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  1. #14
    Join Date
    Jun 2004
    Location
    Dallas
    Posts
    179
    Originally posted by newhomeboston
    The ducts are well sealed to my knowledge...mastic and wrapped with foil tape. Have a little leakage that needs to be address around the air handler/acquacoil unit still.

    Old Yeller, still not sure why I wouldn't get the attic heating up in the winter from solar gain and melting snow that will freeze over the eaves and cause an ice dam. Its a low slope roof. I've been thinking roof heaters might alleviate this possibly though.
    I don't have real-world dam experience in Texas, so I am only speaking from knowledge of what causes them, not experience.

    It would seem to me that if you had enough solar gain to go through the snow and warm your attic, it would surely warm the eaves, also. In theory, ice dams are caused by warm roof and cold roof eaves, with the roof being warmed by leaked air from the house AND solar gain. That is why good ventilation is needed- to keep the attic cold in winter. BUT- Your attic should stay cold enough because of the icynene. Your solar gain should warm the WHOLE roof, not just the non-eave part.

  2. #15
    Join Date
    Aug 2003
    Posts
    626
    The reason the code wants vented attics is for ice dam prevention in winter. The code was not written for hot air removal in summer. When you ventilate in winter the idea is that by letting cold air in, one will prevent the thawing of the snow on top of the roof by having the same temperature on both sides of the roof plane. This , of course is most critcal at the roof/sidewall junction where the heat from inside the home is the closest to the roof plane. With the foam both insulating and air sealing well at this location, the freeze/thaw cycle should not be a problem.

  3. #16
    Join Date
    Jun 2004
    Location
    Dallas
    Posts
    179
    uktra- I can hear MLK now.. "We can all just get along!"

    sorry newboston- "inside joke"

  4. #17
    Join Date
    Jun 2005
    Posts
    167
    Maybe I'm just thick, but why won't the solar gain warm up the attic air hotter on the inside causing the snow to melt even if its just below 30 outside where the melt will freeze down?

    Also, isn't my ac going to work a lot harder than it should in the summer as I can envision 150 degree days once it hits 100 around here if it gets to 130 on sunny 80 degree days. That's OK???

    Thanks guys.

  5. #18
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Your attic

    I am in Houston and have never lived north of St. Louis, so take this with a grain of salt. It seems to me when your roof is covered with snow, there will be little radiation reaching your shingles and that will make for a minor difference in winter attic temperature. It seems to me that leakage of house air into the attic would be a far greater threat for creating an ice dam.

    I do think your attic temperatures are way too high and some way should be found to reduce them. I am near Houston and days typically get to 95 degrees outside this week, and my attic temperature measures a little over 110. My attic is the ventilated type, I have added some soffit ventilation but still think I may want more ventilation at the ridge line.

    Another thread tells us at least one or two people have benefitted from power ventilation attic fans. It seems to me one should approach this very cautiously but it may apply in your case. However the general conventional wisdom is these things suck up energy without delivering good results -- for most people. Any other method for moderating your attic temperature ought to be considered first, IMO.

    Hope this helps -- P.Student

  6. #19
    Join Date
    Aug 2003
    Posts
    626
    Heat is transfered in three ways--conductive, convective, and radiant transfer. Hot air has little radiant transfer, but transferes heat by conductive (heat flow from one material to another by contact) and by convective (hot air going through holes from hot to cold). Since you put foam in the attic as insulation, there will be little heat transfer from convection. Attic temps in properly vented attics don't rise much more than + 30 degrees higher than the outside temp. with shingles on the roof. Lower with metal and tiles roofs. The big transfer to the drywall ceilings in rooms below attic is from radiant tranfer. This comes from the sheathing radiating down through the insulation. Different types of insulation are better or worse at slowing radiant transfer. Loose fiberglass is not as good as cellulos. Foam is excellent. Foam also does not lose R value when the temperature drops, like fiberglass does. The roof sheathing temperature can vary more than the temperature of the air in the attic. For example, when the sheathing temps climb from 100 degrees in the morning to 180 degrees in the hot afernoon, the radiant heat transfer increases over 5 times (from 28btu/h-per sq.ft. to 147 btu/h-per sq. ft. This is the dominat transfer of heat to the rooms below. Hope this helps.

    [Edited by uktra on 06-17-2005 at 10:32 PM]

  7. #20
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Those temperatures

    Uktra, do you have any knowledge of the conditions which can raise roof sheathing temperatures as high as 180 degrees? Is this in the Arizona desert by any chance? While mine have not approached that temperature, other people live in rather more intense climates than me.

    I emphatically agree with the things you have said about radiant barrier(RB). Every new house of any quality in S. Texas seems to have RB now. Such a product would basically nullify the problem you allude to, of heating from radiation via the plywood or OSB roof decking. While I don't have handy access to research with numbers, I can feel the difference in my own attic, partially shielded by foil type RB (work in progress). And using a digital thermometer can easily measure higher sheathing temperature behind the RB vs. the unshielded portion. It tells me the RB must be bouncing away a considerable number of BTUs. The best research I have seen tells me a typical result from RB is lowered cooling needs by 8-12% as measured by electric usage. That is modest rather than spectacular, but well worth going after IMO.

    I have heard there is research showing that in many summer attics, after a long day a major radiant heat source for attic air is the top layer of insulation itself. Presumably the insulation has spent all day absorbing BTUs radiated from the roof decking. Seems strange but maybe plausible. Can you confirm this?

    Best wishes -- P.Student

    [Edited by perpetual_student on 06-18-2005 at 12:09 AM]

  8. #21
    Join Date
    Jun 2005
    Posts
    11
    Originally posted by newhomeboston
    Maybe I'm just thick, but why won't the solar gain warm up the attic air hotter on the inside causing the snow to melt even if its just below 30 outside where the melt will freeze down?

    Also, isn't my ac going to work a lot harder than it should in the summer as I can envision 150 degree days once it hits 100 around here if it gets to 130 on sunny 80 degree days. That's OK???

    Thanks guys.
    Radiation coming from our sun is less during the winter months. As the earth tilts away from the sun we go from summer to winter and get shorter days. The atmosphere actually acts as a barrier reducing the amount of radiation reaching your roof. Your attic shouldn’t get much warmer than the outside in the winter because of its heat gain is coming from your home. If it is properly sealed up you might get a 10 degree gain, but that would be rare.

    Snow is almost a perfect “black body”, except in the visible light spectrum in which snow reflects the energy. A black body absorbs radiation, I don’t think you will have a winter ice problem with you roof it your house and attic are properly insulated and ventilated. I think you have the insulation covered and now need a bit more ventilation.

    You can find out more on heat transfer at how things work, or search Google using heat transfer.

  9. #22
    Join Date
    Aug 2003
    Posts
    626
    The temperature of the sheathing depends on amount of rdaiant energy absorbed, and the type/color of material on the roof deck surface. Rdaiant barriers don't nullify radiant transfer, they lower it by reflecting radiant energy to some degree. As far as how much savings are generated by radiant barriers, the big difference is if the ducts and air handler are in the attic. In terms of insulation storing radiant energy, the flow is constant depending on several factors. Heat will dissapate from different materials at different rates.

  10. #23
    Join Date
    Jun 2005
    Posts
    167
    Thanks...so

    a. I will try to work on adding more ventilation one way or another and would still love ideas on which methods could complement the current but poor ventilation from the soffits to ridge vent.

    b. Which is a better rb, installed foil or the metal paints that act as such?

    c. just an fyi for those southerners...snow often blows off the top of a roof and collects on lower areas or valleys as I have in my house. When the house was under construction, snow stayed in the valleys for quite a while while the rest of the roof was clear which is why I am concerned about the roof heating up from sun hitting the shingles exposed not necessarily going through the snow.

    More tests tomorrow to see if I'm getting any outflow from the ridge vent up there and crawling around on the moonscape of foam carefully to look down those baffles and see if they are blocked or not.

  11. #24
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    uktra: very LITTLE heat is transfered through a ceiling via radiation -- maybe 2%!!

  12. #25
    Join Date
    Aug 2003
    Posts
    626
    Cem-bsee--I don't know how you came up with that idea--but I can tell you for sure its wrong.

  13. #26
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    What seems priority

    >>...poor ventilation from the soffits to ridge vent.

    It seems to me there is evidence some of those soffit vents are blocked off. If there is any reasonable way to unblock those, I would consider that an important repair.

    >>which is a better rb, installed foil or the metal paints that act as such?

    Clearly the foil is higher performance, blocking about 97% of the radiation. If you use it in new construction, there is no added labor cost when you have foil backed OSB or plywood roof decking. However stapling up foil as a retrofit to an existing attic, is very labor intensive. Look into the costs and you will probably confirm this is out of sight for your house.

    The spray-on radiant barrier paint from a good manufacturer, will block 75% or more of radiation. Combined with much lower labor costs for installation, this is today's conventional wisdom for existing homes. Do not be surprised if you can get a contractor to quote a job with the spray method, and have trouble getting a quote for stapling up foil. So in your shoes, I would accept the trade-offs with the paint method.

    If you do things just exactly right, and under the right conditions, a power attic fan has helped some people. But for the majority, the item consumes more power than it saves. Conventional wisdom says this is more likely to be a placebo purchase rather than a solution to a problem.

    If you can get any consulting advice from Building Sciences Corporation, which I believe is headquartered close to you, I would trust their opinion fully.

    Hope this helps -- P.Student

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