[Greenbuilding] spray foam/fiberglas
Bob Klahn
Home-NRG at dnaco.net
Tue May 10 18:19:12 CDT 2011
Tom,
2 cents worth:
The usual rational for limiting the thickness of spray foam is based on
a payback calculation. Foam is expensive; more is more so. The first
half inch or so is usually enough to achieve an adequate air seal -
assuming that they covered the critical joints.
It would help for you to know the length of time and energy costs that
the insulation company used in their calculation. I would guess that
it's based on current energy costs and the "average" time before a
homeowner sells. The turnover rate is still wildly skewed by the
collapse of the housing market; utility costs are very likely to keep
climbing, unless you are going off the grid.
If you are going to stay there a while, or if effective insulation/lower
utility bills are a perceived added value in you area.
It is more cost effective to add insulation now, assuming that the
ceiling hasn't been installed yet, because you have access at no
additional cost. Any insulation will work the fill the remaining space.
Batt material, usually fiberglass, is easiest but must be carefully fit
to fill the space without gaps or compression. (You can actually
compress it slightly but it's inadvisable to add downward pressure on
the ceiling.) Other materials are available as batted insulation in
some areas.
If the ceiling is up, a skilled installer can still blow cellulose, or
special fiberglass, in through relatively small, easily patched holes -
at least one per cavity. It should be lightly "dense packed" so it
doesn't settle and shift over time.
I hope this helps.
Bob Klahn
On 5/10/2011 4:07 PM, tom at honeychrome.com wrote:
> Asked this once before but didn't get any answers, so I'll try again...
> Had spray urethane insulation installed in the cathederal ceiling of a
> small house being renovated (upstate NY). The rafters were only 2x6
> and the installers sprayed in 4", claiming that anything more wouldn't
> be of benefit. So, there will be about 1.5" of space left between the
> urethane and the ceiling. I'm wondering if it is worth filling that
> space with insulation (fiberglas or rockwool) or is that possibly
> asking for moisture problems? In fact the house is pretty tight and
> pretty easy to heat, but I'm about to install the ceiling paneling and
> wonder if I might regret not filling all the space with insulation
> later...
> Thanks
>
>
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