| Air Conditioning and Duct Systems | Floors | Hot Water Heating | Insulation |
| Purchasing a Home | Radiant Barriers | Roofs and Attics | Walls | Windows |
Air Conditioning and Duct Systems
Q: I'm confused about what the minimum required a/c SEER rating is for new homes; I've heard numbers given from SEER 10 to SEER 13. Would you please advise me which SEER rating is correct?
A: The new Florida Energy Efficiency Code (2004 Glitch Cycle) for Method A specifies that air cooled, split system AC's sized < 65,000Btu/h (i.e. the vast majority of systems installed in residences, and probably what you are concerned with here) have to be SEER 13.0 or higher. SEER 13 is also the national minimum. Table 607.1.ABC.3.2A in section 13-607.1.ABC.3.2 of the Florida Code specifies this and other equipment efficiency minimums.
Q: Is the rule of thumb of one ton of cooling per 400 square feet of living space still appropriate for the energy efficient building code of three years ago and/or the one effective in March of 2002?
A: Rules of thumb should not be used. Our energy-efficient Lakeland house for example (http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1044-98/index.htm), uses a two-ton system to cool the 2400 square foot residence (or 1200 square feet per ton), and does a better job than the comparison home’s 4-ton unit. A good sizing procedure that takes into account a house’s construction type, insulation levels, glass area and orientation etc., such as the Air Conditioning Contractors of America’s Manual J procedure should be used to size cooling systems. Even then it’s important that the procedure is used as intended and the size it recommends is not rounded up (or down) or otherwise modified.
You may also be interested in reading how much of a problem over sizing is in Florida at http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-328-97/index.htm
Q: What tightness criteria, if any, exist in the Florida Energy Efficiency Code for air conditioning ducts located in unconditioned attic spaces?
A: Although the energy code has indicated "Ducts
must be sealed" and
specified certain sealing methods, in practice, homes continue to have
leaky ductwork. See:
http://securedb.fsec.ucf.edu/operation/press_display?pressid=1950.
In the new 2001 Code (effective March 2002) the ducts are assumed to be leaky in the Code calculation and one can receive tight duct credit only if the duct system is sealed and tested by a qualified Rater (see http://securedb.fsec.ucf.edu/engauge/engauge_search_rater for a list of raters). Other key changes include an airtight air handler credit and requiring a return air path to closed rooms. Highlights of code changes are provided here: http://energygauge.com/flares/new_code.htm.
Q: For homes which are air conditioned, and have ventilated crawl spaces in hot humid climates, is insulating the underside of the floor worthwhile and is a vapor barrier recommended?
A: First I would recommend grading the soil well so that there is no standing water during rains and from improperly directed gutters or lawn sprinklers. Then put a vapor barrier (6mil poly) on the ground and secure it well and ventilate the crawlspace very well e.g. >25% free vent area by lattice or staggered brickwork.
If you do this then you will have eliminated one of the main driving
forces, water from the ground. The next thing you need to worry about
is making sure you have absolutely tight air distribution ductwork. We
don't want any supply duct leaks which create negative pressures in the
house and can suck moisture through the floor and cause mold under kitchen
and bath vinyl flooring (which we see in manufactured housing). Also make
sure you have adequate return air pathways in the bedroom doors so that
you can not create negative pressures by closing off the interior doors.
After doing all this insulate the floor (really for winter time comfort and preventing dust mites in winter). How to do it is tricky due to all the penetrations involved and then the cable guy will tear it up again anyway. I would recommend a vapor permeable foam type sprayed insulation and no more than R-11. You can probably get away with R-5. Or use 1" skinless (semi vapor permeable) rigid extruded polystyrene (e.g. DOW blue board and you have to special order without the skin) attached to the bottom side of the floor joists and do not vent the space between the floor and the insulation under the joists.
Q: How do I know if I'm buying a good solar water heating system?
A: Buy a system that has been tested and certified by either the Solar Rating and Certification Corporation or the Florida Solar Energy Center. Get details from the contractor on the warranty for the system - what it covers, how long it is in effect and where you can get parts and support.
Be sure the installers are licensed solar contractors with good references. Contact some of their customers and make an appointment to look at the installations. Ask the customers if they are happy with their systems and the contractor's service.
For more information, go to http://www.fsec.ucf.edu/en/consumer/solar_hot_water/index.htm and http://www.fsec.ucf.edu/en/consumer/solar_hot_water/q_and_a/index.htm.
Q: What is the relative insulation and advantages of block versus frame houses?
A: We have done considerable modeling of homes, as well as monitoring many homes. We ran a number of lab experiments during the 1980s. The bottom line is that in Florida there are other components of the home that one should concentrate on before the walls, such as window shading and roofs (see our Priorities page).
Block versus frame – the important item is that either construction can be done well. With block, we generally recommend an insulation level of R-5 or more in Florida. Usually, it is installed adjacent to the block and then a 3/4-inch air space is furred out on the inside. Make sure the top of this air space is blocked off from the attic. With frame walls, the concern is good insulation installation and reducing air infiltration. Seal gaps (especially in the wall top plates where electrical and plumbing penetrates) and make sure insulation is cut correctly. If it is cut too long, it tends to bend. If it is cut too short, it doesn’t cover the gaps. For specific detailed drawings, you may want to purchase the Builder's Guide for hot humid climates. You can find the guide at http://www.eeba.org/bookstore/cat-Builders_Guides-4.aspx.
For general guidance regarding design decisions, you may want to get the Florida Solar Energy Center’s Energy Efficient Florida Home Building manual (GP-33). Go to http://www.fsec.ucf.edu/en/publications/html/FSEC-gp-33-88/. Or, if you want detailed energy and economic information regarding specific design decisions and you are building and computer savvy, you may want to purchase one of our EnergyGauge software packages (see http://energygauge.com/).
With either wall system, the finish is important. Dark finishes lead to greater solar absorption. Consider finish colors that are light or white and preserve or plant trees to help shade the walls.
Q: How do I know I am buying a home that is energy efficient?
A: Buy one with an ENERGY STAR® Label. ENERGY STAR® Homes are designed to use at least 30 percent less energy than required by the national Model Energy Code.
To achieve ENERGY STAR® status, a home must be validated by a Home Energy Rater - a certified, third party inspector who audits and tests the home.
Home buyers can also save money on their mortgages. ENERGY STAR® Mortgages
give home buyers easier access to preferred financing.
Look for advertisements featuring the ENERGY STAR® Homes Program logo
in your local newspapers or ask builders and real estate agents if they sell
ENERGY STAR® Homes. You can contact the ENERGY STAR Homes Program hotline
at 1-888-STAR-YES.
The ENERGY STAR program is cosponsored by the EPA and the Department of Energy. FSEC is an ENERGY STAR® Home ally.
For information on Florida's Home Energy Rating System (HERS), go to our Home Energy Ratings page and for a list of Florida's certified home energy Raters, go to http://securedb.fsec.ucf.edu/engauge/engauge_search_rater.
For energy efficient mortgage information, go to http://www.fsec.ucf.edu/en/consumer/buildings/homes/ratings/eem/index.htm.
For additional information on ENERGY STAR® Homes, go to http://www.fsec.ucf.edu/en/consumer/buildings/homes/programs/energystar.htm or check out the Environmental Protection Agency's (EPA) Web site at http://www.energystar.gov/.
Q: Our house seems to get overly hot. We have a split-level house with the upper level being totally exposed to the sun in the afternoon. We have one unit to cool the entire house. Could a "radiant barrier" be a possible solution?
A: Your situation is unlikely to be solved by a radiant barrier unless most of your heat is coming through the attic and you have very poor ceiling insulation. You may have an airflow balancing problem or excessive solar radiation coming through windows or leaky duct work.
If you are in Florida, you may want to contact a Class1 Rater to come investigate
your house and make recommendations. You can visit the links below to find
raters.
Florida Raters |
Other
Locations
For more information about radiant barriers, take a look at the video listed below. Although the video was created in the 80s, the technology is still applicable.
Note: The video is provided in multiple sizes to accommodate different connection speeds. Quick Time 7 is required for viewing but can be downloaded for free by clicking the button below.
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Q: I have heard conflicting reports regarding attic ventilation. Some say that very good attic ventilation is an absolute requirement in hot climates and some say that sealed attics can actually outperform vented attics. Which is it?
A: The answer is both. But this is a trick answer. The sealed attics referred to by this question have their insulation at the plane of the roof instead of at the ceiling plane. This means that these homes actually do not have an attic in the conventional sense. Since the home's insulation envelope is at the roof instead of at the ceiling, what would normally be an "attic" is now really part of the home's "conditioned" space. This is because normal ceiling drywall provides practically no resistance to heat conduction. So what you might normally call your "attic" is now actually inside your home (for air conditioning purposes, that is). Thus, you would not want to vent this space to the outdoors any more than you would want to open your windows while you were air conditioning.
On the other hand, if your insulation is in the ceiling plane of your home, then you have a "real" attic. For this case, you do want to vent your attic well, preferable with equal amounts of both low air inlets at the eaves of the roof and high air outlets at the peak of the roof. The net free vent area (usually about one-third of the total vent area) should be at least 1/300th of the attic floor area.
In many cases sealed attics can have energy performance advantages over ventilated attics. Air conditioning ducts are most often located in the attic of slab-on-grade homes (most Florida homes). If the insulation for the "lid" of your home is located in the roof instead of in the ceiling, then these ducts remain much cooler. This results in air conditioning energy savings. Duct leakage is also problematic in many homes, often with much of the air leaking into the return side of the air handler system coming from the attic. A sealed attic will reduce the energy waste associated with these duct leaks. Additionally, on beach-side properties, sealed attic systems can have additional advantages related to keeping wind-blown moisture and salt-laden air out of attics.
Side-by-side roof research tests, one with dark gray shingles (solar absorptance of 92%) over a vented attic compared with dark gray shingles over a sealed attic, have shown 9% cooling energy savings for the sealed attic. Tests of vented attics comparing the dark gray shingles with white shingles (solar absorptance of 76%) found savings of 4% for the white shingles. This indicates that combining white shingles with a sealed attic is likely to produce greater cooling energy savings. In addition, these tests found significantly greater savings (17-23%) for white tile and white metal roofing systems. Measured energy performance savings of 9% have also been reported in separate field tests for attic radiant barrier systems.
Measurements also have shown that sealed attics and attics with radiant barriers have hotter roofs. This occurs because heat can not readily leave the inboard side of the roof sheathing if it is insulated. For the sealed attic roof with dark gray shingles, the measured top-surface peak shingle temperatures are about 7°F hotter than the otherwise identical vented attic. The temperatures at the bottom of the roof, between the roof decking and the roof insulation, however, are about 23°F higher at peak than in the vented attic. For attic radiant barrier systems, measured peak shingle temperatures are about 2°F higher and peak temperatures at the bottom of the roof sheathing are about 12°F higher.
Other tests comparing white and black shingles have shown that shingle color makes a greater difference in peak shingle temperature than the presence or absence of attic ventilation or an attic radiant barrier system. These tests, accomplished at the FSEC flexible roof facility, showed peak temperatures for black shingles (solar absorptance of 97%) to be almost 25°F hotter than peak temperatures for white shingles (solar absorptance of 76%). Thus, if elevated temperatures can result in composition shingle failure, then the problems are likely to be much more pronounced for darker shingle products, especially in climates with large quantities of solar radiation like the desert southwest.
In general, cooling energy savings will be greatest when sealed attic and insulated roof deck construction is used in combination with highly reflective white tile or metal roofing materials. However, if an insulated roof deck and sealed attic are used with composition shingles, consider the following recommendations:
- Select white-colored shingles which tend to be much cooler under full sun. White colored shingles also produce the greatest cooling energy savings among shingle roofs.
- Use thicker roof sheathing and high-quality, heavy-grade shingles, preferably from a company that will warranty their product in a sealed attic with an insulated roof deck.
- Consider using a roof assembly with a ventilated air space above the insulated decking (a double roof). This creates a vented decking surface on which the shingles are installed. This is the installation recommended by the Asphalt Roofing Manufacturer's Association (ARMA) to reduce shingle temperatures for sealed attics (see also the links given below).
Warning: Many composition shingle manufacturers claim that their warranty will be voided or severely reduced if the bottom surface of the roof sheathing is not vented. On the other hand, some shingle manufacturers fully warranty certain of their composition shingle products for sealed roof applications. Note also that these slightly elevated roof temperatures are unlikely to affect tile, metal or single-ply membrane products. Thus, if you wish to use a sealed attic system and you must use composition shingles as your roof surface, make sure to use a product that does not warn against sealed attic applications.
Additional information:
For additional information on the energy performance of various
roof and attic systems see:
http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1220-00-es/index.htm and http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1231-01-es/index.htm
For information on the solar absorptance of various roofing products see http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-670-00/index.htm
For additional discussions of roofing options from Building Science Corporation, see: http://www.buildingscience.com/resources/resources.htm#roofs
For the Asphalt Roofing Manufacturers Association (ARMA) recommendations on composition shingle applications see http://www.asphaltroofing.org/tech_bulletins.html and for specific recommendations on construction details for sealed attics see http://www.asphaltroofing.org/tb_211.pdf
Q: What is the best kind of insulation to use in my attic to reduce my air conditioning bills?
A: Instead of adding more insulation, you should consider adding a radiant barrier system. This will have a greater effect on your cooling bills because it reflects heat away from the attic before it can reach the insulation and ceiling below.
Without a radiant barrier, your roof radiates solar-generated heat to the insulation and, if they are mounted in the attic, your air conditioning ducts below it. The insulation absorbs the heat and gradually transfers it to the material it touches, principally the ceiling. This heat transfer makes your air conditioner run longer and consume more electricity.
If the outside air is 87°F, the roof can reach 148°F and the attic air can be more than 120°. Heat from the hot attic roof radiates downward into the house. A radiant barrier will block 95 percent of the heat radiated down by the roof before it reaches the insulation.
A radiant barrier is a layer of aluminum foil placed in an attic airspace to block radiant heat transfer between a hot roof and conventional attic insulation. It can be stapled or nailed to the underside of the top chord of roof trusses or to the underside of the roof decking.
Your attic should have the proper level of insulation for the climate (at least R-19), adequate vents to allow hot air to escape and a radiant barrier to shield the house from heat. The foil radiant barrier and good ventilation can reduce heat gain through the ceiling by 40%. The home's cooling system should also operate more efficiently and last longer because the ductwork is cooler.
More information can be found at:
http://www.fsec.ucf.edu/en/publications/html/FSEC-EN-15/index.htm and http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1231-01-es/index.htm
Energy Priorities for your home:
http://www.fsec.ucf.edu/en/consumer/buildings/homes/priorities.htm
For more information about radiant barriers, take a look at the video listed below. Although the video was created in the 80s, the technology is still applicable.
Note: The video is provided in multiple sizes to accommodate different connection speeds. Quick Time 7 is required for viewing but can be downloaded for free by clicking the button below.
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Q: Do ceramic coatings or products called liquid concrete or siding save energy? I've seen several ads promoting these coatings and paint additives, such as microspheres or ceramic beads that come from NASA technology, that claim to have insulative properties.
A: The Florida Solar Energy Center has tested ceramic paints and found them to have no significant advantage over ordinary paint in terms of their ability to retard heat gains through exterior building surfaces. These products are generally composed of elastometric coating products to which ceramic beads have been added. When tested side by side with the same elastrometric coating that doesn't have ceramic beads, both products have virtually the same heat-gain retarding performance. The product may have other worthy benefits like durability, but any energy-saving benefits it has can also be achieved without the ceramic beads. In Florida, choosing an efficient exterior coating means picking one with a light color to reflect heat.
For further information on how white and very light-colored surfaces can
reduce heat gains to homes in Florida climates, see http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1220-00-es/index.htm.
For measured properties of various surface materials, see http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-670-00/index.htm.
Q: You recently mentioned the benefits of installing a white metal roof
on buildings to reflect heat from your home. Wouldn't leaving the metal
its natural silver color reflect better?
A: No. White is superior in terms of heat buildup. Unfinished
metal roofs can get very hot. Whatever heat unfinished metal roofs
absorb from the sun is not readily lost back to the exterior environment
because unfinished metal surfaces do not lose heat by radiation very
well. This results in high surface temperatures so that even the
apparent reflectivity advantage of these roofs is not realized. Watching
the reactions to this process is probably how Tennessee Williams
got the name "Cat On A Hot Tin Roof" for
his famous play.
Radiation to the sky is a major part of the cooling process for exterior building surfaces, even while the sun is shining. Remember, there is more occurring in the heat transfer process than what you can judge accurately with your eyes. You see less than half the energy in sunlight and even much less of the total energy transferred by radiation.
Q: In reviewing your cool roof study on the internet, I came up with a question about sealed attics. Why does the sealed attic have a lower attic temperature, but not as great a cooling reduction as the white metal and white tile roofs?
A: There are several factors that come into play here. First, there is no insulation on the ceiling floor with a sealed attic. The primary heat transfer occurs between the attic interior (which is semi-conditioned) and the roof surface rather than between the attic insulation surface and the house interior ceiling. All heat transfer is proportional to UA*dT.
U = 1/R
A = Area
dT = temperature difference
Assuming similar insulation, a sealed attic has greater area due to the gable ends (or larger hip roof segments and the increase due to roof pitch. Thus, "A" is nearly always larger than with a flat ceiling. Typically by about 30% with a 4 or 5/12 pitched roof.
The temperature difference “dT” is the big difference. With a conventional attic, the attic air temperature at the ceiling insulation surface may reach 135°F at peak whereas the home interior is at 75°F, so dT = 60°F. In the sealed attic, the heat transfer is between the peak outer roof surface which reaches 165°F with dark shingles whereas the sealed attic reaches a peak of about 85°F, so dT = 90°F.
Let's do the numbers:
Conventional attic = 2000 ft2 of ceiling, R-19, dT= 60
Q gain = UA dT
Qgain = 1/19 *2000 * 60 = 6,316 Btu/hr
Sealed attic = 2600 ft2 of roof surface, R-19, dT= 90
Qgain = 1/19 * 2600 * 90 = 12,316 Btu/hr
The sealed attic shows nearly double the heat gain due to its higher delta T and greater areas. The higher level of heat gain across the ceiling to the interior was verified with infrared thermography in the study.
The reason the sealed attic saves energy, however, is that the duct system is located now within the conditioned space and its cooling is not lost. Same for duct leakage from segments in the attic. They are not lost. This is large enough to more than offset the increase heat gain.
A cool white reflective roof with a sealed attic would work very well, however. Here the outside roof surface would only reach about 110°F (this was measured) so that dT would only be about 25°F.
Sealed attic = 2600 ft2 of roof surface, R-19, dT= 25
Qgain = 1/19 * 2600 * 25 = 3,421 Btu/hr
Heat gain is less than the conventional case, with the double benefit of having the ducts inside the conditioned space. This is the winning combination in a hot climate: a reflective roof with interior ducts.
Q: I presently have a dark shingle roof. Is there a coating I can apply to the roof surface to increase the reflectivity and lower my cooling bills?
A: Shingle roofs aren't good candidates for reflective roof coatings. It can be done, by applying elastrometric roof coatings, but moisture damage can often result.
Shingles have many edges and water can collect under the edges by capillary action. Standard composition shingles depend on getting hot to protect the underlayment from remaining wet. Even white shingles consist of ceramic particles impregnated in a dark asphalt substrate. Their solar reflectivity is only about 25%. With truly reflective shingles (coated) the roof never gets hot any longer and moisture damage can take place. Thus, except in the arid regions of the U.S., where roof coatings over shingles should do well, coating shingles is not recommended for shingle roofs because of the potential for moisture damage.
The most viable residential reflective roofing systems are white metal and white tile. Having reflectances of 65 to 75%, both of these systems work extremely well at controlling cooling loads. A recent study for a Florida utility shows that a white reflective roof can reduce space cooling energy use by 17- 23%. A large part of the advantage comes from cooling the attic space where cooling ducts are often located. However, if you are re-roofing a home with composition shingles, choosing the lightest white color can reduce cooling costs by about 4% at no added cost.
For more information, see a related paper: Comparative Evaluation of the Impact of Roofing Systems on Residential Cooling Energy Demand in Florida.
Q: Are you familiar with (insert roofing product) and how it performs versus various other roofing materials in regards to energy efficiency? I understand that white metal is the best roof style to have in South Florida but the house we're going to build simply can't use that type of roof. We need to use a barrel tile style and I'm trying to determine if this composite is better than using natural clay tile.
A: Basically our studies have shown that what is key in reducing roof heat gain is a roofing product's solar reflectivity, and the solar reflectivity will largely depend on surface color and texture. For example, a white asphalt shingle roof will not be nearly as reflective as a white metal or white tile roof because of the rough texture of the shingle surface.
Solar reflectances of white metal and tile products of 65% or higher are available (there are also some new non-white roofing products available that are specifically designed to have high solar reflectivity, although I am not aware of any that approach 65%). So if the manufacturer can provide the solar reflectivity of the product you are considering, you can get a good idea of how it will compare with these other roofing products as far as solar heat gain is concerned.
If you haven't already seen it, please refer to our "Cool Roofs" study on our web site at: http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1220-00-es/index.htm . Basically the study shows that in side-by-side tests of real homes, white metal, white barrel tile and white flat tile roofs all performed very well. Again, most of this result is due to solar reflectance of the surfaces. Darker colors (as the terra cotta barrel tile results show) did not do as well.
Q: Is injecting foam into the cores of CBS walls worthwhile for Florida homes?
A: Since the foam only fills in the block cavities there is still very significant thermal bridging around the insulation via the concrete block, making only a very low incremental cost for the insulation cost-effective from an energy standpoint. A rigid insulation that covers the entire wall surface will greatly reduce the bridging problem.
In general though, walls are typically a relatively low part of a Florida home's total cooling load (~ 7 - 10%). Let’s look at an example. The annual energy use savings (cooling and heating) for a 2100 sq. ft. example central Florida house (efficiency just above the 2001 Florida Code minimum), going from R-0 CBS to R-4 CBS (with e.g. a rigid sheet insulation) was $74, and going from R-4 to R-10 was $45. Each house will be different of course, but this example at least gives some idea of the relative impact, and how the benefit decreases as we get into higher R-values.
Q: Which is a better wall system in Florida, block or frame?
A: Either can be done well. With block we generally recommend an insulation level of R-5 or more. Usually it is installed adjacent to the block and then a 3/4 inch air space is furred out on the inside. Make sure that the top of this airspace is blocked off from the attic. With frame walls the concern is good insulation installation and reducing air infiltration. Seal gaps, make sure insulation is cut correctly (too long and it tends to bend, too short and it misses spots). For specific detail drawings you may want to purchase the Builder's Guide for hot humid climates https://www.eeba.org/mall/builder_guides.asp and for general guidance regarding design decisions, you may want to get our own Energy Efficient Florida Home Building manual http://www.fsec.ucf.edu/en/publications/html/fsec-gp-33-88/.
With either wall system the finish is important. Stone and brick facades increase the potential for moisture problems. Dark finishes lead to greater solar absorption. Consider finish colors that are light or white and preserve or plant trees to help shade the walls.
Based on a considerable amount of modeling, lab experiments and monitored studies, the bottom line though is that there are other areas one should concentrate on besides walls, such as window shading, roofs and duct system location and leakage (see http://www.fsec.ucf.edu/en/consumer/buildings/homes/priorities.htm).
Q: Are 8 foot high ceilings a lot better than 10 foot high ceilings for energy efficiency?
A: It’s actually changes in ceiling heights and skylight shafts where we find the most problems. Often times the vertical walls that separate conditioned space from the attic that these ceiling height changes create are not insulated properly, or the insulation falls away from the walls with time. So the first key is to either avoid changes in ceiling height that would lead to vertical walls separating conditioned and attic spaces, or to thoroughly inspect the installation of insulation at these locations.
If you have consistent 10-ft. ceilings the only real energy cost is the extra wall area, which does have some summer and more winter effect. For typical, new, central Florida homes for example, the annual energy use increase will be in the $15 to $20 range going from an 8-ft. to a 10-ft. ceiling throughout the house.
There are several other considerations that may come into play. One is that 10-foot high walls usually have taller windows and thus more window area; if so, the additional energy use could be considerably higher. On the plus side though, a higher ceiling may more easily allow drop-down plenums that would allow ductwork to be installed within the conditioned space, which should provide significant energy savings over ducts installed in the attic.
Q: Should I use a vapor barrier? If so, where do I place it?
A: The location of the vapor barrier depends on the climate. For homes in Florida, we do not recommend a vapor barrier, but highly recommend a rain barrier and an air infiltration barrier. In Florida, nailing on paper (which is black tar paper) backed lath to cdx and then stuccoing over it and then painting with elastrometric paint works well. Don't need to paint cdx.
Q: There seems to be an excessive amount of heat in an area of my house where there are a lot of windows. What can I do to keep heat from entering my home through these windows?
A: The best way to dispel the heat is to block the direct solar radiation before it gets to the window. You can do this with exterior shade created by a tree, fence, trellis, awning, shutters or solar screen. A solar screen absorbs solar radiation outside before it reaches the window.
If you cannot use exterior shading, then there are some special hot-climate low-e glazing systems that can reduce solar gain. The best of these require double-pane windows, in which case you get many additional benefits: improved thermal comfort, acoustic attenuation, and reduced peak load on your air conditioner throughout the year. Look for a window with a higher visible transmittance (VT) than solar heat gain coefficient (SHGC).
Window films can be applied to existing single-pane windows and will reduce solar gain somewhat, but tend to reduce light transmission more. Although, there are few window films that block more heat than light. Look for ones that have a higher VT than SHGC or higher VT than shading coefficient (SC) when applied to single pane clear glass.
Window films applied to multiple-pane windows can void warranties, so check with the manufacturer to ensure the window film you choose is acceptable.
For more information on reducing heat gain through windows, see our Priorities page or check out our Solar Gain page in our Windows Research area.
Q: I'm confused about low-e glass. Some people say it is very important here in Florida, but others say it doesn't make any difference. What is right?
A: Normal low-e glass was designed for cold northern climates and its main function is to admit solar radiation as solar heat gain into the building, while preventing the escape of that heat back outside. It produces an accentuated greenhouse effect and is not what we want in Florida.
A better choice for hot climates is a low-e coating that is specially designed to block some of the solar radiation incident on it. There are various trade names for this insulated glass, but the common feature is a solar heat gain coefficient (SHGC) that is lower than the visible transmittance (VT).
Look for a visible transmittance around 70 to 80 percent if the window is well shaded from the sun or from 40 to 60 percent if it is not. Then get the lowest solar heat gain coefficient you can find. The ratio of VT to SHGC should be as high as possible but definitely over 1.0.
For more information on selecting windows for a hot climate, see our Window Selection Guidelines.
Q: Are storm windows effective in Florida?
A: While storm windows will provide some energy benefit in Florida, they are not nearly as cost effective here as they are in climates with harsher winters. They can provide other benefits however, such as reducing condensation on windows and lowering indoor noise levels from street traffic etc..
© 2007-2014 University of Central Florida. The Florida Solar Energy Center (FSEC)
is a research institute of the
University of Central Florida.
For more information about FSEC, please contact us or learn more about us.
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