[Greenbuilding] Exist insulation in ceiling

[Lisa C. Rigazio]

Hi there!

Your question is one I have wondered about myself in similar, although specifically different situations. Basically the idea of how to change an existing non conditioned attic space into a conditioned attic and the possible repercussions. 

Can you please explain this approach further: "My approach would be to create ventilation above the insulation to allow vapor to continue to
vent to the highest point."

Thanks,
Lisa

Lisa Rigazio AIA 
rigazio designs, llc  |  www.rigaziodesigns.com
240-350-0532  |  lisa at rigaziodesigns.com






> On Oct 28, 2015, at 7:52 PM, conservationarchitect at rockbridge.net wrote:
> 
> I have a client that has renovated a 1940's house by gutting the plaster and
> installed close cell foam and fiberglass (flash and batt) against the
> siding.  This is not what my choice would have been, but I was not involved
> in that decision.  However, I expect that part of the thermal envelope to be
> tight.  They are now living there.  They had the floor above the
> basement-crawlspace and the ceiling above the second floor insulated with
> fiberglass.  The ducts and air handling units are in the attic.  The roof is
> metal with hips and roofing boards with gaps.  Since I expect the ceiling
> with duct penetrations and fiberglass to be leaky, I am recommending foaming
> the rafter spaces to bring the attic into the conditioned thermal envelope.
> This will capture the likely heat losses (winter) from leaky ceiling and
> ductwork.  However, I was told by an insulation contractor that leaving the
> old fiberglass could create humidity problem.  He did not know why.
> However, he had that experience.  I wonder if his experience was closed cell
> foam that trapped the house humidity, therefore raising the dew point.
> Without expressly heating the attic, it could drop below the higher dew
> point from elevated humidity.  However, if we use a vapor permeable foam
> such as open cell foam, the vapor would not be trapped.  My approach would
> be to create ventilation above the insulation to allow vapor to continue to
> vent to the highest point.   I expect that incidental leakage for the ducts
> and the ceiling insulated with common practice fiberglass, would deliver
> heat.  However, expressly providing conditioned air supply and return would
> provide assurance of avoiding a problem.  They did a pretty good job
> installing the insulation for that kind of system.  Since we do not need to
> heat the attic to comfort, could that be an intermediate zone that helps
> insulate the main living areas by leaving the ceiling insulation.  Or is
> there a reason that we must remove the newly installed fiberglass batts in
> the attic floor.
> 
> Hope you all are still out there.
> 
> Eli 
> 
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