[Greenbuilding] [BULK] Recessed Cans in exterior ceiling - No?!

[Lawrence Lile]

>I have clients building a house in Scranton, PA who are being told by
their
electrician that they should install recessed cans in an exterior
cathedral
ceiling, which I've always heard is a no-go.  

It is not forbidden, but it does puncture the sheetrock and therefore
one of the major air barriers in the house.  It is not recommended for
maximum energy efficiency.  If this is a vented attic, then there isn't
anything besides the sheetrock that keeps air from puffing through the
insulation.  If it is a non-vented attic, then that is a better
situation. 



>I had layed out a mix of
linear fluorescent uplight and conventional PAR fixture downlights
(hoping
they'll switch to CFL) which our lighting consultant poo-poo'd for the
quality of light emitted (I can't wait to hear all of the debate that
this
thread spawns).  

Fluorescent can make nice light, and indirect can make nice light too.
My house is almost 100% indirect, and the thing just glows inside.  I
found that the "daylight" color temperatures, 5000K plus, didn't suit
me, and I've tended to like the "soft white" color temperatures, more
like 3000K.  These aren't as common in Fluorescents as the 4100K "Cool
White" 


>Anyway, back to the recessed cans, I'm trying to justify in
numbers why it doesn't make sense to install them.  The units proposed
are
IC and air-sealed Juno fixtures going into a polyurethane spray foamed
2x10
ceiling.  The R-value of the ceiling without lights is R-50 and I'm
imagining the portion over the lights will be about R-10 (1-3/4" thick
of
R-6/inch insulation).  I'm going to make the (probably incorrect)
assumption
that the air-sealed fixture is just that and that there won't be
significant
heat loss to air leakage around the light.  I'm calculating the area of
each
light fixture to be approximately 0.42 sqft so that 9 fixtures will be
3.78
sqft (The whole ceiling for the Living Room where they'll be installed
I've
calculated as 320 sqft).  Taking the 3.78 sqft, dividing it by the
U-Factor
of 0.1, multiplying that by the HDD/ per year of 6150 and multiplying by
24
hours, I'm getting 55,793 BTUs for a years use.  Sounds like a huge
amount
of BTUs except until I multiply that by the cost per MBTU of propane and
I
get a measly $1.19/yr - for all of the lights.  


It's not the insulation, but the gaps.  The fixture may be "sealed" but
there is a gap around the fixture where it meets the sheetrock.  This
gap allows more infiltration than a non-recessed light.  The
infiltration hurts in two ways - directly, by letting expensive air out
of the house, and for moisture purposes, by letting moisture laden
indoor air into the insulation, where it can condense and wet the
insulation over time.  A wet blanket isn't good insulation, obviously.  

If you can inspect the lights and find out if they are really "sealed"
(I am skeptical, they may just be "insulation contact rated" but still
have gaps), and then convince someone to caulk the gap around the light
before the final trim goes on, then there is a ghost of a chance this
will be efficient.  

I caulked around all of the electrical boxes in all of the perimeter
walls in my house.  

--Lawrence