Best roof pitch?

[rebar]

All good stuff thanks.

On the plan we buy propane for $1.18 but could buy bulk for 1.09.

I don't know.. That's so close to the break even cost, Id feel more comfortable with a furnace because that's what I know, and repairs are possible instead of replacements. I think I want both for redundancy, but equipment keeps getting more efficient and mini splits are new to the heating world. So since I have the luxury of time, Ill watch from the sidelines unless my bills change drastically. I'm lucky to have such reasonable rates but thing can change.

Id love actually to have a flat roof so that I could have a roof top terrace for these views.

[J.P.M.]

Super thanks JSchnee21!

Yes my PoCo has net metering, but also 10-13 cent kWh for now. I'm unaware of the terms, but HSB185 is in the works from what I understand. Yes we can get allot of snow.

Having additional living space in the attic is handy, but also tricky venting condensation from inside the roof. The other option for cathedral ceilings is unvented, but you have to install layers of insulation board on top of the roof deck, and then another layer of plywood for the roof.. Much more involved than just raised heal trusses, but you get more space.. If I need 35 degrees, that's about 8:12 - 9:12 pitch which would be a roomy attic, but unwalkable roof. I not sure if putting them on the roof is the best approach because I do have room on the ground.. Probably should have researched that first but I only have 1 acre and it would be nice to have them out of the way. I'm just pre planning and getting opinions.



Design will be a small retirement house, with a large tall attached garage with a car lift. Ribbon boards will be let in the garage walls to expand upper level living space when I or next owner don't need height for the lift and want another bedroom. Goals are to have good curb appeal and resale value, good energy efficiency with dual sources, propane/electric. Maybe a wood stove just in case.. r10 basement r30 walls r60 roof. No foam

In my opinion air sealing and windows/doors are your best money spent after 10/30/60 have been met. Ive read that 13 cent electric might be able to out perform propane with the latest gear, but I'm skeptical. Regardless, you never know when they'll jack the price of anything so its good to have dual or triple fuel, so to speak. And I'm not convinced passive or suntempered homes have any advantage over a super insulated home. The larger windows equals less insulation for most of the day when the sun isn't shining. And then you get weather like the last week with absolutely no sun..

So I guess I'm ok with a small efficient home with conventional heating/cooling for now. But like my spare bedroom ribbon board, be prepared for the future by having built the home ready to generate some or all of the energy needed.

Thank you for the responses.

Your opinion that conservation/use reduction measures vs. alternate energy measures to supplement conventional HVAC methods in cold(er) and/or less sunny environments are a better first option - and maybe to the exclusion of any alternate energy methods for heating a residence in such climates - is pretty much spot on with a lot of the smart and informed opinion on the subject.

Bottom line: For most applications in cold and more cloudy climates, money spent on conventional energy reduction methods goes much farther than most every alternate energy production method.

Sensible, time tested and conventional conservation just ain't as sexy as throwing panels or the latest darling gadget of the greenwash media and the tree hugger solar snob set at what's often an energy sieve made so by (lack of) design.

My point, probably poorly written, was that there's a lot of room between the two. Sometimes playing heads' up ball with some sun tempered features and tricks can be useful. Examples would include site and building placement considerations or some consideration for perhaps moving (but not necessarily adding) some fenestration to a more southerly orientation while using overhangs or other arch. features to help reduce summer heat gain. Or, reducing westerly fenestration while increasing easterly fenestration for some passive solar effects on HVAC loads. Or, depending on lifestyle choices, some moveable (and effective) window insulation. Or, adding some (effective) thermal mass to a dwelling, but that can get tricky. But, you need to know some things to do as possibilities before you can consider or act on them. Hence my suggestion on the Google search.

On the cost of various fuels as a heating source, I'd start by finding the heating value of each fuel and then comparing cost of delivered heat for each fuel.

Example: Propane has a heating value of between 90,000 and about 83,000 BTU/gal. I'll use the higher heating value of 90,000 BTU/gal. burn it at 95 % eff. in a high eff. (condensing) furnace for this example. That translates to (90,000 BTU/gal.)*(.95)/(3,412 BTU/kWh of electricity) = ~ 25.1 kWh of electricity used as resistance heat per gal. of propane.

So, as an example only, if electric resistance heat is used as the heating source and it costs $0.13/kWh for that electricity, if propane costs < ($0.13/kWh)*(25.1kWh/gal.) = $3.26/gal., then propane may well be the cheaper fuel.

However, if a heat pump is used, that cost/gal. must be divided by the seasonal C.O.P. (note: not the SEER) of the heat pump, and the breakeven cost for the propane will drop, probably and very approximately by a factor of between 2 and 3 or more.

So, for this example, if a heat pump has a SEER of, say, 12.0, to get the breakeven cost/gal. of propane vs. electricity to power the heat pump, first divide the SEER of the heat pump by 3.412 to convert SEER to a seasonal C.O.P. and then divide that result by the breakeven cost using electric resistance heat to get the propane breakeven cost vs. using a heat pump:

Divide 12.0/3.412 to get 3.52 seasonal C.O.P, and then divide the $3.26 from the example above by 3.52 to get $0.93 per gal. as the breakeven cost of the propane. If you can find propane for that cost or less per gal., in terms of fuel costs only, it'll be cheaper to burn propane than use a heat pump with the numbers used in the example.

I'm reasonably confident you can find a decent heat pump with an SEER => 12, and equally confident that you'll have a hard time finding propane for ~ a buck a gal. or so anytime soon, even if bulk purchased in the off season.

An alternate to the above for heating a dwelling is to consider the cost of delivering, say, 1,000,000 BTU to a dwelling using various methods considering equipment efficiencies (or inefficiencies, and the cost of the fuel as well as the NPV capital cost of the equipment to use that fuel and also assumptions about future fuel costs.

Take what you want of the above. Scrap the rest.

[rebar]

Latitude of Iowa City, IA is 41.6611° N

41.6611 * 0.76, plus 3.1 degrees = 34.76 degrees for optimal (~71% of 2 axis yield) year round harvest.

Is your PoCo Net Metering? How frequently due they true up (annual)? Can you set/change your anniversary/commissioning date?

Do you get much snow?

If I had the option to build a new house from scratch, I would definitely opt for a steeper pitch so that I might be able to walk upright in the attic or perhaps turn it into a spare room. Similarly, I would not use pre-fab trusses for the same reason.

Super thanks JSchnee21!

Yes my PoCo has net metering, but also 10-13 cent kWh for now. I'm unaware of the terms, but HSB185 is in the works from what I understand. Yes we can get allot of snow.

Having additional living space in the attic is handy, but also tricky venting condensation from inside the roof. The other option for cathedral ceilings is unvented, but you have to install layers of insulation board on top of the roof deck, and then another layer of plywood for the roof.. Much more involved than just raised heal trusses, but you get more space.. If I need 35 degrees, that's about 8:12 - 9:12 pitch which would be a roomy attic, but unwalkable roof. I not sure if putting them on the roof is the best approach because I do have room on the ground.. Probably should have researched that first but I only have 1 acre and it would be nice to have them out of the way. I'm just pre planning and getting opinions.

I'd ask: What are your design goals for the dwelling ?

A conventional stick house with PV ?
A solar monstrosity that's an eyesore ?
Something in between that looks semi normal but has suntempering features that take advantage of building orientation and site features and uses sensible measures to produce a healthy and comfortable year round indoor environment with lower utility bills made possible by sensible conservation measures and solar utilization to meet a cost effective portion of the (remaining) lower utility bills ?

Google "sun tempered building designs" if interested.
There's a lot of information, possibilities and ideas between the extremes of an energy hog and an eyesore.

Design will be a small retirement house, with a large tall attached garage with a car lift. Ribbon boards will be let in the garage walls to expand upper level living space when I or next owner don't need height for the lift and want another bedroom. Goals are to have good curb appeal and resale value, good energy efficiency with dual sources, propane/electric. Maybe a wood stove just in case.. r10 basement r30 walls r60 roof. No foam

In my opinion air sealing and windows/doors are your best money spent after 10/30/60 have been met. Ive read that 13 cent electric might be able to out perform propane with the latest gear, but I'm skeptical. Regardless, you never know when they'll jack the price of anything so its good to have dual or triple fuel, so to speak. And I'm not convinced passive or suntempered homes have any advantage over a super insulated home. The larger windows equals less insulation for most of the day when the sun isn't shining. And then you get weather like the last week with absolutely no sun..

So I guess I'm ok with a small efficient home with conventional heating/cooling for now. But like my spare bedroom ribbon board, be prepared for the future by having built the home ready to generate some or all of the energy needed.

[J.P.M.]

I'd ask: What are your design goals for the dwelling ?

A conventional stick house with PV ?
A solar monstrosity that's an eyesore ?
Something in between that looks semi normal but has suntempering features that take advantage of building orientation and site features and uses sensible measures to produce a healthy and comfortable year round indoor environment with lower utility bills made possible by sensible conservation measures and solar utilization to meet a cost effective portion of the (remaining) lower utility bills ?

Google "sun tempered building designs" if interested.
There's a lot of information, possibilities and ideas between the extremes of an energy hog and an eyesore.

[Ampster]

All the above is good advice. I agree that a steeper pitch is better for best production. Also see if you can design plumbing vents to be on the north pitch to maximize your South pitch for simplest installation and future proofing.

[littleharbor]

Try the Solar Tilt app on your phone. Really cool and a keeper.

https://play.google.com/store/apps/d...olartilt&hl=en

[JSchnee21]

Latitude of Iowa City, IA is 41.6611° N

41.6611 * 0.76, plus 3.1 degrees = 34.76 degrees for optimal (~71% of 2 axis yield) year round harvest.

Is your PoCo Net Metering? How frequently due they true up (annual)? Can you set/change your anniversary/commissioning date?

Do you get much snow?

If I had the option to build a new house from scratch, I would definitely opt for a steeper pitch so that I might be able to walk upright in the attic or perhaps turn it into a spare room. Similarly, I would not use pre-fab trusses for the same reason.

[JSchnee21]

Optimal solar inclinations, rough approximation, by latitude

https://www.solarpaneltilt.com/

[J.P.M.]

Thanks.. The PV watts calc says tilt in degrees should be 20 for Iowa City Iowa..

Would there be a advantage building a 4:12 (18.5 degrees) or 5:12 (22 degrees) roof?

PVWatts says no such thing. That's a default value.

Before you go any further, download a free but slightly dated version of "Solar Power Your Home for Dummies, or get a new hardcopy for ~ $25 at bookstores/Amazon. You need more information to make more informed/better decisions.

The array azimuth and tilt PVWatts uses are user inputs. You vary both along with the other user inputs for optimum annual production or what ever you want to optimize.

Reads to me like you may not know how to use the model.

I'd SWAG your best orientation for a residential situation with net metering will be at an array tilt of approx. the local latitude and an array azimuth about true (not magnetic) south.

But, there are lots of other considerations that will modify the optimum orientation/placement and also the optimum system sizing.

Read the book. and come back with questions to fill in any gaps your knowledge acquisition creates.

If you are building, know that your energy $$'s will go a lot farther and be much better spent on conservation measures before any sexy solar devices.

Getting your building heat gain/losses/energy use as low as possible before PV or other active solar devices is a whole lot cheaper and so more cost effective (and easier) that throwing what's about the most expensive and so close to the least cost effective method of supplying energy to a larger than necessary energy load.

Welcome to the forum of few(er) illusions.

[rebar]

Not questioning your results, but IIRC for maximizing annual production at 1 angle, that surprises me. USUALLY, an angle approximating your latitude is optimum ... about 40 degrees. Recheck your input data.

But I'm questioning the results of pvwatts.. 20 degree's sounds to flat for Iowa City. Is there another non profit calc I can use?

[Mike90250]

Because of the long days and clear weather in the summer, I generally have a daily surplus, and I run my array at winter angle all year round, to harvest whatever sun there is in the winter.
(off - grid)

[gbynum]

Thanks.. The PV watts calc says tilt in degrees should be 20 for Iowa City Iowa.

Not questioning your results, but IIRC for maximizing annual production at 1 angle, that surprises me. USUALLY, an angle approximating your latitude is optimum ... about 40 degrees. Recheck your input data.

[bcroe]

The best angle for summer production is likely very poor at rejecting snow. That may
be fine, because roof mounting is difficult to clean, write off winter production. A
summer and a winter angle is much better, but quite difficult on a roof. Bruce Roe

[peakbagger]

PV Watts looks at long term weather conditions for specific areas and calculates what a possible yearly output is for a fixed array. If you want to put in an adjustable angle array its a different story.

[rebar]

If you have snow I would err on the steep side. On the other hand a shallower pitch saves a bit of plywood and shingles plus its easier to walk on.

A BTW, do yourself a favor and put ice and water shield from the edge of the roof all the way to the top of the panels. The area under the panels is not the place for roof leaks.

To bad no one makes a standing seam panel with PV built in.

So when the PV watts calc says tilt in degrees should be 20 for Iowa City Iowa.. Is that a average angle between winter and summer?