bpnkrtn
New Member
Posts:60
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| 08 Sep 2011 07:24 PM |
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Howdy. I had a company out to my father's house yesterday to get a quote on radiant barrier in the attic ... OUCH! But the gentleman had an infrared thermometer that "played with" a little, was surprised at the cost of one at Lowes (pleasantly) and got one this morning.
A 4 PM reading on the outside west wall showed 159 degrees and the inside wall was 93 degrees. This is in south west Texas. As I was thinking about some of the things I've learned lurking on this forum (ok, read ... maybe, maybe not learned/understood) I got to thinking about a house with wrap around porches ... I tend to think of them as Australian houses? I suspect that a 10 foot wide porch would keep most all sun from striking the wall of a one story house. This would especially keep the sun off windows and doors. That would seem to offer serious advantages to keeping the house cool in 100+ degree heat ... is that generally true?
Perhaps a big advantage in a climate where one would like the windows open in the fall and spring ... and maybe a few days in the winter. I spent more than a few Christmas school breaks swimming in a tank (stock stock or muddy hole in the ground for those unfamiliar with the term.)
Among the many things I know nothing about is the relative cost of wrap around porches verses 'assumed' energy savings. I would appreciate any thoughts folks might have about this. A link to any specific information concerning the concept, a few hundred years old, would be greatly appreciated. |
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superduty
New Member
Posts:42
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| 08 Sep 2011 09:28 PM |
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The wall you measured had the sun beating down on it. It is amazing how hot such a wall can get. In my old home I had a stucco wall that faced west. In the afternoon and early evening you couldn't keep your hand on it very long - probably similar temps to what you saw. Obviously if the sun is not beating on the wall (if it is blocked by a porch), the temp of the wall would be much lower. |
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jonr
Senior Member
Posts:5341
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| 08 Sep 2011 10:11 PM |
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Planting trees is probably cheaper than porches. And the trees reach over and cover some of the roof.
Say that the average temperature of the outside wall over 24 hours is 113F without shade and 93F with. If you keep the house at 73F, then you need to double your wall insulation to have equivalent heat loss. That too could be cheaper than a porch. Edit note: I'm ignoring windows - they will need awnings or something. |
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bpnkrtn
New Member
Posts:60
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| 09 Sep 2011 12:01 AM |
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Interesting comments ... and they seem logical. One thing that complicates the issue is that I like porches. I find them very peaceful, and maybe a little nostalgic. This has nothing to do with the economics or efficiencies of porches I fully realize.
Thanks for the thoughts. |
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jonr
Senior Member
Posts:5341
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| 09 Sep 2011 09:41 AM |
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Despite my comments, I like porches too. They are also great at keeping water away from the house, even with big winds and driving rain. It's not clear to me how one can protect a house from big tree roots. |
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Lee Dodge
Advanced Member
Posts:714
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| 09 Sep 2011 02:06 PM |
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If you look at the old homes built before air conditioning in the small towns in south Texas, you will notice that both large porches and big trees were used to make the houses more livable through the three warm seasons of the year. Using deciduous trees allows for some winter heating. Visit the Lyndon B Johnson birthplace homestead in Johnson City and you will see an example. You will also hear stories of them sleeping on the porches during the summer nights. Plus porches are a nice place to spend time. Porches on the south and west sides of the house in that area are useful for blocking direct solar radiation, but are generally too hot to use in the afternoon and early evening.
Jonr, you can help protect the house from tree roots by proper selection of the trees. For example, you avoid like crazy those d**n Arizona Ash trees. A good plant nursery should be of help in the selection. |
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Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
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Dana1
Senior Member
Posts:6991
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| 09 Sep 2011 03:04 PM |
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CRRC rated cool-roof mop-on finishes (or just about any white/light paint) can lower the peak temps of walls too, as does the use of somewhat thermally conductive walls of high thermal mass, both of which are old-school tried & true strategies. (Whitewashed adobe, anyone?)
Extremely low-E materials may be very reflective of the incident radiated heat but, can't re-radiate the heat that is absorbed and end up with very high equilibribrium temps. (Bare aluminum or chromium are prime examples.) A finish that reflects most of the incident solar radiation yet can still radiate heat (in the infra-red )as it's temp rises is key. You'll notice that bare shiny metals will give you false-low readings using an infra-red thermometer, since they have much lower emissivity than most other materials use in a house. |
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rbisys1
Basic Member
Posts:142
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| 09 Sep 2011 03:20 PM |
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Greetings,
QUOTE:Say that the average temperature of the outside wall over 24 hours is 113F without shade and 93F with. If you keep the house at 73F, then you need to double your wall insulation to have equivalent heat loss.
Not so fast. Doubling the insulation thickness can also store more heat and increase energy usage. Doubling the wall thickness possibly means a small increase in wall eff. Hardly worth the cost.
Instead use two layer radiant barrier (RB) in the wall, a single layer RB on the inside as a VB and 1/2" z strips horizontal over the studs and then your drywall. Under worst conditions you will only have about 2 btu/hr/sf radiating into the room. I know of no other insulation sys that will even come close to that. And, that is in a 2x4 wall structure.
Use the same sys in the ceiling and your interior temp on a 95 Deg day will about 80 degs with no ac. Windows must be shaded though.
Getting back to porches, I have a south facing porch on a 1300 sf house. It is a sip house upgraded with RB. I been considering using it as a collector and blowing the hot air into the house. Problem is the house is too eff and the porch would add too much energy. It would easily provide all the day time heat and the structure's storage would take it well into the nite.
If I was building new or could redo this house I would use some 2x8 interior walls, install 6" pvc pipes upright, cap and link with tubing that is connected to a radiant sys in the south facing porch. That would give me a couple of days storage.
It has been my experience that only about 5% glass area is needed for solar gain heat in a RB insulated house. |
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bpnkrtn
New Member
Posts:60
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| 09 Sep 2011 03:33 PM |
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Posted By rbisys1 on 09 Sep 2011 03:20 PM
Greetings,
QUOTE:Say that the average temperature of the outside wall over 24 hours is 113F without shade and 93F with. If you keep the house at 73F, then you need to double your wall insulation to have equivalent heat loss.
Not so fast. Doubling the insulation thickness can also store more heat and increase energy usage. Doubling the wall thickness possibly means a small increase in wall eff. Hardly worth the cost.
Instead use two layer radiant barrier (RB) in the wall, a single layer RB on the inside as a VB and 1/2" z strips horizontal over the studs and then your drywall. Under worst conditions you will only have about 2 btu/hr/sf radiating into the room. I know of no other insulation sys that will even come close to that. And, that is in a 2x4 wall structure.
Use the same sys in the ceiling and your interior temp on a 95 Deg day will about 80 degs with no ac. Windows must be shaded though.
Getting back to porches, I have a south facing porch on a 1300 sf house. It is a sip house upgraded with RB. I been considering using it as a collector and blowing the hot air into the house. Problem is the house is too eff and the porch would add too much energy. It would easily provide all the day time heat and the structure's storage would take it well into the nite.
If I was building new or could redo this house I would use some 2x8 interior walls, install 6" pvc pipes upright, cap and link with tubing that is connected to a radiant sys in the south facing porch. That would give me a couple of days storage.
It has been my experience that only about 5% glass area is needed for solar gain heat in a RB insulated house.
If I may ... how did you get your post to show paragraphs!? I can't get my paragraphs to show in a post ... rather annoying ... |
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Dana1
Senior Member
Posts:6991
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| 09 Sep 2011 05:02 PM |
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There is no credible evidence that doubling the R-value of wall &/or roof systems increases average or peak cooling loads, increasing annual energy use in a single family residential building. It's oft-heard yet never demonstrated. It can be true for larger commercial/industrial buildings with lots of interior energy use, typically in more temperate climates than TX where lower outdoor temps can give you some "free cooling" in a lower-R building. But that's literally never going to be the case in a house, unless you're baking bread 24/7 and have 18 Tivos running just in case you wanted to watch way too much TV. ;-)
All reasonably sophisticated building energy use modeling of real designs using site, orientation, and climate-specific data all point in the other direction: Higher-R/lower-U walls reduce the peak & annual heating & cooling requirements of a building. But all heat gained through walls in even a moderate R building is "in the noise" compared to the solar gains through windows. Even the reflected light entering N-side windows can be large if the scenery out those windows is bright & reflective rather than dark/absorptive. Exterior roll-down shades can be worthwhile retrofit (and much cheaper, if less attractive than a wraparound porch and some tall shade trees. :-) ) |
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rbisys1
Basic Member
Posts:142
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| 09 Sep 2011 07:45 PM |
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Greetings,
QUOTE: If I may ... how did you get your post to show paragraphs!? I can't get my paragraphs to show in a post ... rather annoying ..
An: You may and the answer is....I hit the enter key. Hope this helps.
Quote: Extremely low-E materials may be very reflective of the incident radiated heat but, can't re-radiate the heat that is absorbed and end up with very high equilibrium temps. If I'm wrong, please clarify your statement. And what do you mean by equilibrium temps? How can you have equilibrium if the purpose is to prevent a hot temp from radiating into a low temperature area?
You make it sound like the materials is holding more energy than the ambient temp. That is simply not the case and I would like to know where you got that info.
My experience is this: If you have and interior drywall wall temp of say 75 and exterior temp of zero with 2 layers of foil the !st air space temp, between drywall and 1st foil will be the 75 Deg. The foil will be 75 degs minus 1-2 degs (maybe). The 2nd air space will be zero as will be the 2nd foil and the 3rd airspace.
Marks Mechanical Eng Handbook states that the absorption and emissivity is equal. That's one reason FG and cel is such a poor insulator. Their base materials absorb/emit about 95 % of the energy. |
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rbisys1
Basic Member
Posts:142
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| 09 Sep 2011 08:46 PM |
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Greetings,
Missed this one.
QUOTE: There is no credible evidence that doubling the R-value of wall &/or roof systems increases average or peak cooling loads, increasing annual energy use in a single family residential building. It's oft-heard yet never demonstrated.
ANS; It is demo'd every time someone adds material. And where there's smoke, there must be fire.
Years ago I was siding a house for lady who lived in her house for 30 yrs. I tried to get her to let me put in a RB in her attic because the house was hot. When she saw the difference in just wrapping the house with RB first before the siding she said to put the RB in the attic. She had at least 2 feet in most areas. So I install the RB. This was on Friday. When I came back Monday she was a very happy person. She said this was the first time in 30 years the house was comfortable. Seems her husband kept adding to the attic thinking that it would help. So if the house is still miserable after 2 ft of attic FG then I guess we can assume the FG was about useless.
Once again if you have an "R" 19 FG ceiling and the roof is unshaded on a 95 deg day the interior temp is going to go up to about 100 deg +. The ceiling temp is going to be about 110 Deg and if the floor temp is 75 deg then the ceiling will radiate about 37 btu/sf/hr. That's calculated from the data in Marks Mech Eng Handbook. Now if you add a second "R" 19 to the first the increase in eff is so low as to make the add'l FG meaningless. How ever you have added more energy absorbing mass plus the add'l amount of moisture that is in that mass, and you think that's not going to cause problems.
You must keep in mind bulk insulation manufacturers state that weir prods only SLOW DOWN the heat transfer, they just don't tell you how much, or how little.
Now just in case you missed it. Here's a simple demo of how fast energy passes thru a 3.5" FG batt. Take a 15" sq pc of FG with foil or plain and use a 15" sq pc of kitchen foil. Support thr FG with the foil facing down on a card board box or similar with one side cut out so that you can reach in and touch the foil. Hold a 250 Watt heat lamp about 8-10' above the FG.
Turn light on and after 15 SEC reach underneath and touch the foil. Try again in 30 SEC, one minute. Now use 2 pcs of foil separated by about 3/4" and do the same. With the foil you can ck for any temp diff in one hour, 2 hours etc.
The reason foil is used with the FG is because the foil will hold some of the energy that passes thru and if wasn't there the energy would radiate into the space below, just like in a ceiling. |
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Alton
Veteran Member
Posts:2164
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| 09 Sep 2011 09:07 PM |
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Hitting my Enter Key gives me a paragraph while typing but when I hit the submit button, the formatting disappears.
Are a lot of people having this same problem. I think the administrator should take a look at this problem.
I made three paragraphs while typing this. Now after submitting, there is only one. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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superduty
New Member
Posts:42
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| 09 Sep 2011 09:59 PM |
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Posted By Alton on 09 Sep 2011 09:07 PM
Hitting my Enter Key gives me a paragraph while typing but when I hit the submit button, the formatting disappears. Are a lot of people having this same problem. I think the administrator should take a look at this problem. I made three paragraphs while typing this. Now after submitting, there is only one. if u r using internet explorer. Go to tools and then hit compatibility view. That helped me make paragraphs. |
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bpnkrtn
New Member
Posts:60
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| 09 Sep 2011 10:08 PM |
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Posted By superduty on 09 Sep 2011 09:59 PM
Posted By Alton on 09 Sep 2011 09:07 PM
Hitting my Enter Key gives me a paragraph while typing but when I hit the submit button, the formatting disappears. Are a lot of people having this same problem. I think the administrator should take a look at this problem. I made three paragraphs while typing this. Now after submitting, there is only one.
if u r using internet explorer. Go to tools and then hit compatibility view. That helped me make paragraphs.
Yes I have the same problem ... using Safari on a Mac. It's a bit annoying. I did ask about it earlier today on the appropriate sub-forum but have not heard back. |
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Alton
Veteran Member
Posts:2164
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| 09 Sep 2011 11:25 PM |
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Superduty,
I did as you said.
I now have paragraphs.
Thanks. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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jonr
Senior Member
Posts:5341
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| 10 Sep 2011 09:54 AM |
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I've been told that if you have some type of barrier that forces tree roots to go downward, then they don't turn back and head upward. Ie, if you built a 4' vertical, underground wall between a house and a tree, the roots would never be higher than 4' under the house. Is this true (tree roots never grow significantly upward)?
Bare aluminum is a poor example because the surface quickly oxidizes to aluminum oxide, which has good thermal performance. How thick that layer has to be is not clear to me (anodizing works well, but is it required?). |
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bpnkrtn
New Member
Posts:60
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| 10 Sep 2011 10:49 AM |
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Posted By jonr on 10 Sep 2011 09:54 AM
I've been told that if you have some type of barrier that forces tree roots to go downward, then they don't turn back and head upward. Ie, if you built a 4' vertical, underground wall between a house and a tree, the roots would never be higher than 4' under the house. Is this true (tree roots never grow significantly upward)?
It would vary with specie of tree but in general roots will respond to a moisture gradient and/or nutrient gradient meaning they will grow to water and nutrients that are needed for growth and maintenance. |
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Dana1
Senior Member
Posts:6991
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| 12 Sep 2011 03:08 PM |
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For a low-E material such as aluminum to reach equilibrium under a radiant heat flux such as direct sunlight it's temperature will be much higher than the surrounding air (but much lower than the 5700K of the sun, eh? ;-) ) and higher than high-E high-reflectivity materials such as those used in cool-roof finishes. At equibrium it's combinge radiated and convective cooling rate will equal the incident radiant flux, but with insulating films of air around it the convective portion is pretty low, particularly in a horizontal configuration, and that temperature will be above that of something tuned for high solar reflectivity combined with a moderately high emissivity. The equilibrium temperature above ambient for any material can be calculated/modeled for a particular radiant flux and air temperature AND roof angle with high precision if it's reflectivity & emissivity are known, but for ease of apple-to apples comparisons in building materials & finishes the shorthand for the reflectivity vs. emissivity performance enshrined in building codes is a simplified "Solar Reflective Index" per ASTM E1980 standard test conditions SRI (higher==better at rejecting heat). The Lawrence Berkeley Nat'l labs has a handy spreadheat calculator for SRI that allows you to enter the emissivity & reflectivity numbers directly for comparison, downloadable from several places, including here: http://coolcolors.lbl.gov/assets/docs/SRI%20Calculator/SRI-calc10.xls The reflectivity of fresh bare aluminum is ~ 0.97, and it's emissivity is on the order of ~0.03, but aged numbers can be around 0.85 & 0.05, with an aged SRI of ~90-95. Even if you presumed an aged reflectivity of 0.90 and an emissivity of 0.05 it's SRI is only about 100, a figure that is achieved by a number of high-E white paints: http://www.coolroofs.org/products/results.php?keyphrase=&select_type=all&market_type=&company_name=&brand=&model=&select_color=all&min_solar=&min_solar_3yr=&min_therm=&min_therm_3yr=&sri_init=&sri_3yr=100&slope=&crrc_prod_id= The specified ambient air temp for ASTM E1980 is 310K, or ~ 100F. Using the LBL calculator a fresh bare-aluminum roof would stabilize at a temperature of 39C, or ~ 102F- pretty good. An aged aluminum roof would hit around 110F on a 100F day, which also pretty good, but not better than best-in-class cool roof materials. Under a hot roof deck the RB foil can approach but not exceed the temperature of the interior of the roof deck's temp. (The roof deck being the radiating element.) But peak roof deck temps can already be tens of degrees cooler if a CRRC rated cool-roof material is used on the exterior, even one with an SRI of only 30. At lower cost & effectiveness than an installing interior radiant barrier. Per CA Title 24, RB can be used in lieu of cool roof materials where a different aesthetic is demanded, but the code's SRI constraints are not so severe as to demand white-only roofs except on low-pitch flat roofs. Nighttime temps of high-E roofing material can be even 10F below that of the ambient air, the delta-T often limited to/by the dew-point of the outdoor air. In cooling climates this can lower the average as well as the peak sensible cooling loads. |
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rbisys1
Basic Member
Posts:142
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| 12 Sep 2011 05:29 PM |
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Greetings,
Who's talking about alum roofing. No one can afford that.
If you want to know the emissivity you can go on line to a Mech Eng Handbook web site or just look up material emissivity charts.
Aged? give me a break. Pure alum maintains its values. Alloy alum is not as reflective and if there material reaction then that could and probably would affect its performance. You're the 1st person in 30 yrs that's come up with that diversion.
You're beginning to sound more and more like a FG dealer. They start out with yes that's true, BUT........
Who cares what the color of the roof is if your reflecting 97% of the energy.
The sys I recommend emits about 2 btu/hr/sf. Which one of the sys you propose can match that? Actual case. NONE
Which home with your proposed sys can max out the interior temp at 80 degs on a 100 deg day? Actual case. NONE
By the way dana1, just what is your professional background. |
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