Roof overhang?

Are we maybe doing shade overhangs for passive solar wrong?  The typical overhang just juts out.  This means it is a long lever with minimal support structurally.  Plus all the beams and header provide lots of room for thermal bridging.  What if instead we made it into a triangle just down over the window so the top of the window so it didn't even have a frame at the top?  Mirror or paint white that surface.  During the low sun part of the year for several hours each day it would reflect more light thru the clerestory windows.  Spring and fall it would add a bit more light of the early hours of the morning and evening.  Spring it would likely be beneficial and fall it would likely be helping to get the house too hot.  My house has roughly 21 inches between the top of the window and the over hang.  If it was 50% reflective and applying the sun angle it could potentially add just under of 8" worth of worth of light to the 28 inch high window.  More energy in without increasing the surface area to leak at night should add to the total heat in.  Add the surface down would increase the structural strength of the overhang.  Add some insulation clear up to the roof inside the triangle to cover all the thermal bridging area.  The Would this generate enough gain to be worth it?

A possible way to mirror that surface out in the weather.  mirror surface  Likely to expensive to be function?

There are charts available which detail time of the year and the latitude of your building.
It takes the guess work completely out of the situation.

Valid idea. Solar white paint is probably the winner for price/performance. Need to worry about ventilation in that space. I don’t think filling it with insulation is worthwhile unless that is a solid beam header above all the windows.

I am totally stealing the idea!  

R Scott wrote:Valid idea. Solar white paint is probably the winner for price/performance. Need to worry about ventilation in that space. I don’t think filling it with insulation is worthwhile unless that is a solid beam header above all the windows.

I am totally stealing the idea!  

It wouldn't need to fill the cavity.  That would be a silly waste of material.  Anything to hold say 6 inches of insulation against the front of the wall would do.  The goal would be to just cover the thermal bridging area with insulation.  Could be rigid foam cut to fit, batt stapled in or even just window screen or tyvek stapled up and filled with blow in.  Probably most time cost effective would be window screen type stuff run up the wall and then a bit of snorkel stapled on out towards the eves and blown full.  Done properly no cutting and fitting around corners.  I am thinking window screen simply because it would let some air out while blowing it full.  As for ventilation add a vent screen as close to the top underside as you can get.  The drip edge board down the front would shelter it.

I have been watching my light since I posted this.  In my location(45 degree latitude) and given my construction this would get a bit over 2 months of real potential gain.(one month on either side of shortest day of the year)  Starting in early Feb it is gradually reducing and now in mid march all potential heat gain from it is gone.  The sun has moved far enough north that it doesn't shine on that area mornings or evening and the noon day sun is shaded by the over hang right down to the top of the window.  Since my coldest 3 months are typically Dec, Jan and Feb this would be ideal gain here.  November might contribute to having the house too warm.

Now I have been looking for a way to try this without going to the expense of rebuilding the whole front of the house.  The greenhouse plastic that is white on one side and black on the other mounted on a framework is looking like a cheaper way to test this.  The white side is supposed to be 90% reflective.    The framework to mount the the plastic on would be the major expense.

Not sure what your overall setup is like (how the passive solar relates to the rest of the house) but I find that a 90 degree roof is ideal in my passive solar porch https://permies.com/t/173304/Solar-Porch-Passive-Solar-Retrofit
Any more admission of light would get too hot in the spring and fall. The axial tilt of the earth regulates mine simply by putting the sun higher in the sky when it's too hot out to want that direct light into the system.

Matt Todd wrote:Not sure what your overall setup is like (how the passive solar relates to the rest of the house) but I find that a 90 degree roof is ideal in my passive solar porch https://permies.com/t/173304/Solar-Porch-Passive-Solar-Retrofit
Any more admission of light would get too hot in the spring and fall. The axial tilt of the earth regulates mine simply by putting the sun higher in the sky when it's too hot out to want that direct light into the system.

I am northern Wyoming in a house built in 1984.  For its time it was way over built such that it almost meets modern building insulation codes.  11% glass vs floor space on a single floor plus it has a full basement  This is over the normal recommended 7%.  House gets too warm in August and Sept.  Intending to add a soil based AC system to counter act this eventually.(geothermal but NOT heat pump based)  But the passive part of the house is not enough to stay comfortable thru the winter.(the house would never freeze even with just the passive)  So 4 years ago I added an air based active collector to the front wall.  I can always choose whether I bring that heat in or not.  Since I added that collector I have run 2 of 4 of the last winters without heating at all.  Still not totally comfortable but closer.  Just finished adding an interior recirculation setup to pull more heat to the basement acting as a concrete  heat battery.  The goal between the collector and the interior circulation was 10 air changes per hour in the basement and both systems pull right off the floor heating as much of the basement as possible.  Next step is to add another 13 feet high by 8 feet wide worth of glass collector.  This will hopefully give me enough BTU's to heat the basement enough to cost thru longer runs of gray days.  Currently if both active systems are running the fans for them run on 70 watts of power that is mostly only needed when the sun is shining making it ideal for PV.  4 feet of the add on collector will add to the existing collector driven by exist fans.  The other 4 feet will be combined set of collectors to pull air out and push fresh air in thru and HRV core.  Hopefully this much will run pure convection so no added power needed.

Next step is hot water solar primary to preheat for the water heater.  The goal being to make the water heater almost not needed.  During good sun this will also heat the core lines in the dirt giving the soil a preheat hopefully giving enough stored heat to provide for stored heat for the rare heat needed points to keep things comfortable.  2 pumps.  One driving the collector in sun only(30 watts)  And one driving the geothermal which will be run as needed for another 30 watts.