Page 13 of 40 FirstFirst ... 36789101112131415161718192023 ... LastLast
Results 157 to 169 of 513
  1. #157
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    I've also found that my attic cools off fairly quickly after sunset. The peak temperature occurs mid-afternoon, and begins tapering off toward sunset. It is now three hours past sundown and the attic temperature is now two degrees above ambient temperature, at 88 degrees.

    Before observing this pattern I always thought attics took considerably longer after sundown to approach ambient air conditions. Perhaps more voluminous attics do, but my own attic, at least where my temperature sensor is, indicates otherwise. I may move the sensor deeper into the attic at some point to see if the pattern remains or changes.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  2. #158
    Join Date
    Apr 2002
    Posts
    11,808
    Quote Originally Posted by shophound View Post
    I reread the second link and didn't see what you were referring to. Maybe I missed it, but I skimmed over it several times.

    I agree about the folly of letting a roof deck cook the crap out of an attic, and ducts mounted in an attic. My own home has the spray-on radiant barrier. It does not have as good of an emissivity number as the foil based RB products do, but in all honesty I think the biggest difference it makes is reducing heat gain to my supply ducts in the attic. The supply ducts are round metal wrapped in fifty year old foil faced fiberglass. I do not ever feel a hot blast of air come out of the registers when the a/c kicks on after being off for awhile, and I do not read a considerable sensible temperature gain between the supply plenum and supply diffusers in adjacent rooms.

    Yesterday the attic reached 128 with 94 degrees outdoor ambient air. The insulation level is probably around R-40. I went around with my IR thermometer, shooting the ceiling surfaces throughout the house. Nothing read over two degrees above room temperature except in the master bath, which has no supply register. I measured several single pane aluminum frame windows remaining in the house, sat down with the known u factors of my ceiling plane and single pane windows...the percent of gain from the ceiling vs. the window was astounding. Since my house has three foot overhangs the entire perimeter of the structure, most of the windows are in deep shade most of the day. Even so, a single pane aluminum frame window is just an ass kicker when it comes to heat gain vs. a well insulated ceiling plane with spray-on radiant barrier on the roof deck.
    my bad it was the first link, the DOE one
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  3. #159
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by shophound View Post
    I've also found that my attic cools off fairly quickly after sunset. The peak temperature occurs mid-afternoon, and begins tapering off toward sunset. It is now three hours past sundown and the attic temperature is now two degrees above ambient temperature, at 88 degrees.
    That is amazing.

    What do you attribute it to, ventilation setup, orientation, etc.?

    I have full length soffit vents, front and back w/ ridge vents, and can't detect any airflow through the system. So after sundown, the heat radiating from the ceiling is relentless (R30 in ceiling). I'm at 40 degrees latitude.

    When OD ambient decreases, shouldn't I expect a chimney effect (air pulled in through soffits and out the ridge)? Doesn't seem happen.

    Amp

  4. #160
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    Quote Originally Posted by ampulman View Post
    That is amazing.

    What do you attribute it to, ventilation setup, orientation, etc.?

    I have full length soffit vents, front and back w/ ridge vents, and can't detect any airflow through the system. So after sundown, the heat radiating from the ceiling is relentless (R30 in ceiling). I'm at 40 degrees latitude.

    When OD ambient decreases, shouldn't I expect a chimney effect (air pulled in through soffits and out the ridge)? Doesn't seem happen.

    Amp
    I am at 32 latitude. Here we sit less than a week away from the summer solstice, so the midday solar angle is nearly at its annual peak. This means opportunity for solar loading of an attic is also prime.

    To answer your question pertaining to my attic's performance, you should know a few things about it, as you ask in your post. From mid-morning to mid-afternoon, the roof has a good amount of exposure to the sun. Afterward, trees to the west of my house begin shading the roof. This allows the attic to begin cooling down prior to sunset.

    The house is oriented southeast to northwest. It is a single story mid-century modern with a three foot overhang the complete perimeter of the building. Roof is hip design. It has two whirlybirds and soffit venting. Spray-on radiant barrier paint on the roof deck underside, a blizzard of fiberglass blown-in insulation estimated at R-40. About every penetration between ceiling and attic throughout house has been air sealed. As energy_rater_la notes in another post, attic insulation is considerably less effective if the ceiling plane is not airtight to the attic and to the house.

    The trees shading the roof partially explains the cooldown prior to sunset, but after sunset needs explanation as well. I will offer that the volume of my attic is not large. The house is 1800 square feet, the roof pitch either 4/12 or 5/12. It's not an attic you can stand up and walk around in. After sunset, there obviously is no more heating of the roof shingles from the sun. Instead of being a heat absorber, the roof deck and shingles are now a heat sink for the hot attic, transferring heat to a cooler sky. To what extent the soffit vent openings and whirlybird devices contribute to nighttime attic cooling I have not measured, but I wouldn't count them out. Nevertheless, one can't overlook how the roof itself must contribute to heat transfer in BOTH directions. Inward during the day when solar loading is high, outward at night when the sky above the roof is cooler than the attic volume underneath it.

    The spray-on radiant barrier must also be considered, although at best it may have an emissivity number of .22 or thereabouts. Compare that to foil based products of around .05, and you can see that the spray-ons are not as effective at barring radiant heat transfer as the foil based products. Nevertheless, if it retards heat transfer over bare wood below a blazing hot roof during peak solar loading, it also does so as the sun angle declines and the roof receives more shading. After sunset, its theoretical the radiant barrier might actually be a bit counterproductive, in that it retards heat transfer from the hot attic via conduction through the roof deck (since it reflects radiant heat striking the barrier). This is partly why manufacturers, proponents, and installers of radiant barrier recommend attic ventilation alongside the barrier for the best effectiveness. I don't know if the nighttime cooling of an attic with a radiant barrier has ever been compared to a similar attic with similar solar loading without one, but it would be interesting data to look over.

    Nevertheless, as you mention, one would expect a degree of natural ventilation in an attic at night via soffit inlets and ridge, gable end, or whirlybird outlets. I've been in some attics of homes built more recently that strike me as a ventilating nightmare. These homes have steep roofs with voluminous attic space beneath, and meager soffit/peak ventilation provision. More volume = more material that can be heated via radiant heat transfer. More heated material means more material that must be cooled via ventilation and nighttime radiant heat loss to the atmosphere.

    If you are as curious about heat transfer through residential building envelopes as I am, get yourself one of these:



    Home Depot has this one for around fifty George Washingtons. Walking around your house on a hot day with one of these, aiming it at ceilings, walls, windows, doors, etc. is an eye opening experience.

    What I did this past weekend is observe my attic temperature, convert my ceiling's R value to U factor ( 1/R = U ) and then use the formula U factor times Area times Temperature Delta to estimate rate of heat transfer through my building surfaces in btu per hour per square foot. For Area, use the number 1. Once you obtain a number, multiply it by the square footage of the ceiling in a given room, and you can get an estimate of rate of heat transfer from hot attic or outdoors to your room. My calculations revealed that my remaining single pane, aluminum frame windows were transferring much more heat for their size, than the ceiling above, which has considerably more square footage and a much wider temperature delta between the hot side and the conditioned side. For example:

    Ceiling of room measures 12 x 12 (144 square feet). R value above ceiling is 40, which translates to a U factor of 0.025. Window in room measures 4 x 6 (24 square feet) and is single pane, aluminum frame. ASHRAE Handbook of Fundamentals states U factor for a window of this type is 1.23 (!). This window is shaded all day for ease of illustration. Temperature outdoors is 90 degrees, indoors is 75. Temperature in attic is 130.

    Heat transfer from attic into room from ceiling = .025 x 55 x 144 = 198 btuh.

    Heat transfer through window into room = 1.23 x 15 x 24 = 443 btuh.

    That's a mind blower! We spend so much time here discussing the merits/demerits of attic ventilation, but gloss over the dirty rotten bandits of single pane windows with conductive frames! Also note that the attic to room temperature delta was 55 degrees, whereas the outdoor to room delta was only 15. Again, all this worry about ventilating attics, and right in front of your nose is the worst bandit of all, your windows.

    To further illustrate, let's take the same window opening and improve its U factor to 0.30 and then rerun the numbers:

    .30 x 15 x 24 = 108 btuh.

    Holy cow! The window now contributes less heat load to the room than the ceiling!

    Now, let's bump the attic heat level up to 150 (say it is poorly ventilated and has no radiant barrier) and rerun the numbers:

    .025 x 75 x 144 = 270 btuh

    That still outperforms the single pane aluminum frame window by a comfortable margin.

    Conclusion: thick attic insulation and good windows make for a comfortable house (in addition to controlling other factors regarding the building envelope).
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #161
    Join Date
    Feb 2007
    Posts
    999

    Shophound's Roof

    Wow: that was quite a dissertation--thank you.

    A little more than I can absorb at one time, so your post has been memorialized as a Word document entitled: Shophound's Roof.

    Is that device known as an IR thermometer is it sensitive enough to detect cold spots?

    Amp

  6. #162
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    Quote Originally Posted by ampulman View Post
    Wow: that was quite a dissertation--thank you.

    A little more than I can absorb at one time, so your post has been memorialized as a Word document entitled: Shophound's Roof.

    Is that device known as an IR thermometer is it sensitive enough to detect cold spots?

    Amp
    Hot dog I made the big time..."Shophound's Roof".

    The IR thermometer should pick up cold spots as well as hot spots. It is reading the surface temperature of whatever you aim it at. While not accurate for certain applications, it is plenty informative for sleuthing around a house when there's a significant temperature difference between indoors and outdoors.

    Here it is 4 PM local time. Outdoor temperature 96 degrees, attic temperature 128 degrees, ceiling temperature over my head averaging 76 degrees, room air temperature 75 degrees, average surface temperature of single pane, aluminum frame window to my left is 88 degrees. In essence I have a hot attic overhead that is 32 degrees warmer than outdoor ambient, but is transferring less heat into the room than the window to my left.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  7. #163
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,200
    very well done shophound.
    seems I'm always telling folks that the windows
    are the weakest part of the wall.

    carnuk..I didn't do option 4 because we were talking ventilation
    non vented attics are recommended for resistance to uplift here
    in hurricane alley. I'm just learning to apply to outside of my climate..
    but while we are here..
    option 5 nonvented attic, foam on attic floor,with foam sealing
    attic floor to soffits to roofdecking.
    ducts in attic with radiant barrier.
    not as costly as foam roofdecking (depending on pitch of roof this can be very costly) but air seal on attic floor, insulation value with foam - or foam combo..(some people install 2" foam with blown or unfaced batts on top). ducts are consitered "under insulation" with RB. (not to be confused with ducts buried in insulation, which we can't do here due to humdidty).
    while the benefit is not as great with this install as with foam roof decking, there is a middle..and this is it..before just insulating attic floor with conventional insulation.

    boy this is kinda fun...what if we built a sips house...sips roofline,
    sips walls, sips floors. then we put ductwork inside the building envelope I'd bet you could cool 3000 sq ft with 3 tons of a/c
    ..with the right windows.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  8. #164
    Join Date
    Apr 2002
    Posts
    11,808
    Quote Originally Posted by energy_rater_La View Post
    vnon vented attics are recommended for resistance to uplift here
    in hurricane alley.
    I think it is dumb to give the storm "an in" myself.

    Even if it is just soffit vents, it gets like blowing steam, the water defies gravity will go up in those soffits, soak ceiling insulation and bring the whole ceiling down before the roof ever failed.

    But if there were no hurricanes in Louisiana, there is still no reason to vent an attic, it is a strategy to stop icycles
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  9. #165
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    Quote Originally Posted by shophound View Post
    I've also found that my attic cools off fairly quickly after sunset. The peak temperature occurs mid-afternoon, and begins tapering off toward sunset. It is now three hours past sundown and the attic temperature is now two degrees above ambient temperature, at 88 degrees.

    Before observing this pattern I always thought attics took considerably longer after sundown to approach ambient air conditions. Perhaps more voluminous attics do, but my own attic, at least where my temperature sensor is, indicates otherwise. I may move the sensor deeper into the attic at some point to see if the pattern remains or changes.

    This is pretty consistent with the results I got when I did a spray on barrier in my last house. The temperature in the attic stayed pretty flat until mid morning, peaked mid afternoon, then fell quickly as the sun set, and was at/near ambient before midnight. The peak temp was about 20-25 lower than before the spray on barrier.

    In the same 100 weather and sun conditions before spraying, the attic temp climbed quickly shortly after sunrise, stayed in the 150 range from around 1pm until the sun actually started to set, then cooled slowly. It would still be rather warm in the attic at midnight.

    I actually sprayed the barrier on in the middle of July a few years ago when we had a long string of virtually identical cloudless 100 days, and data logged the attic temperature for 2 weeks before and after spraying.

    Unfortunately the logs died with my computers hard drive not long after recording.
    No changes to ventilation were made when the spray on barrier was put in.

    I have all these plans for recording with multiple sensor location and types on my current house, but so far I've been way to lazy to work on it, lol.
    Every time I get up in my attic to do much of anything, my spirit is quickly broken by the fact that this house is built with an annoying truss system, and has a lowish pitch roof.
    If more government is the answer, then it's a really stupid question.

  10. #166
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Quote Originally Posted by mark beiser View Post
    This is pretty consistent with the results I got when I did a spray on barrier in my last house. The temperature in the attic stayed pretty flat until mid morning, peaked mid afternoon, then fell quickly as the sun set, and was at/near ambient before midnight. The peak temp was about 20-25 lower than before the spray on barrier.

    In the same 100 weather and sun conditions before spraying, the attic temp climbed quickly shortly after sunrise, stayed in the 150 range from around 1pm until the sun actually started to set, then cooled slowly. It would still be rather warm in the attic at midnight.

    I actually sprayed the barrier on in the middle of July a few years ago when we had a long string of virtually identical cloudless 100 days, and data logged the attic temperature for 2 weeks before and after spraying.

    Unfortunately the logs died with my computers hard drive not long after recording.
    No changes to ventilation were made when the spray on barrier was put in.

    I have all these plans for recording with multiple sensor location and types on my current house, but so far I've been way to lazy to work on it, lol.
    Every time I get up in my attic to do much of anything, my spirit is quickly broken by the fact that this house is built with an annoying truss system, and has a lowish pitch roof.
    I am curious about just how you measured the temperature. In particular where did you take the measurement. Over time I have had 3 different locations measured, and have found that each reports a different number. One is a wired probe just above the insulation in a closet, another is a wireless weather sensor hanging a foot below the radiant barrier, and the last one is on the hot side of the RB.

    Thank you -- Pstu

  11. #167
    Join Date
    Jun 2001
    Location
    DFW
    Posts
    684
    Quote Originally Posted by energy_rater_La View Post
    boy this is kinda fun...what if we built a sips house...sips roofline,
    sips walls, sips floors. then we put ductwork inside the building envelope I'd bet you could cool 3000 sq ft with 3 tons of a/c
    ..with the right windows.
    You don't need sips to do that.

    2 ton heat pump, N. Central Texas
    2,230 sq. ft. living space.
    1,800 sq. ft. conditioned walk out basement - 85% above grade.
    According to HVAC calc, largest heat gain is through the walls at 4,500 BTUH
    windows contribute 2,100 BTUH.
    R49 cellulose in the attic with radiant barrier sheathing, 1,500 BTUH

    inexpensive low-e vinyl double pane windows - completely shaded in the summer.
    all the windows are either casement or fixed to keep infiltration to a minimum
    All the duct work is in conditioned space in the basement.

    The nice thing about all of this is that we can ignore the cost of setting the thermostat to what is comfortable for us. Our average monthly heating / cooling cost is about $50 a month at $.14 Kwh. We keep the thermostat at 74 degrees and 42% humidity in the summer.

  12. #168
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    Quote Originally Posted by pstu View Post
    I am curious about just how you measured the temperature. In particular where did you take the measurement. Over time I have had 3 different locations measured, and have found that each reports a different number. One is a wired probe just above the insulation in a closet, another is a wireless weather sensor hanging a foot below the radiant barrier, and the last one is on the hot side of the RB.

    Thank you -- Pstu
    The numbers will likely vary with the sensor location, but I think the pattern both Mark and I reported seeing would hold true even with the sensor placed at various locations in the attic. What I have seen so far in my attic (and am pleased to read Mark's similar experience) has caused me to reevaluate how the spray-on radiant barrier performs. While I imagine it has reduced peak solar gain for each daily cycle, what I did not anticipate was how it also appears to assist nightly cooling of the attic.

    Yesterday I was home a good part of the day, waiting for departure time for the airport to arrive for a much needed vacation. My attic sensor behaved as Mark posted about his...temperature stayed pretty flat after sunup (and in the seventies, no less) until mid to late morning, where it then began rising. I have been in many attics at mid to late morning hours in my time in summer, and they weren't even close to being that comfortable. Between 11 and noon the temp was over 100, and peaked at 128 with outdoor ambient at 96-97. The highest temp I saw last summer was during our hottest day of the year, where the outdoor ambient was 105 and the attic was 135.

    At the attic's peak temperature yesterday, ceiling surface temperatures averaged 76 degrees throughout most of the house, with my remaining single pane windows doing much worse, with surface temps around 88, even in deep shade. The low e double pane sliding doors were about the same surface temperature as the ceiling.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  13. #169
    Join Date
    Jan 2009
    Location
    Colorado
    Posts
    88
    Hi,
    What is the recommendeed ventilation with a cement tile roof system. Currently I have gable vents with some soffit venting. Is it possible to install ridge vents with a tile roof?
    Thx,
    Gary

Page 13 of 40 FirstFirst ... 36789101112131415161718192023 ... LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event