Any benefit to having an attic w/ insulated roof in hot climate? - GreenBuildingTalk - GreenBuildingTalk - Green Building Forums on Insulating Concrete Forms (ICF), Structural Insulated Panels (SIP), Radiant Heating, Geothermal Heat Pumps, Solar Power, Green Construction Projects - Green Building Forums - Green/Energy-Efficient Design and Planning

Forums

Unanswered Active Topics

Forums Search Members

Forums > Green Building Forums > Green/Energy-Efficient Design and Planning

Any benefit to having an attic w/ insulated roof in hot climate?

Last Post 01 Mar 2011 01:51 PM by Dana1. 12 Replies.

Sort:

	Prev Next
You are not authorized to post a reply.

Author

Messages

MountainStone User is Offline

New Member

Posts:12

22 Feb 2011 06:47 PM
We intend to eventually build in Texas. It is my understanding that current wisdom recommends insulating the roof and keeping the attic as a conditioned space in a cooling-dominated climate. If I am insulating the roof, is there any benefit to even having an attic? For instance, if I used SIPs for the roof could I finish the interior side of the SIPs and provide every room with a vaulted ceiling? Am I missing some benefit by eliminating the conditioned attic in a hot climate? Thanx!

cmkavala

Veteran Member

Posts:4327
Avatar

22 Feb 2011 09:12 PM
Yes natural vaults are a benefit of SIP construction, we typically leave most ceilings vaulted, dropping halls & closets to distribute HVAC ducts

Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br />

JeffD

Basic Member

Posts:282

24 Feb 2011 10:36 PM
If you did have an attic it would be a great climate controlled storage space. Plan for some of the living spaces to be vaulted and some to have dropped ceilings. Make sure the ceilings are framed stout enough to have accessible storage above and provide a convenient attic stair to access the space.

Metal SIP Building Designer<br>jeff@panelfusion(dot com) See us on Facebook

arkie6

Veteran Member

Posts:1453

25 Feb 2011 12:26 AM
Posted By MountainStone on 22 Feb 2011 06:47 PM We intend to eventually build in Texas. It is my understanding that current wisdom recommends insulating the roof and keeping the attic as a conditioned space in a cooling-dominated climate..... It depends on where your HVAC ducts are located. If the duct work is located in the attic, then yes, an insulated roof may perform somewhat better than an insulated ceiling with vented attic. However, if you locate your HVAC duct work in the conditioned space or crawlspace, then an insulated roof is unlikely to outperform a well insulated ceiling with vented attic. In fact, Building Science articles indicate that the insulated ceiling with vented attic will perform ~2-3% better than the insulated roof in this situation in a hot climate. And the insulated roof, assuming spray foam insulation, will cost significantly more than loose fill insulated ceiling. http://www.buildingscience.com/docu...-the-south IMO, the optimum solution is to locate your HVAC ducts in the condition space if possible, then spray 1" of closed cell spray foam on the floor of the attic to air seal it, then blow in R38 or better loose cellulose to provide the bulk of your R value. Around here, the cost for 1" CC spray foam is ~$1/sq ft (~R6) and R38 blown cellulose cost is ~$0.50/sq ft. That would give you ~R44 for your ceiling. To get near the same amount of R value on your roof would require either ~7" of closed cell spray foam or ~12" of open cell spray foam. The cost here for spray foam is ~$1/board ft for closed cell and ~$0.50/board ft for open cell. That is $7/sq ft for R42 closed cell foam or $6/sq ft for R42 open cell foam. That compares to ~$1.50/sq ft for the R44 foam + cellulose mentioned above. And the square footage to insulate the attic is less than the square footage to insulate the roof. As an example, assume a 32'x60' home with 6:12 pitch roof. The square footage of the attic floor is 1920 sq ft. Accounting for the roof pitch, the square footage of the roof is approximately 2160 sq ft. To insulate the roof to R42 with CC spray foam would cost ~$7/ sq ft x 2160 sq ft = ~$15,000. To insulate the attic to R44 with 1" CC spray foam + ~10-11" blown cellulose would cost ~$1.50 / sq ft x 1920 sq ft = ~$2900. Another thing to consider for a roof in a hot climate is the installation of solar radiant roof decking with foil facer on one side. This however wouldn't add any advantage with the spray foam insulated roof unless you provide ~3/4" airspace adjacent to the foil facer. I would definitely consider this option if you have to locate your HVAC ducts in the attic and elect to go with the insulated ceiling and vented attic. Solarbord OSB Thermostat Plytanium

Dana1

Senior Member

Posts:6991

25 Feb 2011 12:30 PM
In any home with a decent amount of insulation (or even code minimum), unless you have uninsulated ducts are in the attic the payback on low-E facers on roof decking or radiant barriers is between limited to non-existent: http://www.ornl.gov/sci/ees/etsd/btric/RadiantBarrier/RBFactSheet2010.pdf Most of the time adding R to the roof deck is a better value. In most of TX you can insulate cathedral ceiling style entirely with spray cellulose rather than foam or use an inch of closed cell foam on the and the rest spray cellulose at worst-case in all other areas of the state: http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems

arkie6

Veteran Member

Posts:1453

25 Feb 2011 09:22 PM
Dana, I agree that savings from installing the radiant barrier roof decking would be modest, but the cost is also modest, especially for new construction. Per the ORNL link you posted, the estimated savings from installing radiant barrier on the roof of a 1540 sq ft home in Austin, TX with code min insulation (R30) in the attic plus insulated tight ducts in the attic was ~$40/year. For that 1540 sq ft 27'x57' home with 5:12 roof pitch I came up with 1667 ft sq of roof area. That is ~52 4'x8' sheets of roof decking. The last time I priced Solarbord OSB, it was ~$3/sheet more than regular OSB roof decking. So the additional cost of the radiant barrier in this home would be ~$156. The labor cost for installing regular OSB or Solarbord would be the same. In this case, the radiant barrier roof decking pays for itself the first 4 years. That is a pretty good investment in my opinion. Most home energy improvements often take >10 years to return the cost of the improvement. The ORNL link uses 20 years as a reasonable payback period. But savings can also come in the form of reduced HVAC cooling equipment size. The ORNL link above stated in the Appendix that experimental work in central Florida showed that the addition of the radiant barrier on the roof resulted in a 16% reduction in peak cooling load. Here is what it said "Extensive experimental work has identified the energy savings and peak-load reduction benefits of radiant barriers in attics in the southern climates of the U.S. Eight homes, all with air-handling equipment located in the attic, were retrofit with radiant barrier systems in 2000 in central Florida. Subsequent monitoring and data analysis showed cooling energy savings of 9%, peak load reduction of 16%, and an improvement in indoor comfort.i" As an example, if a new home heat load calc determines that a 3 ton HVAC unit is needed to handle peak cooling load and you can reduce this by 16% with the addition of the radiant barrier, then you can reduce the size of the HVAC cooling unit to 2.5 tons (3 x 0.84 = 2.52). That savings in HVAC equipment alone will more than pay for the radiant barrier roof decking.

cmkavala

Veteran Member

Posts:4327
Avatar

25 Feb 2011 09:40 PM
duct work running thru a hot attic will draw in hot /unconditioned air in the connection joints because of negative pressure, this will reduce the efficiency of the unit Dust and allergans can also get drawn into the house the same way. An insulated / unvented roof assembly eliminates those problems.

Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br />

arkie6

Veteran Member

Posts:1453

25 Feb 2011 11:37 PM
Posted By cmkavala on 25 Feb 2011 09:40 PM duct work running thru a hot attic will draw in hot /unconditioned air in the leaky return connection joints because of negative pressure, .... I added the "leaky return" part. You wouldn't expect to have any measurable negative pressure on the supply side ducts which is what is most commonly found in attics, at least around here (typical single story slab on grade construction with air handler in a central closet with air return below air handler). Properly installed and sealed duct work shouldn't allow any measurable amount of air exchange with the attic. A couple of years ago I saw a new home where they had sprayed closed cell spray foam over all of the metal duct work in the attic while they were air sealing the ceiling with the same foam. Seemed like a good idea - air seal the duct work and insulate it all in one shot.

cmkavala

Veteran Member

Posts:4327
Avatar

26 Feb 2011 07:34 AM
Posted By arkie6 on 25 Feb 2011 11:37 PM Posted By cmkavala on 25 Feb 2011 09:40 PM duct work running thru a hot attic will draw in hot /unconditioned air in the leaky return connection joints because of negative pressure, .... I added the "leaky return" part. You wouldn't expect to have any measurable negative pressure on the supply side ducts which is what is most commonly found in attics, at least around here (typical single story slab on grade construction with air handler in a central closet with air return below air handler). Properly installed and sealed duct work shouldn't allow any measurable amount of air exchange with the attic. A couple of years ago I saw a new home where they had sprayed closed cell spray foam over all of the metal duct work in the attic while they were air sealing the ceiling with the same foam. Seemed like a good idea - air seal the duct work and insulate it all in one shot. arkie6; actually it is not the return side I was refering to and the negative pressure draws-in hot humid air *on the supply side* it mixes with the conditioned air - warms the conditioned air and raises humidity *........... as air flows thru it creates the negative pressure*

Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br />

MountainStone User is Offline

New Member

Posts:12

28 Feb 2011 11:44 AM
It seems the issue is more one of where the air handler and ductwork is located. If I understand correctly that containing the HVAC system within the conditioned envelope is more important than whether that envelope is bounded by an insulated ceiling or by an insulated roof, then does it make more sense to insulate the ceiling and create a smaller conditioned envelope? Good information here; thank you all for contributing! EDIT: After re-reading Arkie's post and the links he provided, I see that he answered the question before I asked it. It is slightly more efficient to contain all HVAC components in the conditioned space IF the ceiling is air sealed and insulated, vs. an insulated roof with conditioned attic. Thanks again!

Dana1

Senior Member

Posts:6991

28 Feb 2011 02:17 PM
Posted By arkie6 on 25 Feb 2011 09:22 PM Dana, I agree that savings from installing the radiant barrier roof decking would be modest, but the cost is also modest, especially for new construction. Per the ORNL link you posted, the estimated savings from installing radiant barrier on the roof of a 1540 sq ft home in Austin, TX with code min insulation (R30) in the attic plus insulated tight ducts in the attic was ~$40/year. For that 1540 sq ft 27'x57' home with 5:12 roof pitch I came up with 1667 ft sq of roof area. That is ~52 4'x8' sheets of roof decking. The last time I priced Solarbord OSB, it was ~$3/sheet more than regular OSB roof decking. So the additional cost of the radiant barrier in this home would be ~$156. The labor cost for installing regular OSB or Solarbord would be the same. In this case, the radiant barrier roof decking pays for itself the first 4 years. That is a pretty good investment in my opinion. Most home energy improvements often take >10 years to return the cost of the improvement. The ORNL link uses 20 years as a reasonable payback period But savings can also come in the form of reduced HVAC cooling equipment size. The ORNL link above stated in the Appendix that *experimental work in central Florida showed that the addition of* the radiant barrier on the roof resulted in a 16% reduction in peak cooling load. Here is what it said "Extensive experimental work has identified the energy savings and peak-load reduction benefits of radiant barriers in attics in the southern climates of the U.S. Eight homes, all with air-handling equipment located in the attic, were retrofit with radiant barrier systems in 2000 in central Florida. Subsequent monitoring and data analysis showed cooling energy savings of 9%, peak load reduction of 16%, and an improvement in indoor comfort.i" As an example, if a new home heat load calc determines that a 3 ton HVAC unit is needed to handle peak cooling load and you can reduce this by 16% with the addition of the radiant barrier, then you can reduce the size of the HVAC cooling unit to 2.5 tons (3 x 0.84 = 2.52). That savings in HVAC equipment alone will more than pay for the radiant barrier roof decking. For spending the same $156 on adding cellulose beyond the R30 code-min attic in Austin would have similar effects on cooling loads as the RB, yet reap a heating-season savings as well. That $156 would buy you another 300-350lbs of cellulose (installed). Assuming it's 1540' of attic floor that works out to 0.19-0.23lbs/square foot for your 1-story 1540' house. At a settled density of ~1.4lbs/cubic foot (typical spec from a few manufacturers) that's 3-3.5" for another R10-R14, for a total R in excess of R40. Going from R30 to even R40 cuts the heat gain through the attic directly by 25%+, but also cuts the heat loss in winter by a similar amount. A 16% reduction in peak load per the central Florida retrofits cited with R9-R30** low density fiberglass insulation and ducts in the attic, isn't likely to haves as comparable an effect on cooling equipment size in most ~1500' code-min R30 houses, (even less if it's higher-R than code min), or with the ducts & air handler located in interior space. Rather it's a "best" (worst?) case scenario that makes for at true double-digit difference in peak load. It may knock off as much a half-ton of compressor size in low-R buildings, but for better thought out better than code-min, designs, probably not. Spending the money on cool-roof materials would have as large or larger effect on cooling loads in TX. Designing the ducts & air handlers out of the attic, and going to R40 cellulose burying the joists/chords instead of the R30 (or less) in low density fiberglass between the joists (as in the in the central FL retrofits would reduce the benefit of RB benefit by at least 75%. Note, even the "Fairey et al., 1988" study cited in the FSEC document, unoccupied identically controlled side-by-side identical homes with R19 batts showed only an 8% reduction in peak load for the radiant barrier case. With even and higher R and using a fiber isulation that is fully opaque to infrared don't count on even that much of a boost. At most a 4% improvement may be close to reality were it R38 fiberglass, and even less would be the case for R30-R40 blown cellulose. The semi-translucency of fiberglass to infra red makes the hottest part of the insulation 1-2" below the top, where it runs appreciably hotter than the attic air temp when there is no radiant barrier. With cellulose or rock wool the hottest part of the insulation is the top layer, and much closer to the attic air temp. (Texas A & M as well as FSEC studied this phenomenon to death back in the 1980s.) Bottom line, radiant barrier may be it's good for low or maybe even mid-single-digit reductions in peak load for a TX code-min house but in most cases you'll get similar or better reductions in cooling load out of deeper cellulose for the same money, with a higher annual reduction in utility costs.

arkie6

Veteran Member

Posts:1453

28 Feb 2011 06:07 PM
Dana, If you look back, my recommendation for radiant barrier roof decking was primarily targeted at attics with HVAC ducts through them. Adding more insulation to the ceiling doesn't improve the ~R6 fiberglass insulation over the ducts unless you can bury the duct work in the loose fill insulation - possible with 6" branch lines, but not likely with the main trunk line. Adding the radiant barrier roof decking will reduce attic temperatures on peak cooling load days which will reduce heat transfer into the duct work. If the air handling equipment and duct work is located outside of the attic, then I agree that adding more cellulose to the attic insulation will give you more benefits than adding radiant barrier at similar costs. However, at some point assuming roof trusses on 24" centers, you have to consider the weight of the cellulose insulation on the gypsum board on the ceiling and potential sagging. I have started a couple threads in the past talking about this. R40 levels shouldn't be a problem with 5/8" drywall or 1/2" ceiling board; however, amounts above R50 would begin to concern me unless the cellulose was stabilized.

Dana1

Senior Member

Posts:6991

01 Mar 2011 01:51 PM
"...my recommendation for radiant barrier roof decking was primarily targeted at attics with HVAC ducts through them" And so you did (sorry if I was harping heavy handedly!) There can be condensation issues in the fiber insulation if you bury the attic ducts, particularly with a vented attic. With 24" o.c. trusses splitting the R between the roof deck and chord can take the weight off the chord and ceiling gypsum at high-R, but the rationale for R50+ is pretty slim in most of TX. In cooler climes you'd have to design it such that the average January temp in the attic space stayed above ~37-40F (the dew point of 68-70F, 30-35% RH conditioned space air), but that a done deal in most TX locations.