Tips

Screen Ceilings

I recently visited 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. 

Please, always demand 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.  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. 

By the way, any time you specify a system that creates an airspace between the conditioned side of a building cavity and a piece of fiberglass insulation, you are begging for high heat bills, condensation issues, probably mold, and likely ice dams.   What is the spec for those problems?  I have seen some framing systems where trusses are set 2 ft on center, and furring is run perpendicular to the trusses 16 inches or 1 ft on center.  The spec will call for insulation, and the contractor installs fiberglass between, and parallel with, the trusses.  This leaves an air space between the ceiling material and the fiberglass that is the thickness of the furring.  When the building heats, the sheetrock gets hot.  It heats the air in the cavity between the insulation and the rock.  That air is not retained by any air barrier, so it rises into the attic.  This allows attic air that is cold to drop down against the rock.  That circulation is called a convective loop.  Convection is a very efficient method of transporting heat (think hot air furnace systems).  High bills, ice, etc.  This problem is also caused by ‘tenting’ fiberglass insulation.  We use infra red on a weekly, and sometimes daily, basis in the winter.  I have thought there was no insulation in a spot in an attic several times only to find there is insulation there when I get to the attic.  The problem is the batt in that location was suspended off the sheetrock by a framing piece like a wall nailer or wire or pipe.  A secondary tenting problem is occasionally insulation that is thinner than the bottom cord or collar tie in an attic is run parallel with a truss, then a second layer of insulation is run perpendicular to the truss.  This creates an airspace between the two layers of insulation; the top layer is not doing anything.  It is simply too easy for air to get heated and get momentum enough to travel through the second layer.  When we see this, we have to remove the top layer and use blown insulation as top fill.  That leaves no airspace in the middle of the insulation, and it makes for a lot of happy homeowners. 

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