you feel that the air will hit condensing temperatures on the under side of the rigid insulation in the winter?
Assuming there is a vapor barrier on the existing insulation, and it is on the interior side, the rigid would create another vapor barrier on the exterior side. This is never a good practice. If the op lives in a hot climate, and there is no existing vapor barrier, having a vapor barrier on the exterior side of the insulation is sometimes accepted practice.
Originally Posted by Carnak
Well there will two issues.... the interior moisture in the winter... is sure to be trapped and saturate the drywall...
Originally Posted by Carnak
Then in the summer all the moisture can't defuse and will be trapped under the plywood attic floor deck....
Could be year round petri dish
Whats your thoughts Carnak ???
From Dr Joe Lstiburek.... Good Stuff
Conditioned Attics Save Energy in Hot Climates
Do homes with vented attics use less cooling energy than homes with unvented, conditioned attics in hot climates? With little formal research available to answer the question, common wisdom and most building codes have favored venting. But a recent field study in Las Vegas challenges existing assumptions about the virtues of venting in hot, humid climates. The study shows that moving the thermal and air barrier from the plane of the ceiling to the sloped roof plane improves airtightness and can save cooling energy by eliminating heat gain to ducts located in the attic.
Joseph Lstiburek of the Building Science Corporation (BSC) in Chestnut Hill, Massachusetts was the primary researcher for the Las Vegas study. He said that one goal of the study was to validate observations of energy savings at homes and schools that BSC had retrofitted in Florida and Hawaii to eliminate moisture problems. The buildings had leaky ducts and air handlers in vented attics that caused the buildings to be depressurized. In hot climates, negative pressures in the house cause infiltration of warm, moist air from outside. This led to mold growth throughout the house. Having given up on getting large builders to install tight ducts, Lstiburek decided to do the next best thing--put the ducts inside the conditioned space.
The retrofits (sealing the attic vents and insulating at the roof deck) solved the moisture problems because, with the ducts leaking only to the inside, the houses were no longer depressurized. They also showed reduced energy use. According to Lstiburek, having the duct system within the conditioned space saved more energy than was used to condition the attic.
To confirm these results, Lstiburek and Armin Rudd of the Florida Solar Energy Center designed a computer model to describe what was happening, which they then tested with real data from the Las Vegas study.
The Las Vegas Study
The field study was performed on three houses in a Las Vegas subdivision. Two of the houses had attics with roof air barriers of sheathing, along with R-30 fiberglass batt insulation under the plywood roof deck. The third house (the study control) had a conventional vented attic with insulation above the ceiling gypsum board. The three houses had the same floor plan, elevations, and orientation.
The researchers installed temperature sensors at several locations and elevations--from the roof tile top to the inside air space--to record temperatures throughout the day. They measured air conditioner energy use at the compressor units, and performed air leakage tests for each home. Lstiburek and Rudd then analyzed the monitoring data for two weekend days on which the outside temperature peaked at 92°E The days also had similar solar radiation peaks.
Even though the maximum temperatures recorded at the bottom of the plywood roof deck were higher in some instances for the conditioned attics than for the vented attic, air leakage rates and energy consumption were significantly less in the conditioned attic homes. The duct systems for the sealed attic houses had no measurable air leakage via the attic to the outdoors. There was also reduced heat gain to the ducts, since the attic was now cooled along with the house. The two conditioned-attic houses used an average of 19% less cooling energy than the vented-attic house.
Lstiburek and Rudd used these data to calibrate their computer model, which compares space-conditioning energy use and roof temperatures in hot climates for homes with vented attics and conditioned attics (see Figure 1). The model also incorporated extensive laboratory test data on the ways in which conditioned attic space and venting affect temperatures in the attic, roof, and roof deck. These test data were provided by William Rose of the Building Research Council at the University of Illinois.
Annual simulation results for Las Vegas showed that, compared to a vented attic, an unvented, conditioned attic could save 4% on space-conditioning energy assuming no duct leakage. However, when modeled with typical duct leakage (10% return leak and 5% supply leak), the conditioned attic could save 10% on space-conditioning energy.
In addition to energy savings, Lstiburek noted that conditioned attics also reduce pressurization and depressurization problems associated with opening and closing doors to interior rooms. The attic acts as a pressurization equalization system for the house. Much of the air leakage in houses built in the South is between the house and the attic, and this leakage is eliminated by conditioning the attic. Thus, if a bedroom has a supply register in it and the door is closed, the air can travel through the attic to the rest of the house, instead of pressurizing the bedroom. The balanced pressure eliminates uncontrolled air leakage, backdrafting of gas appliances, and soil gas infiltration.
Contending with Codes
Traditionally, building codes in most locations require vented attics in houses. Lstiburek has been battling the general assumption among building officials that homes with vented attics use less cooling energy than those with unvented, conditioned attics. For instance, the Florida code now allows the option of a conditioned attic; however, it requires a higher level of insulation in the conditioned attic.
Lstiburek expects that "building codes will eventually be rewritten to reflect [the Las Vegas] findings." He says that the building department in Las Vegas waived their venting requirement when he showed them the results of the study. The builder of the 100-home Las Vegas subdivision where the study was performed is using unvented conditioned attics for the entire development.
When to Vent
The benefits of having ducts in conditioned space also apply to cold and mixed climates. But in these climates attics must be vented. In 1995, Rose tested attic construction in which the insulation was placed along the underside of the roof sheathing with various venting configurations. With a conditioned attic space, Rose found that "energy transfer through the ductwork, both convective and conductive, is no longer a loss to the exterior," while "air-tightness requirements for the ceiling plane are reduced or eliminated." However, for moisture control, Rose concluded that having an air chute to maintain an air gap between the sheathing and the top of the insulation is critical for keeping the sheathing dry.
Lstiburek recommends completely unvented attic assemblies only for hot climates with mild winters. These areas include Florida; the southern parts of Louisiana, Alabama, Mississippi, and Texas; and the southern tips of California and Arizona. Homes in areas with cool or cold winters should vent the roof to avoid moisture problems and prevent the formation of ice dams (see "Out, Out, Dammed Ice," HE Nov/Dec '96, p. 21).
Ted Rieger is a freelance writer based in Sacramento, California, who specializes in energy issues
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I think you have to go through the summer/winter scenario and the dewpoints of the room air.
Originally Posted by kenny mac
So in winter maybe you are 68 to 70F (probably a smidge warmer as air will stratify) with RH up around 40%, you take that dewpoint and figure out how cold the underside of the foam would be with a cold attic above it.
So you have an air film on top of the plywood maybe its R0.65, 1/2 inch plywood , R 0.62,an inch of Dow Sm R5, call R30 and 24 inch wood trusses R28, R0.45 for 1/2" of sheet rock, and then take an air film value of 0.65 so you have an over all R-value around 35.37. I was too lazy to look up exact air film values here, good enough for reasoning it out though.
So now it depends on how cold it is where the OP is
But for 20F in the attic, and 70F air below the ceiling plane heat loss per sqaure foot is delta T divided by R
(70-20)/35.37= 1.41 Btu/hr per square foot
So now work out the temperature for the underside of the foam
The R value is 0.68 +0.62 + 5 = 6.3 to the underside of the foam
T-20 = 1.41 x 6.3
T = 1.41 x 6.3 + 20
Dewpoint of 70F @ 40% = 44F so would get condensation
would need to keep RH down like 20%
So jax was right, in winter it violates the "vapour barrier on the warm side of the insulation" rule
You would get condensation occuring inside the R30 in this scenario
They got one where they go through the various attic scenarios and see where condensation will occur
Originally Posted by kenny mac
As far as sealed attics go in a hot humid climate, I would be your strongest ally on this forum in agreeing with the concept. Except I would say do the full monty and put the insulation on top of the roof deck and it works in any climate, even an indoor swimming pool for the Ice Road Truckers
Originally Posted by Carnak
I'm not familiar with that method. Do you have a link where I could read more?
its in every mall with a flat roof , built up deck and a membrane, you can do the same on a slope, I have
Originally Posted by jax1
try building science search for BSD-102
look at figure 7, photo 4 and figure 8
They did an interesting study of the LA superdome after Katrina, as its roof was similar to figure 8, they now recommend filling in the flutes with foam.
The roof of the superdome had hundreds of bullet holes in it before Katrina hit.
water flowed all over in those hollow flutes
My attic is 1.5 inches of foam above a sloping concrete deck, there is white metal roofing. Maybe R7 total.
As far as keeping the heat out I think it outperforms R30 at the ceiling plane in a vented attic.
Before I turned on the AC the attic was a degree hotter than ambient. I believe that was better than a 140F sauna with a fibreglass air filter in between the ceiling joists.
great link Carnak!
I like that method. Leaks should be easy to find before doing structural damage. Is it cheaper to build than spray on the underside?
I bought a half built, abandon concrete structure that was to be 4 apartments. The majority of the materials to finish the job were on site.
I had 2 @ 20 ft containers with about 15,000 reddish spanish roof tiles on the site that came with the property. They would have been difficult to install and I was going to blow the foam on the underside of the deck.
I ended up abandoning the scheme, as I figured the dark tiles, the concrete beneath it would have been a very substantial thermal mass, and the colour would of encouraged it to heat up. The foam benenath would have slowly allowed stored up heat to trickle in and by the time the mass cooled off the sun would be up again.
So I decided to keep the heat out of that mass as best as I could in the first place.
So, we hilti nailed pressure treated 2x4s on the flat to make a surface to screw the metal roofing to, In between the 2x4s is 1.5 inches of foam. I put tar paper over that then the white metal standing seam roofing. After going through a category 5 storm, it was very obvious that standing seam roofing is the best to have.
It was a financial decision, however it was the cost of trying to stick roof tiles to smooth sloping concrete that was the problem. It would have been very labour intensive.-
I sold about 3000 tiles, we held out for too much money on the rest, and the roof repair rush after a hurricane ended. Now I have 12,000 tiles I cannot give away.
I am new to the site, so I am going to let this be my first post.
I have this stuff called Hy-Tech Thermal Solution. It is a paint laced with some type of ceramic. This helps to cut down anywhere from 20-80% of the radiant heat in the attice space. You simply paint it on the entire roof, right on the shingles. I have it on mine, and am THUROUGHLY impressed.
Check out the link.
here we go again...rb paint vs rolls of rb attachec to roof decking...
The cure of the part should not be attempted without the cure of the whole. ~Plato