[Greenbuilding] spray foam/fiberglas

Bob Klahn Home-NRG at dnaco.net
Tue May 10 16:19:12 PDT 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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