Attic Insulation Question
Geothermal a/c with air handlers in the attic. Metal roof with ridge vents. system is struggling to produce adequate a/c.
HVAC contractor believes the attic insulation is insufficient (system designed assuming R-38 on attic floor but R-30 was installed in error).
Builder is offering to put rigid insulation on attic floor and recover with plywood (so it would be R-30 fiberglass that is in the attic floor now covered with plywood, a sheet of rigid insultation, then another sheet of plywood). the reason they dont want to pull up the existing plywood and install R-38 is because they are worried about the stress this might put on the plaster ceilings below.
Q: would you do the above OR would you just get the spaces in the roof rafters sprayed with icenyne to get the attic temperatures down?
The heat radiating at your air handler and insulated duct work could be robbing you of 10% of the cooling and is a lot more significant than R30 vs R38. Maybe ask if an allownace was made for the heat picked up by installing HVAC equipment in a hot solar collector.
You need a complete load calcualtion done and you need to make sure that the attic duct work is not leaking. The HVAC guy should be verifying the capacity of the equipment as installed. Measuring the room air temperature/humidity entering grilles, the temp humidity of the air entering the unit, leaving the unit and coming out of the supply diffusers needs to be looked at.
Icynene is great in the right climate but is meant to work with a sealed attic
Would you pls explain what you mean by "hot solar collector"? are you speaking about my metal roof?
Your attic gets hotter than the outside air temperature when the sun beats down on it.
It is a solar collector. People do not like to give up floor space so they install equipment in a hot humid attic, then get upset later if it does what it naturally does-- sweats.
My own home I have a white metal roof, insulation beneath it,the attic is sealed, it works great. Before I started up my AC, during the heat of day the attic was about 1 degree hotter than the outside air.
Maybe your attic heats up to 130F or more. The metal roofing will be radiating heat directly at the equipment installed in the attic. Attics are a piss poor place to install HVAC equipment, but just a fact of life as people want high ceilings and do not want to give up floor space for essential mechanical systems.
Your best option short of those more drastic measures you mentioned (which may not give much bang for the buck) is to have radiant barrier installed underneath the roof decking. Spray-on or foil type, take your pick. Foil type performs better but is more difficult to install. Spray-on highly effective, will reduce heat gain to ducts in attic, and a/c equipment in attic. Make the attic cooler, the house and the a/c equipment cooling the house will be cooler.
Safe bet that none of the supply boots penetrating the ceiling from the attic are sealed where the boot and plaster meet. Safe bet some aspects of your ductwork are leaky. These are typical things so often overlooked in most new and remodel construction.
I would be concerned that the rigid board insulation would create a vapor barrier trapping moisture between the ceiling and the existing insulation.
What is your local code about fire protection? Foam boards usually require a covering as a fire barrier, like gypsum wallboard. Some building codes do not require an additional fire barrier for certain metal-faced, laminated foam products.
In your jurisdiction single family dwelling units require fire rated ceilings? Do the wood trusses require to layers of type X?
Originally Posted by jax1
With a heatload calculation it would be very easy to to compare the change in heat gain from R38 to R30.
No, but there is a concern about toxic gases released when some foam burns.
Originally Posted by Carnak
It may or may not apply to the OP, but it's free to ask.
Also, if you compress batted or blown-in insulation you lose R-value.
"Builder is offering to put rigid insulation on attic floor and recover with plywood (so it would be R-30 fiberglass that is in the attic floor now covered with plywood, a sheet of rigid insultation, then another sheet of plywood). the reason they dont want to pull up the existing plywood and install R-38 is because they are worried about the stress this might put on the plaster ceilings below."
Certainly sounds like a whole lot of work!!
Where are you located?
The cure of the part should not be attempted without the cure of the whole. ~Plato
Why not have Icynene sprayed below the roof deck between the truss system. They would need to seal off all your attic ventilation but it would the best way to solve your problem. The only other issue to that is the heat pumps would need to have fresh air ducts ran to them if they haven't had that done already.
jax1 is right !!!!!!!!!!!!!
That was the first thing that came to mind, ,,,,disaster.... especially in humid climates...
You could have someone spray a couple inches of closed cell foam on the roof deck ..... would help a bunch....
But depending your budget.... It is always a slam dunk to seal & condition any attic.... 4.5" Closed Cell R-25 or for for a little less 8" Open Cell Foam (Icynene)
NOTE "Icynene" is a name brand... from Canada it is a Open Cell 1/2 lb....
And it is over priced $$$$$ !!!
Most all PSF manufactures sales a 1/2 # product...
Foamedix, NCFI, Demalack...... Even a couple of the fiberglass guys are make spray foam now
As mention several times... find and PAY a "HERS Rater" to do a comparisons of what you have.... to a "Seal Conditioned Attic" And have a Manual J done as well
REMRATE (HERS Software) will tell you your Load Calc. perimeters.
We spray both foams and do HVAC and have several hundred sealed conditioned attic.... all are very satisfied customers !!!!