Solar panel question...

Shawn,

I think you would be hard pressed to see a 10-15% decrease in ac use by 'shading' your roof with PV. It certainly couldn't hurt,, but to you would get a much more substantial benefit by adding insulation to the roof/attic. Even a vaulted/cathedral ceiling can have rigid foam added either on the under side with drywall or paneling added over the foam,, or on top of the roof deck, under new sheathing or steel roofing.

I think that any reduction in radiational losses by putting PV would be so slight as to be negligible.

One other thing to remember is the PV efficiency goes down with temperature. The higher the panel temps the lower the out put.

Icarus

 

Our region is subject to several hail storms per year. How well do the panels hold up to hail stones and what about expense to replace on the roof of the house as well as the Prius ?

 

I have heard on almost no damage from hail on solar panels.

The angle of installation of most panels in N.America serve to allow a hail stone to be more of a glancing blow on solar panels. That and tempered glass keeps them from breaking in all but the worst hail events. In case you are worried,, you home owners insurance should cover any loss with no increase in premiums since it is such a rare event.

As for the Gen 3 Prius panels,, I have no opinion, except to say that I think it is more of a PR gimmick than a real contribution to fuel efficiency. I can't remember how big the panels on the Gen 3 are,, but if they are 100 watts it would be big. In one hour it would put ~70 watt hours into the car,,,, The regen car icons represent 50 watt hours for reference.

Good luck,

Icarus

 

Thanks !

 

and obviously very poor PR at that!! since its been long established that they do not contribute to fuel efficiency in any way. they only run the ventilation system while the car is turned off...

as far as solar panels reducing possible AC use. not sure that i understand why they would not make a difference? they collect solar energy that would otherwise hit the house and redirects it into electrical energy.

the real insulating property most materials employ is air. the tiny air spaces heat up reducing the heat that eventually filters into the house. solar panels should take a significant part of that heat away from the roof

 

Most panels are warranteed up to a certain size hailstone. Ours are waranteed up to, I think, 2 inch hail.
We have had Zero damage from the hail we have had in the last couple of years (which has all been no more than half that size at most).
As for shading the roof, most systems don't cover that large of a fraction of the roof. I would suggest white (or even light colored) tiles or shingles and/or more insulation better ventilation (as already suggested) would be of more benifit.

 

As far as insulation and attics, in my climate it's essential to have a properly vented *and* insulated attic space. Otherwise, cold winter temps will not only cause condensation (With rot the result) but there also be ice dams on the roof edge

A climate that is primarily hot all the time, different approaches to attic insulation must be taken. But in a mixed or "4 seasons" climate, you must ensure proper attic ventilation

http://www.iko.com/misc/ArmaPdf/209-RR-86ArmarVen.pdf

For a catherdral ceiling, I would *never* consider a totally closed insulation system in my climate. That would all but guarantee condensation, mold, and eventual rot/failure

 

Shawn,

Without details of your local climate I am just guessing that ventilation approaches will be a better improvement/cost than trying to shade the roof.

 

In the case a vented attic,, I think that any shading (from PV) would have almost no effect on a well insulated space below. (assuming ~R-40 insulation) The point is,, if you don't have you vented attic space insulated that well,, you should do that way before you invest in PV,, if you are in a net heating environment or net cooling environment.

Now if you have an cathedral ceiling with poor or no venting soffit to ridge the effect of covering the bulk of the roof might have a substantial effect on temperature inside the building. That said,,, I go back to my original suggestion that adding insulation, even on a vaulted ceiling might be cost effective.

Depending on local costs,, the idea of adding 2" of rigid foam under the ceiling and adding new dry wall over it might have a pretty quick pay off. It has been a long time since I bid drywall,, but let say you could get some one to hang, tape and finish a typical vaulted living room for $2000,, the insulation cost might be $200. If that took the vaulted ceiling from R-20 to R-50 you might have a pretty good pay off.

In the avenue of energy efficiency,, sometime one has to think a bit out of the box,,, and realize how inexpensive some changes can be,,, how expensive others can be.

Icarus

 

Shawn,

You are on the right track. One other thing to consider is with the down economy in construction,, in may areas finding a after hours drywaller should be pretty easy,,, and a guy with the right tools,, and skills can tape and finish a ceiling in a couple part days. A couple of hangers should be able to hang the vaulted ceiling in a day or so.

Icarus

 

Shawn

I'm not sure if I agree with that assessment

It has been my experience - in my climate - that the single largest contributor to lower roof deck temperatures is a properly ventilated roof. That means vented soffits, vent guides for the rafters, and *ideally* a completely vented ridge

As an example, vent guides for the rafters/trusses

Owens Corning Roofing: Raft-R-Mate? Attic Rafter Vents

For some reason, not too many builders use this product. It's actually pretty easy to block the soffit vents while insulating, especially with blown insulation. The vent guide prevents blocking the airflow

Although standalone attic vents are fairly common, it's far better to use a completely vented ridge system

Owens Corning Roofing: VentSure® 4-Foot Strip Heat & Moisture Ridge Vents

There are many benefits to using the above systems: attic stays cooler in hot sun, shingles last much longer, no ice dams in cold climates, no condensation/mold in attic, etc etc

 

The problem Jay,, is in a vaulted ceiling,, 2X8 rafter,, with a high density r-21 fibreglass batt takes up almost the entire space. Ideally it would leave ~1" of vent space above the insulation,, but over a long rafter run,, the chance of plugging that vent in some or all the rafter bays is pretty good.

The same is true with a r-40 batt in a 2X12 rafter system. The ideal would be to fill the rafter space with rigid foam, with one layer under the framing as a thermal break.

Using baffle like your link is essential, especially with loose fill, blow in insulation.

Icarus

 

Shawn,

How about planting a big tree ? Now that I watch golf because of Tiger Woods I've been amazed at what gardeners can do in no time flat.

 

Kind makes it tough for the PV solar system to operate though!

Icarus

 

I've never liked cathedral ceilings. Never mind insulation, the tall ceiling means air stratifies. It's a trickey situation to put a ceiling fan in such a location

So that means in winter, the hot air rises and the floor is freezing. In summer the hot air can't be removed by the typical in-floor registers

Sounds like Shawn lives in a climate - though not as cold as mine - that gets cold. For tall ceilings, in hot climates, the A/C vents can be located in the ceiling.

In a climate with some form of winter, the vents have to in the floor. Otherwise, the floor will be ice cold