Thermal heat reduction impact of solar panels on roof vs. shading optimization

[SunEagle]

It will all be site specific

higher roofpitch will cause more convection between roof and modules,
most racking has hight adjustments, install a few inches higher will help as well.
effect on the home is also site specific. For example I have energy start shingles which reflect a great deal of the energy, so the PV has less thermal impact than if I didn't have them.
Better insulated homes, better ventilated attics etc, all will decrease the impact of the PV on the home.

The point is that it is a good idea to do these other things before solar as they are usually cheaper than solar and have a larger impact.

Where PV thermal effect on cooling the home would have the greatest impact would be on a home with vaulted ceiling. Insulation is limited, and no attic to ventilate.

I agree.

[rsilvers]

So maybe this is an argument for not using the lowest-profile mount (like the church photo I posted yesterday).

[SunEagle]

So maybe this is an argument for not using the lowest-profile mount (like the church photo I posted yesterday).

If the angle of the roof is steep then whatever heat is collected by the panels may quickly vent up and away from the roof and not get absorbed by the building. A flat roof would have more difficulty venting the heat collected unless there was some type of forced air movement beneath the panels.

[ButchDeal]

So maybe this is an argument for not using the lowest-profile mount (like the church photo I posted yesterday).

well that is one argument and the other more impactful argument is that the higher mount has more cooling for the modules, and thus more production.

There is likely a point of deminishing returns.

[J.P.M.]

It will all be site specific

higher roofpitch will cause more convection between roof and modules,
most racking has hight adjustments, install a few inches higher will help as well.
effect on the home is also site specific. For example I have energy start shingles which reflect a great deal of the energy, so the PV has less thermal impact than if I didn't have them.
Better insulated homes, better ventilated attics etc, all will decrease the impact of the PV on the home.

The point is that it is a good idea to do these other things before solar as they are usually cheaper than solar and have a larger impact.

Where PV thermal effect on cooling the home would have the greatest impact would be on a home with vaulted ceiling. Insulation is limited, and no attic to ventilate.

FWIW, I'd agree with all this, but with a comment on the natural convection (buoyancy driven) convection flow rates and roof pitch.

The rate of natural convection heat transfer improves (increases) as the pitch of a roof increases. That's because temp. changes (heating in this case) in the fluid(the air) cause density changes in the fluid ( increases the buoyancy of the fluid), which act as the driving force to increase flow rate of the fluid, which increases the heat transfer rate between the surfaces (roof deck and back of the array) and the fluid.

However, most all of the time, those natural convection effects are very small when compared to what are the forced convection effects caused by even a slight wind of say 0.5 ft./sec. or so, pretty much regardless of wind vector, with respect to magnitude, direction and variability.

In other words, even a small wind - like a few tenths of a ft./sec. will probably swamp most any natural convection effects. That's the textbook case. Reality follows along quite closely and it gets real complicated real quick, but forced convection usually dominates by at least an order of magnitude if the is any wind blowing at all, pretty much irrespective of roof pitch.

[J.P.M.]

I don't see why the shade from an array would not be a giant benefit. The sun is putting 1000 watts per square meter into your roof. Think of your roof as a solar thermal heater.

I have a FLIR and can do some testing later. I just tested a $30 Duck brand attic stair cover and decided it was not good enough.

Think what you want, but that's probably not the case most of the time.

For any reasonably well insulated dwelling built within, say, the last 40 yrs. or so, the effect on interior temp. caused by shading from a roof mounted array, or any shading device for that matter, is usually quite small. For example, on homes with uninsulated attics or crawlspaces, but with insulated attic or crawlspace floors, the effect on the conditioned space temp. - what's under the attic - will, in all likelihood, be unmeasureable, particularly with proper attic ventilation.

In such cases, and if properly ventilated, the attic will see some reduced temp. from adding roof shading, but probably not much - won't hurt, and a nice bennie, but not the biggest reason to add an array. In such case, the attic will be cooler during sunlight hrs., but, BTW, warmer at night by a bit as nocturnal cooling (what's called radiant sky cooling) is reduced. Where conditioned space and the roof have a common boundary, and that surface is insulated, even poorly, the decrease in heat gain from the shading is quite small. I seriously doubt anyone reading this will be able to quantify a lower A/C operating cost attributable to roof shading alone.

The bottom line is: insulation is effective. Any reasonably, or perhaps even not so reasonably well insulated dwelling will see small heat gain reduction effects from shading devices added to a roof - solar arrays or anything else. If well insulated, the effects will probably be unmeasureable.

There can be some other benefit of shading as well as some drawbacks. On the + side, roof materials under the shading will see lower temp. excursions. On the - side, critters, their scat, and other debris can collect under the shading device. Snow buildup and ice dams come to mind as well their mitigation/correction costs. Roof maint. is more challenging.

If on a poorly insulated dwelling, the temp. effects and effects on HVAC costs, while perhaps noticeable, will probably be slight and can be mitigated or entirely eliminated in a practical sense, quite beasily with insulation.

As a matter of likelihood, the degree to which the effects of adding shading are noticeable is a pretty good qualitative measure of the state of the envelope insulation between the shade and the conditioned space, and probably for the dwelling as a whole.

[NYHeel]

I have a colonial style house and in the master bedroom there is a vaulted ceiling and therefore no attic. The master bedroom is significantly warmer during the day than the other rooms on the second floor of the house. I always assumed this was because the room was directly under the roof with it's dark asphalt shingles whereas the other areas of the second floor had the attic as an insulating buffer.

Reading this thread, I'm hopeful that the solar panels that will be going on the roof above the master bedroom will help normalize the temperature of the master bedroom compared to the rest of the second floor. My wife works nights and therefore often sleeps during the day. She sometimes complains that it's much hotter in our room than the other rooms of the second floor. Any improvement there would be nice.

[rsilvers]

I am getting my attic air-sealed. There are state incentives where it is free. And if you need more attic insulation, there is a state contract price that is very low and the first $2000 is free.

[ButchDeal]

I have a colonial style house and in the master bedroom there is a vaulted ceiling and therefore no attic. The master bedroom is significantly warmer during the day than the other rooms on the second floor of the house. I always assumed this was because the room was directly under the roof with it's dark asphalt shingles whereas the other areas of the second floor had the attic as an insulating buffer.

Reading this thread, I'm hopeful that the solar panels that will be going on the roof above the master bedroom will help normalize the temperature of the master bedroom compared to the rest of the second floor. My wife works nights and therefore often sleeps during the day. She sometimes complains that it's much hotter in our room than the other rooms of the second floor. Any improvement there would be nice.

It will not help much for keeping it warm in the winter

[dc/ac]

I was mulling over that same question last summer and did some experimenting on a recently built shed in back which has the same asphalt roof as the house. Measuring the heat on the roof with a laser gun, the heat was above 160. I did not have enough panels so I tested just regular old barn aluminium, putting it on a temporary harness elevating it about 2 inches over the shingles. The results were dramatic - the roof temp under the aluminum was about 50 degrees less. Measuring the temps in the shed ceiling (already protected with radiant barrier, insulation and drywall) were less dramatic but still consequential.

Of course, panels would have a different profile. It made me wonder if putting an aluminum 'hat' over the top of the entire south facing roof and then putting the transparent LG panels [1] on that would be an interesting (if not crazy, I'm not suggesting this :P) combination.

A radiant barrier in the attic may be an option to look into. Especially if you have AC ducts in the attic.

[1] https://www.solarpaneltalk.com/forum...or-transparent

[NYHeel]

It will not help much for keeping it warm in the winter

Yeah, I figured so but I'm more concerned about the warmth in the summer. We like it a little colder in our bedroom anyway. Our issue is that it's too warm in the summer and we need to keep the AC on a lower setting (the thermostat is in a different area of the second floor) to keep it comfortable in our bedroom.

[J.P.M.]

I have a colonial style house and in the master bedroom there is a vaulted ceiling and therefore no attic. The master bedroom is significantly warmer during the day than the other rooms on the second floor of the house. I always assumed this was because the room was directly under the roof with it's dark asphalt shingles whereas the other areas of the second floor had the attic as an insulating buffer.

Reading this thread, I'm hopeful that the solar panels that will be going on the roof above the master bedroom will help normalize the temperature of the master bedroom compared to the rest of the second floor. My wife works nights and therefore often sleeps during the day. She sometimes complains that it's much hotter in our room than the other rooms of the second floor. Any improvement there would be nice.

Ventilate the room and add insulation between the interior ceiling and the roof. Sounds like an older dwelling ? Roof shading will help, maybe more than for some other dwellings.

Kind of a toss up/Hobson's choice. Either shading or ceiling insulation might well mitigate or eliminate the problem. BTW: Any other heat sources such as west facing widows in the room ? If you insulate the ceiling before seeing/observing any heat reduction from any shading from an array, you'll probably never know if the added roof shading would have mitigated or eliminated the need for the insulation, making the added insulation an unnecessary expense to some degree. On the other hand, insulating a ceiling can be relatively costly for the $$ reduction in HVAC costs, but it may be easier and perhaps with an added benefit if it allows access to the underside of a roof under an array, if for no other reason to get a better chance at hitting the rafters with roof penetrations, particularly if an older dwelling ( or not, and maybe the opposite if recent construction work I've seen is any indication).

[stevenremote]

I see this is an old thread. Hopefully, meaningful input can still be offered. 1960's home, 2,500 feet, single story, black asphalt shingles on a pitched roof, San Fernando Valley (hot) in Los Angeles. Part of my home has vaulted ceilings, 9-15 feet. I have considered a plethora of options including an insulation layer below the existing ceiling line and over the roof.

The problem I'm having is heat transfer into those vaulted areas without an attic I have and still am addressing insulation (not going to happen in the vaulted area) as well as AC modifications, 3,000 cfm gable vent mounted attic fan, a 6,000 cfm whole house fan, etc.

I am in the process of adding 40 Panasonic 330 watt panels to my roof, over the vaulted area. It is my intention to have the installers place the panels at the maximum height, which I'm told is 4-5 inches from the roof. My hope is that the panels will shade a substantial amount of the area over the vaulted ceilings and, provide a meaningful decrease in heat transfer into the house. I have measured the unshaded areas of my roof to be near 180 degrees Fahrenheit and at the same time, the shaded parts are closer to 130 degrees Fahrenheit.

What can I expect? What can I do before the physical installation to increase my chances of secondary benefit? Is there a radiant barrier that can go between the panels and my roof? Should I wire waterproof fans to move air between the panels and my roof?

Thoughts would be appreciated.

[azdave]

There are some benefits to shading your roof of course. Link below.

I would not bother with adding outdoor fans to move air under the panels (but the birds roosting there might like to hang out in the breeze).

https://phys.org/news/2011-07-solar-panels-cool.html

[J.P.M.]

I see this is an old thread. Hopefully, meaningful input can still be offered. 1960's home, 2,500 feet, single story, black asphalt shingles on a pitched roof, San Fernando Valley (hot) in Los Angeles. Part of my home has vaulted ceilings, 9-15 feet. I have considered a plethora of options including an insulation layer below the existing ceiling line and over the roof.

The problem I'm having is heat transfer into those vaulted areas without an attic I have and still am addressing insulation (not going to happen in the vaulted area) as well as AC modifications, 3,000 cfm gable vent mounted attic fan, a 6,000 cfm whole house fan, etc.

I am in the process of adding 40 Panasonic 330 watt panels to my roof, over the vaulted area. It is my intention to have the installers place the panels at the maximum height, which I'm told is 4-5 inches from the roof. My hope is that the panels will shade a substantial amount of the area over the vaulted ceilings and, provide a meaningful decrease in heat transfer into the house. I have measured the unshaded areas of my roof to be near 180 degrees Fahrenheit and at the same time, the shaded parts are closer to 130 degrees Fahrenheit.

What can I expect? What can I do before the physical installation to increase my chances of secondary benefit? Is there a radiant barrier that can go between the panels and my roof? Should I wire waterproof fans to move air between the panels and my roof?

Thoughts would be appreciated.

SWAG is you have less heat gain from that portion of the roof that will be now covered by panels. More SWAG: maybe you'll have ~ half as much heat gain from the roof that will be covered by panels than you had from that portion of the roof before the panel installation.

One way or the other, more insulation, correctly installed will effectively kill most of the heat transfer. Another first line of defense would be to lighten up the color of the roof. A black roof in a hot/sunny climate is asking for high bills.

As for the top of roof deck to underside of panel clearance, it can be whatever you choose. From an underpanel roof deck temp. standpoint, and from a panel efficiency standpoint, more distance is better, but beyond a clearance of maybe 6" or so, the benefits of increasing the distance usually diminish at an accelerated rate. While some HOA's may have max. distances, if some vendor is telling you there's a max. and implying that it's a gov. or legal mandate, they're probably telling you that to save money. FWIW, my HOA has a 6" minimum clearance dimension.