leonmeyer
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| 11/20/2008 1:11 PM |
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I’m in the process of remodeling my home. I live in Houston Texas and I was curious about re-insulating my attic. What is the difference between Radiant Barrier and Spray Foam? Is one better then the other? If I use spray foam and my roof springs a leak will I rot out my roof before I find out about it? Any help would be greatly appreciated. |
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aardvarcus
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| 11/20/2008 3:57 PM |
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There are three different types of heat transfer, convection, conduction, and radiation. Spray foam helps fight Convection and Conduction, while a Radiant barrier Helps fight Radiation.
If you live in a AC dominated climate, a radiant barrier could help more than anything. There isn't really a R-Value associated with a radiant barrier though, so it can be tricky finding the best barrier. Just because it looks reflective, doesn't mean it will reflect at most thermal wavelengths.
If you are more worried about air sealing, spray foam will be your best bet. I would be wary of insulating the underside of your roof leaving no air gar, it can cause your singles to overheat and need replaced much sooner.
Your best Bang/Buck will probably getting more cellulose/fiberglass installed on the attic floor, and laying thin sheets (1/2" or less) of board foam reflective side up on top of your new insulation, taking care not to compress the insulation. That gives you R-Value and Reflective all rolled into one. |
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wes
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| 11/20/2008 5:11 PM |
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Actually, I would disagree with aardvarcus' statement concerning spray foam insulation on the underside of your roof sheathing. You are in a very hot, humid climate in south Texas, the last thing you want to do is introduce extremely humid air into your attic space. Condensation forms on your rafters (mold, rot, etc.) then falls onto your cellulose/fiberglass insulation, causing it to become wet, settling, and losing its insulation value. A sealed, conditioned attic space is a far more sound approach, and the actual shortening of a shingle's lifespan is minimual. if any at all. Many shingle mfg. now warrant their products the same either vented or non-vented.
The overall positive aspects of sealing your attic far outweigh any potential shingle concerns.
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Wes Shelby
Design Systems Group
Murray KY
wandr@ainweb.net |
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Alton
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| 11/20/2008 8:10 PM |
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Wes,
I agree with your view. The best reference for this type of question would be Joseph Lstiburek book for the applicable climate. |
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Alton C. Keown
Residential Designer and Construction Technology Consultant
Auburn, Alabama
E-mail: alton at auburn dot edu |
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FlaICF
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| 11/21/2008 8:35 AM |
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| Spray foam is the way to go, Icynene, Demelec(sp), etc. Sealed attic is great. I agee with wes on this. Biggest problem will be access for application of spray foam in a remodel situation on an existing structure. Pick best method you can to advance the cause of improving your energy efficiency. |
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ReadyToRetire
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| 11/21/2008 4:29 PM |
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Wes, Alton,
My concern with spray foam under the roof deck is detecting the leaks when the do develop. With a traditional deck, I have that handy brown spot on my ceiling to let me know that I have a problem. If the truss and deck are sealed in foam, won't that make it unlikely to notice a problem before it has become serious?
Or am I worrying about the monster under my bed?
VERY respectfully,
Larry
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aardvarcus
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| 11/22/2008 7:49 PM |
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The humidity in the air is not going to condense in your blazing hot attic. Your vented attic will still be at least 20 degrees warmer in the sun than the outside air. I have been in many vented attics in houses 50 years old or older, and have seen no mold or rot on rafters, nor any damage to fiberglass insulation, with the exception of houses with serious roof problems. The real question is on a 90 degree day, do you want to insulate against a 110 degree attic or a 140 degree roof deck(assuming you have light tan/grey shingles)? Also, you will have to insulate a good deal more area, all roof planes and all side walls. If you increase your surface area by double, you will have to insulate double the R-value to get the same heat loss as before.
My current house has a 2" air gap between the roof deck and the R-Max board insulation, with powered fans constantly blowing air through the gap when the sun is out. I have routinely inspected this area, and have seen no signs of condensation damage. However, on a sunny day if you go up to the gable vent, it feels like a hair dryer with all the hot air coming out the top, even on a cool day. Also, the R-max acts as a radiant barrier.
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CFL-ICF
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| 11/25/2008 10:04 AM |
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just use open cell foam. the cost of closed cell vs open cell is not worth the extra cost for the small amount of extra insulation claimed. nice an thick like 4-6 inches. open cell will also let you know when/if there is ever leak and get rid of that monster under your bed. :)
when you do goto redo your roof. use a peal and stick instead of the paper. have a nice day! |
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aardvarcus
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| 11/25/2008 10:29 AM |
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Posted By CFL-ICF on 11/25/2008 10:04 AM
just use open cell foam. the cost of closed cell vs open cell is not worth the extra cost for the small amount of extra insulation claimed. nice an thick like 4-6 inches. open cell will also let you know when/if there is ever leak and get rid of that monster under your bed. :)
when you do goto redo your roof. use a peal and stick instead of the paper. have a nice day!
Considering the R-Value of open cell insulation is about R-4/inch, 6" of open cell insulation will give you an R-Value of 24, which is not acceptable.
Do not expect water to freely run through the open cell foam, especially if it is just a small leak. It will take a lot longer for this leak to dry out with the limited airflow that will make it's way through the foam, but the roof heating up in the sun should eventually dry it out.
Peel and stick is awesome, my dad has it under his tile roof, but you still have to be wary around flashed areas, his roof leaks a little around his step-flashed chimney.
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CFL-ICF
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| 11/25/2008 10:44 AM |
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r-24 is not acceptable???? hmmm interesting. its about air and vapor exchange. not some R-value. going from say r-24 to r-48 there is not much gain there. so what ever r-value you think is acceptable you really need to re-examine that line of thinking.
there is a diminising return on r-value. so whats more important is stopping vapor and air exchange into the attic space to begin with.
just make sure roof is in good shape before hand. the foam will actualy greatly improve the strength of the roof. |
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aardvarcus
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| 11/25/2008 6:53 PM |
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Here, I will explain my reasoning.
Let's say you have a 1500 ft^2 house, 30*50 with a 6/12 pitch, gable ends on the 30' side in a typical 90 degree day in Texas. You are going to condition it to 80 degrees inside.
A spray foam roof deck insulated to R-24 will give a U-Value of 0.04167, the roof deck and gable wall area will be 1902ft^2, and the temperature of your roof deck will be about 140 F in the sun. Your BTU loss/Hour=U-Value*Ft^2*Delta_T=0.04167*1902*60=4755 BTU/Hr
A attic floor with blown cellulose to R-48 will give you a U-Value of 0.02083, the attic floor has an area of 1500 ft^2 and the temperature in a vented attic will be 110 Degrees. Your BTU loss/Hour=U-Value*Ft^2*Delta_T=0.02083*1500*30=937 BTU/Hr
Yes, the foam will completely seal the attic space, however you are not going to have a lot of air blowing through about 14" of cellulose. At that point, you are just splitting hairs. Also, the cellulose is still going to be cheaper.Lastly, yes there is a diminishing return on R-Value, so I guess we should stop after just one inch of insulation, because the second inch will never give the same returns as the first. |
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wes
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| 11/25/2008 7:47 PM |
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Guys,
I don't think we ever convince aardvarcus of the error of his ways. And I certainly don't want to get into a battle of Delta T's. I speak only from my experiences as I make my statements about conditioned attic space. And
it seems that I'm not alone in my beliefs.
By the way, aardvarcus, I have seen dew form on the top of attic insulation in the middle of the summer. Don't believe that just because it's hot, that moisture isn't present. Just as importantly, think what those wild swings in temperature and humidity levels is doing to your roof structure. A stable environment means are more stable roof structure. Less stress means a longer lifespan, with less likelyhood of damage to your roof. Now I can't give you a mathmatical formula for this last statement, but it really does work that way.
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Wes Shelby
Design Systems Group
Murray KY
wandr@ainweb.net |
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aardvarcus
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| 11/25/2008 8:34 PM |
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Ok, you all are right and I am wrong. Don't let my fancy equations, and numbers dissuade you from what you believe.
Just answer me this-
I know that just because it is hot that there is still moisture present. However, I don't understand where in the blazing hot attic that the moisture is going to hit the dew point and condense. The only time I have seen attic condensation in the summer is when there have been problems with the insulation and AC is leaking in, cooling a localized area to the dew point.
Aren't your rafters and roof deck still outside of the insulation envelope? No matter where you insulate, with the exception of on top of the roof deck like they do timber frames, the insulation is not keeping the rafters and roof deck from being exposed to the outside elements. Your roof and rafters are still outside, and still subject to wild temperature and humidity swings. I admit that if your roof system was truly (completely) insulated, it will last longer, however insulating it out of your house is not going to help it at all.
If I am incorrect, please just explain it to me, show me your calculations and data, and I will openly admit I was incorrect.
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heatoftheearth
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| 11/26/2008 10:09 AM |
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hey leonmeyer you still there?
ardvarc is correct in his definitions and calculations regarding heat transfer
for your case do you use your attic for storage? can you walk up there ? if you have no functional use for the space his bang for buck is best
if you use the space its not because unless you biuld another floor structure on top of added insulation (costly)
i think spray foam as a product is great in many applications i wouldnt worry about leak scenarios
if your doing renovations ,having a quality roof install is your best combat of leaks,after that visual inspection of the roof should reveal when its time for repair |
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leonmeyer
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| 11/26/2008 10:31 AM |
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| I would like to thank you all for your input it was very informative. I will have to some reading on it for the climate that I’m in but I think the radiant barrier and blow in after will give me the result that I’m looking for. Once again I would like to thank you all for the information and look forward to hearing from you again when I have another question (Trust me I will have more as time goes on) |
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RockStoneSteel
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| 12/04/2008 11:35 PM |
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I live near Fort Worth, TX and recently went through the same building decision process this past year. I built a 2,700 sf ranch using ICF and a conventionally framed roof. I ultimately decided to use the open cell spray foam on the underside of the roof and create a sealed envelope. It worked out wonderfully and I have no regrets. I did not use any radiant barrier and I must admit the attic stays very cool in the summer and so far has maintained a warmth so far this fall. The attic stays within about 10 degrees of the interior temperature of the inside of the house. It never reaches the outdoor ambient temperature, except on evenings where the outdoors cools down rapidly and the house/attic hold some heat and lag slightly. But that is the nature of a well insulated home; the temperature is very stable with only slow fluctuations.
I think there are multiple ways to build an efficient house that will provide only minor variations in that efficiency. My sealed and floored attic is a great bonus as the consistent temperatures make it excellent for storage. Plus, if I ever want to finish it out with a big game room or media room, it is already insulated. I would only need to finish out the walls.
As for shingle life, I have the Class 4 wind resistent shingles which provided for a HUGE insurance discount here in Texas. The concrete ICF walls provided for an additional discount as they are classified as masonry construction. So the home is very weather resistant, and I believe the foamed roof decking provides a significant additional measure of racking strength for the roof.
One change I may make to my house is to insulate the ceiling. Currently it is just sheetrock between the air-conditioned living space and the sealed attic. I am guessing a barrier of insulation on the ceiling would make this home all that much more efficient. I did have them foam the garage ceiling and around the attic stairwell. But for the ceiling I would probably use cellulose or a batting with foil/radiant barrier backing. Another inexpensive option would be just an inch or two deep layer of spray foam.
From everything I've read, the foam significantly outperforms the radiant barrier when it comes to lowering the heat in the attic and house. With a vented attic (just radiant barrier), you can pretty much expect the temperature to stabilize at the ambient outdoor temperature. In Texas, that is not good.
One drawback of the foam is the expense. It cost $9,000.00 to foam the entire attic and garage ceiling (I think it came to about 4,500 sf when counting over the living area, 3 car garage, two large patios, and large eves). However, the cost of that amortized over the 30 year loan is probably more than offset each month by the money I save in energy costs. For a remodel, that may be a bit more difficult to afford.
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RockStoneSteel
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| 12/04/2008 11:38 PM |
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leonmeyer...
I responded to this thread in a separate message below. I wouldn't worry about the rotting roof. Read about my foamed attic in the separate message, and let me know if you have any questions. ...although you may have already had the work done.
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Jelly
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| 12/05/2008 8:25 AM |
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| RockStoneSteel, I'm on the side of sealing the attic and insulating under the roof deck, but you said you wished you could go one step further and insulate the ceiling. If you were to insulate the ceiling from the attic space, wouldn't you risk totally isolating the attic and therefore possibly getting a mold issue? The sealed attic needs to be conditioned, that means being connected to the conditioned living space. But I may have misunderstood your post. |
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engineer
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| 12/05/2008 7:48 PM |
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Sprayfoaming the underside of the roof sheathing confers additional advantages not yet discussed here
1) Attic ductwork is moved into the conditioned envelope, resulting in substantial savings. One can verify this using Manual J load software available from several sources can be 15% of cooling load. On an older house (ducts in hot attic) I measured a 6 deg differential between supply and discharge through registers distant from the supply. That's 30% of the cooling lost in a typical 20 deg system. The ductwork was R6 board and flex, newly installed.
2) Closed cell sprayfoam is very strong and rigid - adds substantially to strength and rigidity of roof system.
3) Sealing attic makes it more liveable for storage - not too hot or cold
4) Sealing attic zeros out the often substantial infiltration / exfiltration of conditioned air through every plumbing, electrical vent, can light and drop stair ceiling penetration.
I tucked a thermometer beneath a shingle over an uninsulated eave on the west side of my roof last October here near Jax, FL. Temperature indicated 180 in ambient air less than 80. I wonder what it would read in July? I'm very glad that heat is kept out of my attic.
I'm told that shingle manufacturers now understand the popularity of insulated roof decks and certify their products and warranties hold up despite temperature concerns. |
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Without data, you only have an opinion. |
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RockStoneSteel
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| 12/05/2008 10:29 PM |
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I'm not sure I agree with the statement that a "sealed attic needs to be conditioned". It's not necessary to actively heat and/or cool that attic space. In fact, counting the space over my garage would more than double the current a/c load for my living space. That's not practical and would defeat the purpose. I see the primary purpose of sealing the attic as the first line of defense at keeping the heat out of the living space.
On the other hand, I suppose one could consider it "semi-conditioned" since there is just a sheetrock barrier between conditioned and unconditioned space. There is no formal "connection" between the living space and attic where air can pass freely. Because of the typically dry climate here near Fort Worth, I'm not too worried about mold. More than mold, I'm mostly worried about the air simply getting stale or dusty in the attic. I have thought it would be nice to have some type of louvered blower for occasional use to draw fresh air into the attic. Perhaps an intake on one side of the attic and exhaust on the other side. However, the concern there is, it's nearly impossible to find a ventilation fan that is sealed/insulated when not in use.
With just the sheetrock between the living space and attic space, and a slight temperature variation between the two, it just seems that insulation in the ceiling would help seal the cool air into the living space during these hot Texas summers. On the other hand, perhaps that lack of insulation currently permits heat to rise from the living space into the attic where energy is not expended in removing it. This is one of those situations where I wonder if it is splitting hairs to go to the n'th degree in insulating a house. I think time and experience will tell what the best techniques are in applying these various building methods. I also believe it will vary significantly based on climate.
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