When I read about spray foam under the roof and creating a sealed attic there is reference that the attic now becomes part of the conditioned space. If the attic now becomes part of the conditioned space, which I assume means it will be getting conditioned air (either heat or cold) via the many leaks in the ceiling, doesnt that mean the HVAC has to work to cool or heat the entire attic area? If you have a large house, particularly a single story with a large footprint, that attic could very well be a large area! What am I getting wrong in my analysis?

work to cool or heat the entire attic area... that attic could very well be a large area

That is correct.

Posted By jonr on 03 Sep 2011 09:03 AM

work to cool or heat the entire attic area... that attic could very well be a large area

That is correct.

So, if I am correct - how can spray foam be economical? I am going to be building a 5100 sq ft single sotry house in so cal. Summers see temps up to 110 degrees, so the attic gets hot. Obviously, a 5100 sq ft single story home has a very large attic area. Does the spray foam keep the attic SO much cooler that cooling (or heating)it is now not that much of a resource hog?

Posted By superduty on 03 Sep 2011 10:10 AM

So, if I am correct - how can spray foam be economical? 

It generally isn't economical because the spray foam costs so much more per sq ft than other options like blown cellulose.  The only benefit I see to spray foaming the roof line is if you have your HVAC equipment and air ducts in the attic. 

If you can have your HVAC equipment and ducts located inside the conditioned space or in the crawl space, then loose fill insulation over your ceilings will be more economical by far.  Combining that with a low emissivity roof decking such as Solarbord OSB or Thermostat plywood will further reduce your cooling costs.  The cost for the low emissivity roof decking is only fractionally more expensive than regular roof decking and the labor costs are the same for either.

Tell us more about the design of your home.  Slab on grade, crawl space, or basement?  If crawl space, do you intend to seal and insulate the crawl space?  What is your wall construction type?  What kind of insulation do you plan to install in your walls?  What type of exterior sheathing are you planning?  What type of exterior finish?

I previously discussed some stuff regarding this house over here  since that post, I have read alot more and learned (I hope).

The house location is in Zone 3b (Hot-dry).

The house will be slab on grade. 5100 sq feet + 4 car attached garage. The house is 10' interior walls, so the exterior walls will be considerable square footage.

My current plan for the walls is 2x6 with blown cellulose. Structural engineering requires 1/2" plywood on all exterior walls. I would like to tape the plywood seams. I would then like to add 1" of foam on top of the shear wall. The foam on such a large project may be cost prohibitive - not sure yet. I need to figure out the exterior wall square footage and how much the foam is per square foot. Do I need a house wrap if I use the foam and it is taped at the seams? The exterior finish will be siding. Debating between the Hardie or some other brand of engineered type wood??

THe house will likely utilize at least 2 HVAC units. The HVAC equipment will be going in the attic. Still trying to figure out if there is any way to put it somewhere else. It may be possible to put one HVAC unit in the garage. However, it is likely the ductwork will all be run in the attic. The interior design of the house does not lend itself well to soffits in different areas. So, I am trying to plan based on the HVAC and ductwork being located in the vented attic.

My roof will be standing seam. I think hard foam below the standing seam (as suggested by DANA1 in the other post) will be cost prohibitive for the gains? I was going to a radiant barrier plywood, but I think DANA1 says it is a waste of money if you have a cool roof. I hope to use a cool roof (assuming we can get a color of our liking).

Attic - I plan on cellulose in the attic above the ceiling. We will likely have recessed lighting in a variety of places (I know not recommended - but in a house like this - it is almost mandatory). I have read about spraying foam above the ceiling. Is that economical in my situation? Does it get sprayed right over the can lights or do you have build boxes for each one? Since the ductwork is in the unconditioned attic, I suppose R8 is recommended for the ductwork. I have also read about burying the ductwork. I thought Los Angeles Building and Safety requires the ductwork to be suspended in the attic, but I will double check on that.

I am looking for an overall plan. I am looking for the perfect balance of energy efficiency, cost effectiveness, and aesthetics. I want my cake and eat it too.

After doing some more research today.....I am rethinking what I said above about recessed lights. Seems they are bigger detriment to energy efficiency (in terms of air leakage) than I originally thought. It almost seems, why go through all the trouble if I am just gonna install recessed lights.

So I will be looking into some alternatives.

Note that most standing seam metal roofs are required to be placed over solid decking - not purlins as discussed in your previous thread. That is unless you are going with commercial grade heavy gauge metal which can span several feet unsupported, but then it costs $$$.

If you go with a certified cool roof, then you can avoid the added cost of the radiant barrier plywood decking.

With a vented attic, duct insulation should be as high R value as you can find and with an outer foil radiant barrier. If at all possible I would find a way to put the HVAC air handlers inside the conditioned space. Surely with 5000+ sq ft you can find two spots ~10 sq ft in size to put two vertical air handlers. It is easier to air seal and insulate the duct work than the air handler with removable panels most of which probably have no more than R3 to R4 of insulation if that. When the blower is not running, within several minutes the entire air handler assembly will be approximately the same temperature as your attic. Then when the air handler cycles on, it blows all of that hot air into the home.

I could probably get the air handler in the conditioned space somewhere.  But the ductwork will likely be in unconditioned.  Is it worth the effort to put the air handler in a conditioned space if the ductwork will be in unconditioned.  This will obviously mean longer duct runs.


I plan on putting the roofing over solid decking.

What are your thoughts on the rest of the "plan?"

I am constantly confused as to why people who have duct work located in the attic would not spray foam the roof line. I know that cost is always a factor but would you like to pay for this wasted energy every month or do it right and spray the roof and save money every month? People will go crazy and add extra insulation to the ceiling and bring it up to r-40 [in my area] but will leave the duct work up in the attic with only a r-6 or r-8 rating. When you factor in the leakage most units have at ductwork seams the heat or cooling lose is huge. My suggestion is to seal the whole envolope of the house and keep everything within that sealed envolope. You can save money on something else that can be easily upgraded late and seal the house the best you can. Remember it does count what is unseen in the walls and ceiling/roof as much if not more that the finish of the walls. Good luck with you new home.