Make an unvented semi-conditioned space around air handler and ducts in attic?
If the builder can use rafters instead of roof trusses, I want to make the attic space around the air handler and ducts semi-conditioned by putting rigid foam board on underside of rafters, and buttoning up the envelope completely, leaving the space behind the foam to be vented by soffit and ridge vents.
I think it would be expensive to button up whole 2,000 square feet of attic with rigid foam, but does anyone have an idea what ballpark cost might be to seal about 900 square feet up with the higher R foam, ISO, maybe 4″ thick slabs? We will be building in humid South Carolina climate, and not sure yet what R value will be needed. Thanks.
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Rob,
Any construction estimate, including the one that you mention in your question, is calculated the same way. You have to price out the materials, using the prices charged by local suppliers; you have to estimate the number of hours of labor; and you have to assign an hourly cost to the labor.
If you won't be doing the work yourself, then you need to get estimates from local contractors.
Thanks, Martin. I just thought someone in the trade might have done that type of insulation job and remembered what they charged. I'm trying to get a cost comparison to closed cell spray. If the builder uses trusses instead of rafters, I'm not sure what approach we will take to creating a ventless space around mechanical and ductwork. Any suggestions? I wanted to keep some space for storage, so didn't want to put insulation on attic floor, which is why I wanted to seal the roof.
Fiber faced roofing iso typically runs 10-cents per R-foot f.o.b. the distributor's yard. At 4" it's about R25 give or take R1, call it $2.50 per square foot plus delivery, installation, and scrap rate multiplier. To be code legal you'd also have to paint the interior facer with intumescent paint or put up a thermal barrier equivalent to half-inch wallboard for an attic that was used for storage.
Closed cell foam is typically 17-18 cents/R-foot, installed price. In high labor cost areas that is sometimes cheaper than rigid polyiso, but it's not nearly as green. Due to the radically different global warming potentials of the blowing agents used, closed cell polyurethane has on the order of 100x the greenhouse gas footprint of polyiso.
In a SC climate, unless you have high-SRI "cool roof" shingles or light metal roofing, you can safely use open cell foam on the underside of the roof deck, which typically runs about 10-12 cents/R-foot, installed price, which would usually be cheaper than polyiso in this type of assembly, even in low labor cost areas. That too would require an intumescent paint or thermal barrier to meet code in a storage attic. To meet IRC 2012 code min takes R38, or about 10-11" of open cell foam, but it needs to be installed in lifts no greater than 6" at a time (otherwise the curing foam is a fire hazard, and can have shrinkage issues as it cools.) Unlike it's closed cell cousin, open cell polyurethane is usually blown with water, which has a very low GWP.
With simple roof lines and putting the foam above the roof deck iso is still competitive though, and that would be the preferred, best performing solution, since there are no thermally bridging structural timbers. Two staggered layers of 3-1/4" roofing iso (R20 per layer) beats code min on R, but soundly beats the performance of R40 open or closed cell applied to the under side of the roof deck.
Thanks, Dana. 10-11 inch thickness of spray foam means rafters would have to be that wide in order to provide a channel to which the foam can adhere. And, unless you spray the rafters also, you would get the thermal bridging, as you said, but I have no idea how much that heat/cold infiltration would amount to. You recommended to use two 3 1/4 thicknesses of rigid foam staggered, which is 6 1/2 inches of foam. Can that be stapled or screwed in to the rafters? It seems like a lot of foam to try to attach to underside of rafters.
Also, if the builder can't build with rafters but uses trusses, I'm guessing that another way of semi conditioning the attic will be needed. I can't picture how to do it with rigid foam, but can picture using spray foam on roof deck only. If you have any thoughts, I would appreciate them.
Putting 6-7" foam is attached above the roof deck all the time. It requires either a nailer-deck of OSB or ply over the foam through-screwed to the rafters with timber screws on which to attach the shingles, or 2x furring through-screwed to the rafters, with a nailer deck screwed to the furring, with conventional soffit-to-ridge venting for the nailer deck, or purlins onto which suitable metal roofing is applied. At 6.5" it's probably going to be easier to go without the vented nailer deck, since the screws get to be pretty long. But it's just as valid to put 3-4" of rigid (easier) above the roof deck and 6" of open cell on the underside, if that's any easier/cheaper. (That gets done often enough as retrofit upgraded in built out attic rooms with cathedral ceilings that have only 2x6" rafters. They would otherwise be stuck with R15 batts to be able to still have a vented roof deck.)
Adding that much rigid foam to the underside of the rafters is awkward at best, but could be done using lateral 1x furring 24" o.c. through screwed to the rafters, on which you'd hang the half-inch gypsum to make it code-compliant. It's a lot simpler/easier to put it over the structural roof deck, bringing the whole roof deck fully inside of the conditioned space.
Open cell polyurethane doesn't need rafters bounding it to be able to stick- it's basically a foamed-glue, (chemically similar to Gorilla Glue) and is essentially self supporting as long as it has a decent bond to the roof deck. It's pretty common to apply a foot of open cell to trusses with thin rafter elements, installing it in 2 passes. It can get pretty lumpy and uneven if the installers aren't used to doing it in double-lifts, but it doesn't take a spray foam Michelangelo to do a decent job of it.
Dana, I thought I needed to keep channels of air open on the roof side of foam to let the soffit and ridge vents circulate air. I guess I'm not understanding why you might seal the roof at the decking. That would give you a sealed attic, but then the soffit and ridge vents have to be covered. Do you not need some air to circulate next to the roof decking?
Rob,
There are two kinds of insulated sloped roof assemblies: vented or unvented. Either approach can work, as long as you get the details right. For more information, see How to Build an Insulated Cathedral Ceiling.
Martin,
I read the excellent article you linked me to and read responses to the piece. I realize now that almost everyone who comes to this site is someone who already knows a lot, pros. I knew little before reading the various articles on this site, but I feel like I am on the verge of clarity about insulation, vapor, thermal and air barriers, and the one that is hard for me: vented vs. unvented attic. Your article gave me a few aha moments though. If I am asking newbie questions on a site for pros, I apologize and will understand non-response, although you've been very helpful so far, and I really appreciate it. I love to learn about this. Here are some more questions and aha moments:
1. You said roof ventilation does not significantly lower roof temp, but it does contribute to roof deck/sheathing drying out faster. So, I guess when considering sealing an attic, you have to consider that if you spray foam roof decking, a water leak onto the decking from outside could trap water and rot it without you being able to see the leak from inside attic. However, if you spray to button up attic, you wouldn't have to install soffit vents and ridge vents. On the one hand, you take a chance about a leak leading to rot, but on the other hand, you would save money by not installing vents that really aren't very helpful in the first place. Is that right?
2. An aha was to find out that moisture rotting the roof decking wasn't just from outside water infiltration, but also from vapor coming up from living space through can lights, wire runs, etc. and condensing on the colder roof decking inside house. So, if I understand this correctly, closed or open cell spray foam on roof decking will form a vapor barrier to protect decking from inside house (in addition to insulating), but outside water infiltration can still take place leading to rot unless you also use rigid foam on the outside of decking/sheathing. Would a house dehumidifier connected to heat pump not be enough to eliminate the vapor rise into attic?
3. If I were able to add a small mechanical room to first floor plan and put air handler, ventilator and dehumidifier in it instead of the attic, could I just wrap the ducts in attic with insulation and not be concerned with creating semi conditioned space up there? Would the wrapped ducts be cool enough in attic to prevent heating the cool air coming through them in summer? (Of course, if I didn't have attic conditioned, then I guess I have to have soffit and ridge vents for drying out decking).
Rob,
Q. "On the one hand, you take a chance about a leak leading to rot, but on the other hand, you would save money by not installing vents that really aren't very helpful in the first place. Is that right?"
A. The cost of the soffit vents and ridge vents is low, and ventilation can be helpful. But you are right that it is easier to inspect the roof sheathing in a vented unconditioned attic than an attic with spray foam installed on the underside of the roof sheathing.
Q. "Would a house dehumidifier connected to heat pump not be enough to eliminate the vapor rise into attic?"
A. Trying to limit moisture damage in your attic from air leaks through your ceiling by installing a dehumidifier would be nuts. Running the dehumidifier enough to prevent the problems you describe would be expensive. It would make far more sense to just plug the air leaks in your ceiling. After all, once the air leaks are plugged, my solution requires no electricity (unlike your dehumidifier).
Q. "If I were able to add a small mechanical room to first floor plan and put air handler, ventilator and dehumidifier in it instead of the attic, could I just wrap the ducts in attic with insulation and not be concerned with creating semi conditioned space up there?"
A. That is correct. Air handlers don't belong in an attic. (By the way: Ideally, your home shouldn't need a dehumidifier.)
Q. "Would the wrapped ducts be cool enough in attic to prevent heating the cool air coming through them in summer?"
A. The best location for ducts is inside your home. If you have ducts in the attic, the duct system will never be as efficient as it would if the ducts were located indoors. However, sealing the duct leaks and wrapping the ducts in insulation will improve the situation compared to many older homes with leaky attic ducts. More information here: Keeping Ducts Indoors.
Thanks, Martin. I'm learning a lot, and still need to understand more, so I'll read more articles on your site. I still have to grasp the whole permeable, impermeable, semi-permeable, retarder vs. barrier for vapor, air and thermal for the humid climate we will build in. But I do better understand the vented vs unvented choices we have.
If the mechanical room doesn't add too much to cost of house, we'll have it added onto house. Problem is that the only space available is next to master bedroom, so I have to investigate whether the noise factor in mechanical room will penetrate wall into bedroom.
To sum up some of what I read or inferred from your answers: If I want to use some of the attic space for storage, I need to, by code, semi condition whatever portion I need. I don't need to insulate roof deck to do this, since mechanicals would be downstairs. I can just drywall in a certain amount of space with batts in those walls, sealing off that room from the rest of the unconditioned attic. I should keep the soffit and ridge vents for drying and partially for air flow. I need to tight seal the downstairs ceiling, and then no moisture/vapor barrier will be needed in attic. You didn't say why dehumidifier was not needed in hot humid SC, but I guess you mean that the heat pump itself should dehumidify. The HVAC man recommended a dehumidifier, but he's trying to sell me something. Hope I have understood your points correctly.
Rob,
It's news to me that some codes require that an attic must be "semi-conditioned" (what does that mean?) if it is used for storage. Here in New England, many homeowners use their unconditioned attics for storage.
Thought it was somewhere in your posts, but obviously not. If the info is true, maybe it's because of flammables being stored in a hot space.
Sorry, I didn't answer your question. To me, semi-conditioned means insulating attic, but without an HVAC register run to the space.
Martin,
You said, "By the way: Ideally, your home shouldn't need a dehumidifier." I took that to mean that the heat pump could remove humidity in a tight house in a hot humid climate. I read your article on HRV and ERV, where you said neither one is good in humid climate. Then you mentioned getting a stand alone dehumidifier. Seems contradictory about the dehumidifier, so I'm confused.
1. Is a heat pump alone enough to dehumidify or is a dehumidifier needed in humid climate in summer?
2. If ERV is not effective in that climate, what is a better solution for a tight house in hot humid climate? And just a basic physics question: Why does humid air brought in the house by a ventilator not raise the level of humidity in house, furthering the problem?
Rob,
Dehumidifiers use a lot of energy, and are a blunt and undesirable weapon in the war against high indoor humidity. Your first defense is to build a tight building envelope to minimize the infiltration of humid exterior air. Your second defense is to install and use exhaust fans in humid locations -- in bathrooms and above kitchen stoves. Your third defense is to operate your air conditioner or heat pump in the cooling mode.
In almost all locations in the U.S., a central air conditioner does an adequate job of dehumidifying your indoor air. In a few locations in the Southeast, some energy-efficient homes have a hard time keeping indoor humidity under control during the swing seasons (spring and fall). In these homes, a stand-alone dehumidifier may be the best solution.
However, installing a dehumidifier is always a last resort. For more information, see All About Dehumidifiers.