Caution: (1) One can of worms about to be opened…
I’ve contracted to build a simple stucco over block single story “developer” house in Tampa. The developer doesn’t much care for upgrades aside from countertops and tile. I’m interested in a couple things that would be easy to do (if they’ll let me) and go much further toward my enjoyment & the energy efficiency of the place.
The house will have a trussed roof & a 12 seer heat pump system, all ducted in the attic space. If I can “wangle” it – I’m going to have “attic trusses” put on in lieu of the typical truss attic – a tangle of chords and bracing.
Afterward, my plan is to keep the miniscule R19 insulation they provide (laid on the ceiling sheetrock) for sound control, then spray the underside of the roof sheathing with icynene. I’ll plug the attic vents & have a conditioned attic. I’m speculating that the wimpy R6 duct insulation will leak enough to make the attic a quite enjoyable space.
My concern is this: the mechanical system is sized for the hot attic inefficiency. When I spray the icynene I want to spray the proper amount, to find a balance between thermal efficiency and NOT making the mechanical system my worst nightmare. [I’m afraid that, despite the added space in the attic that is being cooled, the result could be an oversized ac system – due only to the efficiency of the insulation – and thus the system may fail to run long enough to dehimidify the house properly]. The result could be mold, and that concerns me a great deal.
Does anyone work in this region with an engineer or mechical subcontractor that I could speak with to apply some science to the project – Ideally somebody I could hire to offer up some suggestions or better yet calculations to determine whether this will work?
Comments welcome.
PS: My grand plan is to have enough room for a pool table in the attic, and I’d add windows in the gable ends or possibly some of those Velux “step through” windows for egress…
Replies
My guess is that your builder is putting in an el cheapo HVAC unit, and probably isn't properly sealing the walls of your house. So I doubt adding foam to your roof + adding more living space is going to DECREASE your load. And I also don't think "passively" cooling that attic space is going to cut it. And not to be totally negative, but that r19 fiberglass isn't going to do anything at all for sound transmission. You might consider leaving it off all together and just take care of the foam immediately after closing.
Just some thoughts.
MERC
interesting comments - thanks for weighting in.
Just to clairfy: The AC unit is 12 seer, so it's not total crap. The walls are gyp board over rigid insulation inside, on 8" block, cement stucco outside. My gut says they'll seal pretty well. The ceiling will be sheetrock with R19 batts. Seems to me that's pretty similar to what's commonly recommended by the FG manufacturers for sound attenuation? Not trying to argue; but I'm interested in why you think otherwise?
I'm no expert on this by any means. It's my understanding that fiberglass insulation is pretty far down on the list of things to attenuate sound. I'm sure some others will chime in and give us some schooling. I'm pretty sure that split stud/joists constuction is a big gainer, as is double sheetrock. Maybe insulation comes next. But insulation alone won't make a significant difference.
I'd be wary of information from the FG manufacturers recommending their product. Of course they will. They might suggest it for dinner if they thought we would eat it!
MERC.
First off, sorry I did not see this post sooner. There is a reputable HVAC contractor in the Tampa area, CGM Mechanical. They also, apparently, can offer engineering solutions for the type of problem that you describe. I apologize for not being able to provide their phone number but it's been several years since I spoke with them. They are in the yellow pages.
Best Regards,
Neil Wilhelm
Thanks for the name!
All I have to do is keep an eye on the developer's hvac sub, and then brainstorm this thign through.... If the trusses cannot be changed, the whole thing will be a moot point anyway. Always interesting to kick ideas around though.
You might want to get the HO version of the heat gain/loss calculator from http://www.hvac-calc.com and see how much your changes affect the loads.
Thanks - that's a good idea.
My main thought is that R6 duct insulation is so minimal that most of the cooling must go straight to the atmosphere (read: attic) rather than into the house. Insulating the outer envelope would capture all that lost cooling - hopefully not to the detriment of the much needed humidity removal.
Based on what you are wanting to do I assume that you are familar with these resources, but if not I will point them out.
http://www.buildingscience.com
Has lots of recomendations for doign exactly what you are talking about, conditioned attic space for hot climates. You can do a search on conditioned attics and also look for the Houses That Work for your climate.
JLC has an article about 1 -2 years ago on roofing and insulation in hot climates by some one from Florida Solar Institute and I think that the data is online, but probably not all packaged like it was in the JLC artical.
You can buy the JLC article for $5 (jlconline.com then Research)
Don't think that article went into conditioned attics, but talked about different types of roofing material, reflectiivity, and the affects of conventional attic insulation.
This will be your residence? You're paying the bills and whatever necessary for upgrades?
It's not that big of a deal to change truss engineering and run it by the engineer of record for the house. You will end up with a steeper pitch to accomodate the bonus room (attic) trusses. Maybe even a gambrel.
Unless you have a certain type of roof/ceiling arrangement where R19 is allowed, as when the rafters are also the ceiling joists, your ceiling batts or final insulation value would normally be a minimum of R-30. You will also have to run the rest of your insulation ideas through the engineer of record. All the energy calculations will have to be submitted along with the building permit. I'll bet they're going to require venting and airflow directly under the roof sheathing so your spray idea may not please the building department officials.
Not really up on the HVAC end but going to the vendor recommended above would be a good move. Certain newer developments, such as variable speed compressors and improved airhandlers, sized for your particular needs, will cost more initially but will pay off in the long run.
Hi Ralph,
Yes, I've contracted to build this one. Not my dream home, but a stepping stone. It's already an 8 pitch gable roof, about 40' wide so the headroom will be available w/o changes that'd make the developer nervous.
I'm going to check the FL residential code regarding the R19 - that's what the "specs" say they build with. (What they call specs are laughable, frankly - so I'm a little suspect).
Icynene's website posted a new announcement recently - the ICC's acceptance of spray on the underside of the roof sheathing. Check it out: Link to Icynene site