building. 

Secondly, if you are going to have a plenum between a truss bottom cord and a drop ceiling tile, then put the ducts there, not in the attic.  Why put air conditioning ducts in the hottest part of the building?  Especially when there is a space for them that is in the conditioned area? 

Another frequent abuser of this rule is tongue and groove wood finishes on ceilings.  Don’t do that unless you specify some sort of air barrier between the wood finish and the rafter or truss cavities.  The tongue & groove acts as a decorative screen, and air will travel rapidly through the fiberglass to the roof deck, high heat bills, ice damming, and occasionally condensate leaks on the floor inside are the result.  Preventing this problem is simple; specify urethane foam to insulate these ceilings and the air barrier is the foam.  You don’t need any membrane, and you don’t have to worry that some carpenter or electrician will shred your air barrier.