1500′ ranch house in the hot, humid south. Gable roof with dark asphalt shingles. Summer weather superheats attic to well beyond outside ambient temps. Radiant intensity challenges even the most well insulated ceiling, requiring 2.5 heat pump to work harder than normal.
Conventional solution: Insulate roof deck and/or power ventilate attic area.
Proposed solution: Provide a weeping system at the roof ridge that keeps roof surface damp (imagine a soaker hose stretched across the ridge and activating only above a preset temp). I theorize that the latent heat necessary to convert the dampness to vapor would remove a substantial quantity of BTU’s, thereby significantly reducing the radiant intensity and subsequent heat load in the attic area.
What are the flaws in this concept?
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Replies
If you're in a dry climate, it might work--same principle as swamp cooler. Waste of water, IMO, but to each his own. May encourage growth of mold and stuff on roof and may leave minerals, depending on hardness of your water. What about light colored shingles or metal (reflective) roofing?
Too hard to get the lawnmower up there.
SamT
Moss.
I read somewhere that applying a radiant barrier, something on the order of a thin sheet of foil, directly to the underside of the roof sheathing would be effective.
Anyone else see that?
Yes, a radiant barrier can be very effective.Look at some of the research from the Florida Solar Institue. Also from Southface (Southpoint?) in Atlanta.JLC has run several article based on teh FSI research..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
There is another way of doing this.And that is using a Green Roof. Actually growing stuff on the roof. And depending on the plants and climates some claim that after it get started you do'nt need artifical watering.But that is not an easy retrofit..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Does maple trees growing in the gutters count?
Only when they get big enough to shade the whole roof..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
(It'll work, at least to a degree, in a reasonably dry climate, but will shorten the life of the roof. Whether it's an efficient use of water depends on the local water situation.)
Flaws -
you use energy to pump the water up there
you keep the roof damp which feeds algae and moss causing other problems
God forbid you have the slightest flaw in the roof system because all this water will cause rot inside
Better choice is to ventilate the roof soffit to ridge vent, then staple radiant bubble stuff ( Astrofoil is one brand) to bottom side of the rafters. This creates a channel for the airflow and radiates heat away from the interior
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One more - the basic principle that any active system which this is, will inherently be more expensive to maintain than any passive system, such as the venting and radiant barrier
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BUt if you are going to an active system, might as well make it a solar water heating system
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You have an excellent idea--take all that heat and use it elsewhere! I have thought about collectors to heat domestic water, but I don't think it would work in central Michigan all that well. Even thought of putting refrigerator coils in a tub of water to preheat water going into water heater, but figured it would be a lot of messing around for little result.
One more point: This probably is a reasonable approach for maybe ten days a year when it really is insufferably hot and the AC can't keep up. What's questionable is it's use on a continuous basis.
reasonable approach for maybe ten days a year when it really is insufferably hot and the AC can't keep up
LoL! I'll guess that SC has a few more of those days than MN <g>.
Ok, searched a bit, looks like they are getting DP of 67-68 to 72-73 from June through all of August. That'll yeild RH above 60 most days (and above 85% most mornings).
It's actually very similar to my climate, other than not having a seabreeze effect as the ydo.Occupational hazard of my occupation not being around (sorry Bubba)
> One more point: This probably is a reasonable approach for maybe
> ten days a year ....I see you're in Minnesota. The OP stated he's in the south. I worked for 9 years in Atlanta. The temp in an attic down there will exceed 120 degrees nearly every day from May to October. I doubt that you can imagine what it's like in late July. You don't stay in an attic long then.George Patterson
Yeah, but the AC is sized for that. Most everywhere you get some days hotter than others, the 95th percentile days when the AC can't keep up.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Strip the shingles, install galvalume metal panels and then either seal the attic and spray foam the rafter bays for a 'hot roof' (best idea), or vent the attic at the ridge (second best).
Guarantee your cooling bill drops by a third.
Well, you're on the right track. I have rigged such a system here, and it has both pluses and minuses.
Your area needs to have low humidity. The secret to your success is that the water evaporate - not drip off the roof.
To get just the right amount of water, you need to have really fine control of the flow; it is surprising how little water you need. I'm still looking for an appropriate valve ... a simple ball valve doesn't work well at that low of a flow.
The soaker hose will have a tremendous difference in the flow along it's length, with most water coming out in the first few feet. This is a matter of simple physics. You can "halve" the problem by feeding the hose from the middle, rather than one end.
You will need something on the roof to keep the hose in place.
The method works extremely well; I seriously doubt that you will want the system to run more than five minutes an hour.
Power vent.
Don't waste water, I couldn't begin to calculate the usage, but its got to be high.
"..in the hot, humid South." This idea won't work where it is humid.
Bill
It'll work, just not as well. The roof is probably 20F warmer than ambient, meaning that the dewpoint immediately adjacent to the roof is will be at most 20F below roof temp. So the water has the potential to lower the roof temp down to ambient.To lower the temp well below ambient does require relatively dry air, though.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
What are the flaws in this concept?
What's the "delta" to the humidity level? If it's above 50%, you won't get very much evaporative cooling at all. Above 75% and it's going to be near nil.
We've had a raft of traveling sales types (not tinkers, but some poor imitation thereof) about town lately trying to sell "porch misters" to spray brick veneer houses on the theory that, if the veneer is wet, it will "cool off."
The problem being that you have to start the water as soon as it is light--this is also when the RH is highest, too.
There might be a tiny benefit in the water making the wet surfaces a trace more reflective, but that's not going to be a lot of help.
Oh, and without a good airflow, the water vapor created by evaporation won't "go" anywhere, which will slow evaporative cooling.
Solar constant is about 240W/m**2, and about 3m**2 per "square." Even if this is attenuated at surface level to about 24W, that's still around 75W per square per hour--or about 245 BTUH per square of roof.
Solar constant is about 240W/m**2, and about 3m**2 per "square." Even if this is attenuated at surface level to about 24W, that's still around 75W per square per hour--or about 245 BTUH per square of roof.
huh? think those numbers are off a bit......
Solar constant is about 1 KW per meter squared at sea level, higher than 1.4 KW/m^2 in space at times.
100 sq ft = .0929*100 sq meters = 9.29 sq meters; so
1 sq of black roofing collects nearly 10 KW!!! = 34, 130 BTU/HR!!!
1 gal di-hydrogen oxide takes 8*985 = 8865 Btus to evaporate, so the amount of water to carry all the solar irradiance away in vaporization is 34,130/8865 = 3.84 gal/hr or about a pint a minute or say 20 gallons for all day.
Power vent at air = sp ht of .24, vent air temp rise of say 15F, so about 13 cu ft of air carries off only about 4 BTUs. 34,130BTU/hr*13 cu ft divided by 4 = 110,000 cu ft/hr of a 1800 cu ft min fan, the size of a fair sized furnace fan.
Say the fan burns 30 cents worth of electricity during a 6 hr hot spell, if water costs you more, use the fan, otherwise a cup or so of water (somebody else already said it was not a lot) a minute is more cost effective.. Cant imagine a cup of water evaporating off 100 sq ft can cause long term roof damage,
There are a lot of second order effects, tha above calcs are for gross worst case and the conversion numbers are off the top of head, so may be an error there too. .
Oops, I was remembering the daily average at the top of the atmosphere, and then left the kilo out of the watts.
Say the fan burns 30 cents worth of electricity during a 6 hr hot spell
Ah, but, I'll bet that SC has longer than 6 hour "spells." Ambient this time of year is probably 90 by 0900 and does not drop below that until after 2100 (I could look up average daily hour values, but I'm feeling lazy <g>) By 0900, the sun has been at the roof envelope for three hours.
But, given humidity levels around 89-95% at 0700, it might be hard, even with 8800 btuh to get any more water to evaporate. (And, if I'm the neighbor, I'm not going to be too keen on anybody fully saturating the local air to whatever maximum #/cf it will take, either <g>).
My experience and measurements are that 20-30º is the attic delta over ambient, too. So, I'm not sure a power vent will help much, either. Pumping volumes of moisture-laden air over the roof framing kind of rubs the wrong way, too--but, that's me (my attic was cool over lunch, repeater showing a mild 128º @ 61% RH).
Galvalume room over foam still is likely best, if under-likely in remodel work.
I've been noodling about, trying to think of some way to harness all of the energy created in all of the attics in Texas (and other locations). We have all these solar "batteries" storing up KW that we have to then "fight" to be comfortable beneath.Occupational hazard of my occupation not being around (sorry Bubba)
I live where swamp coolers work well. The problem with wetting the roof is you will eventually have lime deposits build up and they are ugly as hell on a dark roof. It happens out here all the time when a swamp cooler starts to leak on the roof.
We use a system that vents cooler air from the house into the attic (Updux) and this can reduce the heat radiated down from the ceilings by 5F-10F. Your house isn't pressurized by a swamp cooler so you might try a different approach to forcibly remove the superheated air and reduce the attic temperatures. Solar vents seem like a natural solution that won't waste water or force you to grow a lawn on your roof.
"Solar vents seem like a natural solution that won't waste water or force you to grow a lawn on your roof."
I've considered installing one of those. Do they really collect enough solar energy to be effective heat exchangers? New knowledge is priceless.
Used knowledge is even more valuable.
Do they really collect enough solar energy to be effective heat exchangers?
I'm guessing that he meant the solar-powered roof ventilator fans.
The solar powered units usually get plenty of juice. Their weak points are the fan motors being worn out in long cooling seasons (like either of us have). Also, if they have a rechargable nicad in them (as a bunch used to), the nicad eventually conks out, and the fan won't start because the there's no energy to power the on/off switch.
Still back to the problem of what volume of, let's see, Darlington is reporting 92.8º air will cool an attic probably 110 to 115º? Wow, shows only 45% RH, ah, DP is 67º--better day there than here.Occupational hazard of my occupation not being around (sorry Bubba)
How much insulation do you really have? I think you should have at least R30 in the south.
Just put a big tent over the house.
Put in a few massive towers around the perimeter and stretch a canvas between them so it's a couple of feet above your ridge.
Nothing to it.
:)
Rich Beckman
Coming to the Fest? Don't forget pencils!!!!!!!!!!!!!
As mentioned, I have such a system, and it works surprisingly well here in Reno. But, South Carolina might be something else ....
Instead, may I suggest another method I've seen used to great effect?
Add a second roof. Not a second layer, a second roof.
Imagine laying 2x4's on edge, atop the existing roof, running from ridge to eave. Cover with sheathing, then shingle. Only, be sure to leave the eave ends open, and the ridge area open.
The additional layer will essentially put your house in the shade all the time.
The open channels between the 2x4's will act as chimneys, encouraging steady air flow over the roof.
That's an interesting concept. But the aesthetics would be challenging and the compliance questionable, especially this close to the coast (hurricane prone). Certainly a unique strategy, though.
Thanks.New knowledge is priceless.
Used knowledge is even more valuable.
pull the shingle off . install a light gray metal roof. it would drop your temp 30 degrees
Edited 7/18/2007 8:50 pm by brownbagg
The cold roof works but the radiant barrier would do the same for you, and a lot cheaper and easier
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Actually unless you have a radiant barrier under the top layer I don't think that kind of roof system will have much affect on heating.The top layer will radiate heat into the 2nd layer, which intern will radiate it into the house..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
No - it warms the space there between radiant barrier and sheathing ( which is not great for the shingles, but not disastorus either) and that heating accelerates the airfow from soffit vent to ridge vent, that flow velocity bing a function of the temperature differential between exterior air, and that in the space..No, I can't quote a source, but I recall reading on it years ago.
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Bill, my enthusiasm for this 'permanent shade' is entirely based upon a time when I accidentally made one in the Sahara desert. That shade, along with the airflow, was more effective in stopping the 'ceiling radiation' than 4" of plaster covered styrofoam.
Piffin,
The aero foil appears to be about 4 times the cost of Reflectex and other radiant barriers that I've researched online. Is the aerofoil really that superior to other products? New knowledge is priceless.
Used knowledge is even more valuable.
Reflextric is fine too. I just called it as a generic name, like skil saw indicates most circ saws to many
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That is called a cold roof
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