James Fincher is a builder in Oklahoma who’s leaning toward designs with conditioned attics insulated with spray polyurethane foam.
However, he’s not convinced that a conditioned attic is the best approach in a large home — something, say, in the 4,000 sq. ft. to 5,000 sq. ft. range.
The problem, as he puts it in his Q&A post, is the “sheer volume” of attics in a house this large, and whether the increase in volume will force him to use a bigger HVAC system.
The exchange that follows delves into the merits of conditioned attics — that is, those that are heated and cooled just like the rest of the house — versus unheated attics separated from the rest of the house by a layer of insulation on the attic floor.
That’s the subject of this week’s Q&A Spotlight.
First, reconsider the size of the house
Although the size of the houses that Fincher builds is not directly related to his question, some green building advocates may still wince. Brett Moyer is one of them.
“Since this a green building forum, I think I should say a couple of things,” Moyer writes. “You certainly have the right to build these ridiculously large monstrosities. You certainly have the right to install spray foams and foam sheathings, and place HVAC and ductwork in the attic.
I just hope you aren’t promoting these excessive dwellings as ‘green’ homes, because they are CERTAINLY not green.”
True, reducing the square footage of new houses is one way of reducing energy use while consuming fewer natural resources, both fundamental green-building objectives.
“Build a smaller, better, well detailed house that has more amenities and lower operating costs for those that live there,” suggests Corian Johnston. “…A well done small house can still be worth as much as a larger one, so you could have the same profit margin with a much better product. Sort of a quality-over-quantity approach.”
But, as Fincher points out, homebuyers often want big houses, not little ones, leaving builders stuck between the realities of the marketplace and their own goals.
“I didn’t create the market for bigger homes,” Fincher writes. “The market exists in spite of myself and other builders. If and as the market changes to smaller homes (which it is, but maybe not small enough for some of you) builders will adapt as well because we’re not doing this for kicks and grins, we’re doing this with the hopes of making a profit.
“I made a decision to build a better home that uses far fewer resources over its life cycle than the same home built to code. I call that being ‘green.’ That decision has added a whole new level of complexity to my building process which increased both my hard and soft costs which I probably won’t ever completely recover in my market.”
“I’m OK with that because I’m still making enough money to keep me engaged in this business and I feel better about the improved product that I offer,” he adds.
Weighing two energy penalties
When HVAC equipment is installed in an unconditioned attic, the system has to work harder in both the heating and cooling seasons. Soaring attic temperatures in summer greatly lower air conditioning efficiencies; in winter, the problem is reversed but no less problematic.
GBA senior editor Martin Holladay calls that “Energy Penalty #1.” He adds that insulating between rafters to create a conditioned attic creates a different energy penalty — “Energy Penalty #2” — namely, the additional energy required to heat and cool the attic now that the volume of the home’s conditioned space has been increased.
“Here’s what you need to remember: Energy Penalty #1 is always much bigger than Energy Penalty #2,” Holladay writes, “so there is always a net gain (lower operating costs) when you create a conditioned attic (assuming, of course, that we’re talking about houses with ductwork in the attic).”
That said, he adds, there are alternatives. One would be to bring heating and cooling ducts inside the thermal envelope with the use of soffits that hide them. Another would be to build a small mechanical room in part of the attic that could be heated and cooled, rather than making the entire attic a conditioned space.
A related factor, notes Corian Johnston, is the increased surface area of an insulated roof compared to an insulated flat ceiling. “A 9/12 roof will have 25% greater surface area than the flat ceiling,” Johnston writes, “so that even with the same R-value and performance of insulation, there is a theoretical 25% greater heat loss or gain. This doesn’t include gable end walls that will add additional area.
“Second, heat rises, and unless the ceiling is a barrier, which is not likely although possible, heat will rise into the attic instead of being in the living area conditioned space,” he says. “This may be desirable for cooling but not for heating.”
Polyurethane foam is expensive
Spray polyurethane foam has some advantages that make it very attractive to some builders and homeowners, but it’s also much more expensive than other options — closed-cell foam more so than open-cell foam. The price premium, and the limited depth available for insulation between rafters, may result in less-than-optimum insulation levels in the house.
“If a builder’s idea of a conditioned attic is to move the insulation to the rafters — but to install less — then the homeowner is getting cheated,” Holladay says.
He adds that when spray-foam contractors try to convince homeowners to accept thin installations of insulation in an effort to make their product more competitive on price, they are “cutting corners,” while “code enforcement officials are looking the other way or getting bamboozled by fast-talking spray-foam contractors instead of doing their job. As a result, homeowners are left with homes that have below-code levels of insulation. That’s wrong.”
The problem may be more pronounced with open-cell foam because it has a lower R-value per inch than closed-cell foam, as Fincher himself points out. “If Martin is correct, then perhaps I’ve been bamboozled by the insulation contractor who installed 5 1/2 in. of closed-cell on the underside of the rafters,” he writes. “I questioned him because it was only around R-19 and he confidently explained to me that it would work.
“…It’s really hard to cipher through what is factual and what is not,” he adds. “Without a whole lot of actual empirical testing and data, you almost have to try something and see if it works. That’s a scary proposition for me.”
John Brooks adds numbers to the mix, taken from a recent price quote in Dallas: R-30 open-cell foam at the roof deck for $2.80 per sq. ft. vs. R-30 cellulose at the attic floor for 50 cents a sq. ft.
In Fincher’s 4,000-sq. ft. house, cellulose would cost $2,000 while foam would cost $14,000. He calls this a “$12,000 incentive” to air-seal the attic floor and find a way of putting the HVAC system within the conditioned space.
Our expert’s opinion
Here’s what GBA technical director Peter Yost had to say:
No doubt, any conditioned attic (no matter the pitch or ceiling square area) is better performing, in terms of energy performance and indoor air quality, than exposed HVAC equipment and ducts above the ceiling insulation and air barrier.
And in addition to Martin’s suggestion of constructing a small attic mechanical room, there is Building America research supporting some performance advantages of “buried” attic HVAC distribution systems. Also, see this article .
But I have to agree with Martin when he states near the end of his related blog on conditioned attics, that “creating a conditioned attic is a solution to a fundamental design flaw.”
A high performance home starts with high performance design — if you can’t find design solutions to keep all HVAC ducts and equipment in conditioned space, you missed the earliest and most effective solution. Here is how Steve Baczek, the resident GBA architect puts it:
“Creating a mechanical room that is effectively air sealed within a vented attic is a straight-up challenge. I don’t believe the approach has much merit either. I have been asked to do it a number of times and reluctantly have, knowing the likely outcome. When tested for performance, they just don’t work as planned. It’s one of those concepts that seems easy, gets people on board at the design/planning stage, only to fail at the post-construction/ leak-chasing phase. In lieu of this approach, I would put the air handler in the conditioned space and run sealed ductwork in the attic, buried under R-50+ of cellulose. Not the best solution, but not a bad second choice, and one that is pretty easy on the pocketbook. You could also spray the ductwork with CCSF and cover with cellulose at a cheaper cost than spraying the attic.”
So onto what I would do. You need an architect who understands HVAC design and performance. I understand this is a challenge. I have had too-many-to-mention (or want to remember) conversations with architects who preach out about high performance house design, and just don’t get it.
In a 4,000-square-foot house, depending on the design and site, you are probably talking about one large mechanical room (40 sq. ft.) or two smaller mechanical rooms (20 sq. ft. each – 40 sq. ft. total). Either way, the space accounts for 1% of the conditioned space. This house probably has closets bigger than that.
I remember working with a builder to re-engineer his houses and the architect who designed them. The architect argued vehemently with me about how there was just no room in the plan for mechanicals. I simply asked, have you ever designed a house this size without a powder room? Or a laundry room? Her replied, “never.” I simply said you need to place the same priority on the mechanical room as you do the powder room and the laundry room. With that in mind, I would suggest one or two mechanical rooms in the conditioned space, using dropped soffits and ceiling locations to distribute the HVAC system. If a client wants a 4,000-square-foot house, chances are the house will come with 9- or 10-ft. ceilings. A good architect, using this ceiling height to his or her advantage, can create not only a high performance home, but also an aesthetically pleasing one. It’s simply a win/win!