Call me crazy but I am considering the effects of installing a complete second roofing system above the current roof, with an air space between them much like dual pane windows. In a hot climate such as Phoenix, this would eliminate direct sun and a bunch of heat on the original roof making the house cooler, right? The air space would have to be well ventilated to remove most of the hot air build up. I have seen trailer houses parked under what looks like a metal building with just the roof and no sides but never a regular house. Naturally the second roof would have to be inconspicuous as possible. Does anyone know how effective this would be?
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it could be effective, depending on your system.
Oh..I forgot to call you crazy...I'm not sure but that might bring out the posse that doesn't exist.
Edited 10/7/2009 9:07 pm ET by jimAKAblue
Look at this roof system.
http://www.greenbuildingadvisor.com/cad/detail/unvented-roof-cold-climate-metal-roofing-over-exterior-rigid-foam
It would be very effiective. The same method is used to prevent ice dams in colder climates, called a cold roof, and the principal is the same. The original roof has no practical way of venting soffit to ridge, so a second roof is installed on top with proper ventilation. I imagine the same pricipal applies in hot climates to keep the house cool.
~ Ted W ~
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no your not crazy. Dad & I talked about this a looong time ago but never did any experimetns on it (I drew up some details on paper back when I used paper). Cold roof is the term to google.
Always wondered why that wasn't seen more in use.
How much space would you be placing between the roofs?
Not sure, originally thought of enough space to crawl thru if needed, but in reality, probably 6 or 8 inches would be enough. Those "cold" roofs mentioned above look interesting and don't have much space, but a lot of insulation. I think getting the hot air out of the space the quickest way possible would be very important also.
I think the space between roof decks is only 1-1/2", or 2x4s laid flat. Isn't it?~ Ted W ~
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Code wise, officially, I don't know. But if using a 2 x 4 laid on it's side as joists and placed directly on the original roof, would work, then that seems to be a real easy solution.
2-4" would be adequate. This system would funtion for you in two ways. First, you are reflecting heat away from the house if you use a bare AL or galvalum metal, or other product in white colour.Second, you are allowing acquired heat to vent off with the ventilated space. How deep that is required would depend somewhat on style of home and how much open vent in and out you can provide - IOW, there is no gain going from one inch deep to six inches deep if you still only provide 1" vent in and out.
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>2-4" would be adequate.<
would you think a 1x furring strip would give enough space for good airflow?
Normally, yes, but it depends on all the other factors, and I think he is in hell or arizona or some such hot place whjere more space wou;ld help disperse the BTUs before gained by under-roof
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>but it depends on all the other factors,<
aka choke points
Wow, I came back to a bunch of great comments on having a second roof. The best news is that nobody has said "Heck no, don't do it". And I have seen those tents with the fly on top and been told they are very effective, also have seen some older Land Rovers with a second roof-had about 1/4 inch between the roofs.While I don't actually have that house in Phoenix yet, all of the houses in the area I plan to buy into, have a fairly low pitch. Most of the houses have asphalt shingles but a few remodeled houses have metal. Many of the comments here have suggested metal and I like that.It sounds like: Extending the overhangs and using large vents (same size as the space between the two roofs) in the sofet will be effective and also hide the two roofs to a large degree. The vent along the ridge would also need to be equally as large. It seems like I have seen a large ridge vent being shown in Fine Homebuilding sometime in the past.Thanks everybody, I appreciate all the input.
the devil is going to be in the details on how you vent the one and not the other at the eaves. I'm thinking to get it right you'll be pulling all the soffits off along with the facia to allow room for working the sealing off of the existing attic/roof and then creating a path from the vent to the channels beneath the new roof.
My thumb suck is that 1x (3/4") is probably too thin -- will reduce your airflow to probably about 1/4 of a 2x space. But I don't know what fluid equations to plug in there to say with any certainty.Also, on anything other than a simple gable, remember that you need special fiddling to obtain airflow in the "odd" spaces near a hip end or a valley.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
and one curl of the underlayment that I'd think you'd need on the main deck would effectivley block the entire gap using 1x. Would have to be 2x.
I'd just leave the existing shingles, if they're in good shape, and you shouldn't need an underlayment otherwise. But you're right that minor variations in the surface -- warping and unevenness -- would be enough to reduce the 3/4" down to 1/4" or so in spots.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
If i was going through the trouble to add all that on top of the old, I'd do what it takes to be sure what's existing is in good shape and won't be needing any attention any time soon. And the old shingles are adding weight to something that may be at it's limits of support. Less weight = better IMO.
Practical considerations aside, looking at jobs with homeowners I've found they really have a hard time with renovations where there are still shingles on an old roof. It seems to be something that builders are readier to accept than they are. Same with flooring over old Lino. When you remove a layer and come across it owners often react like you found the past builder buried his wife in the foundation.
It is a very good idea. I have seen a number of Australian houses in architectural periodicals which use that strategy. Several have been featured in Dwell Magazine. The roof seems easiest to incorporate into modernist designs. It might be harder to fit into the styles more commonly found in Phoenix.
I agree, you wouldn't want it to look real obvious and that might be the trick to the whole thing. In the area I am interested in the houses were built in the late 1950's and 60's. Most don't have much pitch.
I'm sure I've seen it written up somewhere, but probably 20 years ago or more. I think it isn't done more because other techniques (mostly versions of the "hot roof") are about as effective and generally cheaper. But it would probably be cost-effective in the right circumstances -- eg, on an existing structure where you needed to reroof anyway, and there weren't any odd roof angles to worry about.
One thing to consider is how you vent the thing without allowing entry to birds and bugs. Also, you need to be confident in the fastening, to avoid blow-off in your local worst-case conditions.
Screw 2x4s to your existing roof, on edge, over the existing trusses/rafters with 6" screws. Then sheet it and roof it like it was any other roof. You'll just wind up with relatively deep fascia.
I guess you could do 2x6 too. Then you'd have REALLY deep fascia.
To vent it, cut a strip out of the original roof sheathing along the eaves and allow it to draw through soffit vents. Big ones. With screens.
What are you going to put at the top to vent, a cupola? It would work well but it might look a bit odd.
I used to do low income rehabs and we'd strap over rotten old roofs and then put metal roofing on it, leaving out the foam closures at the ridge. It amounts to the same thing as you're thinking of. A lot of air draws through that system on a hot day.
j
This project is starting to sound much easier.I was thinking of a ridge vent from one end to the other.I am a little worried about all the additional weight on the old structure.
<I am a little worried about all the additional weight on the old structure.>Is your old roof compromised in some way or not up to standards in the first place? You could strip the old roofing off first. Asphalt shingles? It wouldn't be a bad idea to swing the plan past your local inspection folks before you start. Do a nice drawing of how it would all be. j
Actually, at the present I am renting and looking to buy a fixer upper. One of the things I will be looking at closely is the structure of the roof. Your suggestion about the inspector is a great one, thanks.
It can be easy or fairly hard, depending....I'd need to have a good solid idea of what style and systems you have in place NOW to recommend how to add this to make it work and look decent. Any advice without those specifics is generic and shooting in the dark.
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He could eliminate the larger fascia by creating a wider soffit.
<He could eliminate the larger fascia by creating a wider soffit.>That's true on the eaves but might be tricky on the gables. Guess you could make the whole roof into one big eyebrow. j
"To vent it, cut a strip out of the original roof sheathing along the eaves and allow it to draw through soffit vents. Big ones. With screens."How do you know what sort of house and system he has now?The idea of a ccold roof is to separate the twin envelopes, but you just suggested combining them by cutting thru that roof
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<How do you know what sort of house and system he has now?>That's a good point. I was assuming a gable or cottage style with overhangs of some sort that could be used for soffit venting. Turns out from OP's later post that he hasn't got any house at all right now - it's a hypothetical question. <The idea of a ccold roof is to separate the twin envelopes, but you just suggested combining them by cutting thru that roof>Also true, although in the case of a non-habitable attic - let's say with loose celulose or f-g in it, it would already be outside the building envelope and would need exterior venting at the eave and ridge anyway. The second envelope, also outside the habitable area, needs venting too. Could the two vents not draw through the same port? Air from the two spaces would seem unlikely to mix .Topside venting seems more important to separate. Original roof - let's say a simple gable, could vent through gable end vents and vented soffit at the peak. New 2nd roof vents through a ridge vent or pigeon house or whatever. OK, probably needs an engineer to detail it but engineers need suggestions too. j
Had one of those in the town I grew up in, out in the desert. Works well.
IIRC, It used Rafters sufficient to support the top roof, but laid on edge on the lower roof, which was shingled.
A four hip roof, it had a cupola at the peak.
The tails were long enough to end level with the original lower ones.
Looked kool/wierd up close.
SamT
A Pragmatic Classical Liberal, aka Libertarian.
I'm always right!
Except when I'm not.
very good. This is often done and is called a 'cold roof'
Have you ever camped out with a modern tent?
Remember the tent 'fly'? It is the secondary that does the same job, keeping the interior far more comfortable
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Hi dutchoven,
You're as crazy as I am......hate the heat......a second roof is wonderful and there are many ways to go about it. Personally I'm convinced a hot roof is fine as long as you are using a thermal break and metal roofing.
I would start with a tear off of the existing roof material and add foam sheathing equal to the number of years you plan to be in it.....the higher the r value, the longer it takes to payback so figure about r6 per inch, then top it with 1/2" sheathing, 30 lb felt and "metal" roofing. Even in your climate I wouldn't consider venting as long as I had a good thermal break provided by the foam and a metal roof which will reflect the majority of the suns infrared heat.
To recap.....if you're planning to be there for 15-20 yrs put 3" of foam etc. The foam, extra layer of 1/2" sheathing and metal roofing won't add up to more than 2 layers of asphalt shingles so additional support should not be an issue if the house was built right to begin with.
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What you describe is adding insulation to the existing, which has value on it's own, but the vented plane is a separate thing that has it's own separate value alone.The two combined could be a valuable upgrade to his home. I await details of existing to start speculating best way to do this
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but the vented plane is a separate thing that has it's own separate value alone
Hi Piffin,
Agreed.....hey we can get a radiant barrier in there can't we........
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The sheathing you would want to use over your furring would have the foil face on the underside, facing the air gap. This lessens transmission of radiant heat from the outer shell.
Tu stultus es
Rebuilding my home in Cypress, CA
Also a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
Wouldn't HAVE to be, but it would improve the effectiveness of the sandwich.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
If the source of the heat his is trying to deflect is the sky, he wants the foil facing UP, not down
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Ah, yes. And shoot down passing aircraft.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
While you could use a foil faced OSB/plywood sheathing in that way, it is normally installed with the foil face down on the deck, over the needed airspace. You could put on on the bottom side of that airspace too, but I think most people would think its effectiveness would be reduced by dust too quickly.The aluminum foil is not only a great reflector of heat energy, its also a poor emitter of the same energy. This is why it's used with the foil pointing down, it keeps the heat on the deck instead of being radiantly emitted below. Direct contact with rafters will reduce its effectiveness though, as it will transmit heat through direct contact with the wood.I thought you knew that! You were the one who was big on an airspace of at least 3/4" or more weren't you?
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
"a foil faced OSB/plywood sheathing"Like the Dow insulsheathing? Thats a different product than I was thinking.This whole thing is all over the map because it is a generic question with no specific application - too many variables floating in and out of the conversation
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When I redid my garage roof, I used OSB sheets that had the layout lines on one side (16" and 24" OC) and a perforated foil covering on the other. It REALLY made a difference in the amount of heat in the garage.EDIT - it may just be a hot weather climate product.
Edited 10/10/2009 11:12 am by xxPaulCPxx
I've done this, and it was very, very effective. There are three details to keep in mind:
1) It needs to be open at the ridge and at the eaves for good air flow. The more, the better. Oddly enough, 'hot' is 'good' - the greater the temperature difference between the two roofs, the more airflow there will be between them, so I would not insulate this top cover;
2) You need to screen those openings to keep the critters out. Bigs generally aren't enough of an issue to worry about; squirrels and pigeons are! 1/4" hardware cloth will work; spray it flat black and it becomes invisible; and.
3) When you go on the roof, you'll be walking on this new surface. Keep that in mind. Also, do not neglect the roof underneath; it will still need to divert water. Don't just set the new framing atop it. You'll want some form of cushion (roofing felt works), and align the sleepers with the rafters underneath.
I suggest at least 3-1/2" air space between the two roofs.
If you plan to re-roof your house, you can use simple roll roofing on the house itself, and save the pretty shingles for the visible 'cold' roof.
Edited 10/9/2009 1:07 pm ET by renosteinke
What's with the gigantic space? Why 3.5" instead of 1.5"?
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
Our code increased the minimum depth of roof ventilation spaces to 3" in 1992.
That's interesting that the vent space is larger there, but not for a conventionally framed roof. I suppose higher air exchange rate is better in a cooling environment... and in the winter you are already insulated to the deck.
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
That is for any roof venting. They found that 1 1/2" was too hard to maintain and was often impeded by insulation.
but when you factor in that with the dual pane that the OP is asking about is rigid on both sides of the vent path and probably won't be comprimised by overstuffing the insul I'd bet that 3.5" would be overkill and 1.5 would be all that you'd need.
Honestly I don't know enough about it. You may be right, but the only thing that can go wrong in this roof is that the airspace is too small. If after going to the trouble of building it the roof is ineffective, he might be kicking himself for not turning the 2"x4"s on edge.
True, but 2" more thickness of a roof deck can be really noticeable. Since it's not going to be overstuffed with batts like a conventional roof. If you look at ridge venting systems, they usually only have a spacing of 3/4".I understand that more room will give better flow, and for releasing hot air that was heated in that channel that is a good thing. But I'd like to have a better idea of the CFM differences between a 3.5" channel and a 1.5" or a .75" channel, and how that relatets to being able to BETTER cool.Adding 3.5 inches to a roof that is already probably at least 4" thicker than normal may make for an unacceptable appearence, vs. a 1.5" or .75" add.
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
You may very well be right.
It just seemed that using 2x2's as sleepers did not allow the air to flow freely enough. Use of 2x'4s made a big difference.