Inter-Row Spacing for roof mounted solar

[J.P.M.]

Well I'll admit it... Everyone is right. Doesn't make sense to tilt the panels for the roof. I just don't have the room for it. I've done some more math (I hate trigonometry) and I've figured out that in order to tilt the panels it would take up the majority of my roof space and there would be no room left for additional panels due to the spacing. So I guess I'll have to mount them along the slope. I built my place with a single roof slope to the south and it's only 22x32, so for now I'm going with 3 rows of 5 panels in portrait. This will give me 15 panels now and if they aren't producing enough for my needs at least I'll have enough room left over to mount up to 10 more in the future. I suppose I could still find a way seasonally tilt the highest 5 panels to eek out just a bit more power, but it may not be worth it, who knows.


Kind of sucks though... I'm big on trying to maximize the efficiency of what I've got, versus just getting more stuff. Heck, that's why I'm building my house as small as it is. I guess in this case though, the most efficient way to do it is by leaving enough room now to add more later.

One way to look at efficiency is using the least amount of resources to accomplish a task. Some ways to do that are to insulate and seal the holy grail out of the building envelope, use low energy methods to do the remaining tasks. Smaller loads will make for smaller arrays. It's a matter of thinking out of the box and making choices.

[Animag771]

One way to look at efficiency is using the least amount of resources to accomplish a task. Some ways to do that are to insulate and seal the holy grail out of the building envelope, use low energy methods to do the remaining tasks. Smaller loads will make for smaller arrays. It's a matter of thinking out of the box and making choices.

That's so true. That's why I'm building this house as tight as energy efficient as possible and using all energy star appliances along with lots of insulation, air sealing eliminating thermal bridging and using a galvalume standing seam roof with above sheathing ventilation.

[J.P.M.]

That's so true. That's why I'm building this house as tight as energy efficient as possible and using all energy star appliances along with lots of insulation, air sealing eliminating thermal bridging and using a galvalume standing seam roof with above sheathing ventilation.

So what's the design heat load at what design temp. ? Design cooling load at what design temp. ? Annual design heating/cooling loads ? How many air changes/hr. Did/do you plan on a blower door test @ 50 Pa pressue differential ? Any solar tempering or consideration to minimize heat gain ? What are the insulation R values for the design ? Any foundation/under slab insulation considerations ?

I'm curious.

[Animag771]

So what's the design heat load at what design temp. ? Design cooling load at what design temp. ? Annual design heating/cooling loads ? How many air changes/hr. Did/do you plan on a blower door test @ 50 Pa pressue differential ? Any solar tempering or consideration to minimize heat gain ? What are the insulation R values for the design ? Any foundation/under slab insulation considerations ?

I'm curious.

Lots of questions I'll try to address them all.

It's been a while since I did the manual J calculations, but what I ended up with was a single ductless minisplit that offers 12k BTU cooling and 20k BTU heating. I do plan on having a blower door test done and I'm crossing my fingers for low numbers. I won't really know my air changes per hour until I've done this.

I'veI've taped all of my sheathing seams with 3M flashing tape, foamed all penetrations as well as top plates, bottom plates, and rim joists.

I did double wall framing with staggered studs on 24" centers with a quarter inch between them to allow for a 2x8 double top plate to be used. For the most part this should eliminate thermal bridging. I also have a large south window to allow heat in during the winter, but 2' overhangs to keep the sun off during the summer. I installed my siding and roofing using the rainscreen principal to allow an air gap behind both allowing heat to rise up and out while at the same time letting any water that may find it's way behind the siding to drain.

I'll be using 7.25" (R26) open cell spray foam in the walls and 9.5" (R35) in the (cathedral) ceilings.

As far as foundation considerations go... I built it on a pier and beam foundation and I intend to encapsulate the crawlspace. Then I intend to install a small dehumidifier in there and insulate the crawlspace walls with closed cell foam board.

[peakbagger]

No doubt with that much care taken to seal the place up its time to factor in an air to air heat exchanger into the design as otherwise its going to be bear to open and cose the doors plus you will potential trap in way to many indoor air pollutants. Might want to figure out a way to preheat the incoming air instead of installing the inline duct heater that most techs install. I did hear a suggestion once to just route the return air up under the refrigerator, it wont care about a draft and will run a bit more efficient. Always a bit of bummer to get the place super tight and then have to go drill some big holes for the HRV.

My experience is most AC units are oversized to deal with excess latent and sensible load built in to cool the space down to setpoint. If its just set and forget during high load periods the units can usually keep up but turn them on in a hot damp room and they just cant handle it.

[J.P.M.]

Lots of questions I'll try to address them all.

It's been a while since I did the manual J calculations, but what I ended up with was a single ductless minisplit that offers 12k BTU cooling and 20k BTU heating. I do plan on having a blower door test done and I'm crossing my fingers for low numbers. I won't really know my air changes per hour until I've done this.

I'veI've taped all of my sheathing seams with 3M flashing tape, foamed all penetrations as well as top plates, bottom plates, and rim joists.

I did double wall framing with staggered studs on 24" centers with a quarter inch between them to allow for a 2x8 double top plate to be used. For the most part this should eliminate thermal bridging. I also have a large south window to allow heat in during the winter, but 2' overhangs to keep the sun off during the summer. I installed my siding and roofing using the rainscreen principal to allow an air gap behind both allowing heat to rise up and out while at the same time letting any water that may find it's way behind the siding to drain.

I'll be using 7.25" (R26) open cell spray foam in the walls and 9.5" (R35) in the (cathedral) ceilings.

As far as foundation considerations go... I built it on a pier and beam foundation and I intend to encapsulate the crawlspace. Then I intend to install a small dehumidifier in there and insulate the crawlspace walls with closed cell foam board.

Thank you. Sounds like a lot of thought went into the task.

Two comments, both of which you may be aware of and not meant as snide remarks/knocks, and one trick::

Comments: Open cell R-values tend to decrease a bit more over time than closed cell values. Also, some foams , both open and closed will outgas. Good vapor barriers and external skin looser than internal skin help, but in a tight build, interior air quality and some insulation smell can be a consideration.

The trick: if you want to really find leaks at pressure test or at least have a very visual highlight, pressurize the building positively on a non windy day, and have a couple of smoke pencils around places around sill plates, window casings, electrical outlets, etc, and (perhaps) be ready for a surprise or two. I did that once to dispute a point to an insulation vendor. We watched smoke poue out of every place where two surfaces met (or, more accurately, didn't quite meet).

Good luck.

[Mike90250]

Be sure to lay a layer of plastic sheet (vapor barrier) on dirt in the crawl space, that will reduce a lot of moisture load. Post & Beam allows a lot of outside water, under the structure, that a perimeter foundation blocks

[Animag771]

No doubt with that much care taken to seal the place up its time to factor in an air to air heat exchanger into the design as otherwise its going to be bear to open and cose the doors plus you will potential trap in way to many indoor air pollutants. Might want to figure out a way to preheat the incoming air instead of installing the inline duct heater that most techs install. I did hear a suggestion once to just route the return air up under the refrigerator, it wont care about a draft and will run a bit more efficient. Always a bit of bummer to get the place super tight and then have to go drill some big holes for the HRV.

My experience is most AC units are oversized to deal with excess latent and sensible load built in to cool the space down to setpoint. If its just set and forget during high load periods the units can usually keep up but turn them on in a hot damp room and they just cant handle it.

Already have the big hole in the wall for an ERV. It did me very sad to drill out that big hole after taking such care to seal every crack, but on the bright side it does have electronic dampers and it will keep the air filtered and fresh. Sadly though due to the size of my space, the ERV I chose isn't the most efficient unit, everything else was oversized.

Thank you. Sounds like a lot of thought went into the task.

Two comments, both of which you may be aware of and not meant as snide remarks/knocks, and one trick::

Comments: Open cell R-values tend to decrease a bit more over time than closed cell values. Also, some foams , both open and closed will outgas. Good vapor barriers and external skin looser than internal skin help, but in a tight build, interior air quality and some insulation smell can be a consideration.

The trick: if you want to really find leaks at pressure test or at least have a very visual highlight, pressurize the building positively on a non windy day, and have a couple of smoke pencils around places around sill plates, window casings, electrical outlets, etc, and (perhaps) be ready for a surprise or two. I did that once to dispute a point to an insulation vendor. We watched smoke poue out of every place where two surfaces met (or, more accurately, didn't quite meet).

Good luck.

Good looking out. I am aware of those and hopefully won't have any problems (fingers crossed). I know the R-value may degrade some over tome, but that is true with most types of insulation and at these high R-values, I'll probably never even notice. I'm really hoping the outgassing won't be a problem, it seems to be a hit or miss thing. I have given it some consideration though. My interior skin should end up being >1 perm while my exterior will is probably around 8-10 perms. That might help any vapors that do outgas make their way outside instead of inside. I also have an ERV which should help the air stay fresh and filtered.

Nice tip, I'll have to do that when the time comes.

Be sure to lay a layer of plastic sheet (vapor barrier) on dirt in the crawl space, that will reduce a lot of moisture load. Post & Beam allows a lot of outside water, under the structure, that a perimeter foundation blocks

The vapor barrier will be part of encapsulating the crawlspace. A nice thick vapor barrier will seal the floor and be taped (and/or glued) around the piers as well is up the crawlspace walls. After that, I'll put 2 layers of foam board insulation with taped and staggered seams.

[J.P.M.]

Already have the big hole in the wall for an ERV. It did me very sad to drill out that big hole after taking such care to seal every crack, but on the bright side it does have electronic dampers and it will keep the air filtered and fresh. Sadly though due to the size of my space, the ERV I chose isn't the most efficient unit, everything else was oversized.

Good looking out. I am aware of those and hopefully won't have any problems (fingers crossed). I know the R-value may degrade some over tome, but that is true with most types of insulation and at these high R-values, I'll probably never even notice. I'm really hoping the outgassing won't be a problem, it seems to be a hit or miss thing. I have given it some consideration though. My interior skin should end up being >1 perm while my exterior will is probably around 8-10 perms. That might help any vapors that do outgas make their way outside instead of inside. I also have an ERV which should help the air stay fresh and filtered.

Nice tip, I'll have to do that when the time comes.

The vapor barrier will be part of encapsulating the crawlspace. A nice thick vapor barrier will seal the floor and be taped (and/or glued) around the piers as well is up the crawlspace walls. After that, I'll put 2 layers of foam board insulation with taped and staggered seams.

Thanx for the info. Been where you are in sane energy use. Nice to see that someone still knows what they're doing and takes the time/makes the effort to do it right. Honest.

With the outside barrier more permeable than inside, your chances of outgassing problems will decrease, but sounds like you know that. If that situation does turn up: More you probably already know, but if not: Open the outside skin more with bullet or ~ 1" dia. vents or some such measures. Seems counterintuitive, but 1" dia. minivents top/bottom of each stud space doesn't add (much) to heat loss/gain but and does wonders for vapor "out"gress - if it ever comes up.

On air change/heat recovery: FWIW, and not trying to throw a wet blanket on anything/anyone, while spending about half an engineering career designing industrial heat exchangers of many types and or many applications in what seems like centuries ago. I also spent a fair amount of spare time trying to find a residential heat recovery unit (a heat exchanger) for my own home (in Buffalo back in the day) that was worth more than spit. I failed.

Maybe things have changed in the world of heat transfer. I suspect not much.

More FWIW and IMO only, and most important, for design of residential heat recovery units, if I was still looking to add such a device, I'd be most mindful of ease of access to the internals and the location of any heat recovery unit with respect to removal/access of the whole unit , and I mean for deep access/disassembly. What the heat recovery peddlers don't mention or soft peddle: Units for residential heat recovery applications need frequent cleaning or at least frequent inspection, particularly of the heat transfer surfaces. Reason(s): dust/bugs/hair/skunga/etc. will collect, and condensation on the hot side will occur, making for a nice broth to breed stuff (and as an aside, foul the heat transfer surfaces thereby greatly reducing what's probably already poor heat transfer performance). The drip pans/condensate conduit are usually all anyone considers, but all the internals need to have clear access for inspection and cleaning.

A unit that is easy to get at and can be easily removed and disassembled is not only easier to service but that service is about essential. That's one little factoid that makers of such devices usually don't talk about much. Otherwise, mold/bacteria and unhealthy situations will present themselves.

Also unfortunately, units that can be repeatedly disassembled for inspection/cleaning need to be pretty robust to put up with the frequent movement of the guts. That adds to cost, making a well designed heat recovery unit easily beyond what most consider cost effective in terms of what it will save in heating/cooling costs. I believe they are a great idea. I understand the design principles both thermal and mechanical Well. But most of the residential units I've seen are about on a quality par with the quality of stuff you usually find at Harbor Freight. Aside from and in addition to that, the units I looked at and still see occasionally had heat recovery claims associated with them, that if not in violation of the laws of Thermodynamics and heat transfer, got very close to being unattainable, and that was before any allowance for fouling of the heat transfer surfaces as talked about here. Seemed to me that the (non)designers were about clueless with respect to heat exchanger design/limitations.



Just sayin'.

[Animag771]

Just figured I would close this out. Got the solar panels up today and finished the install. Just waiting for the co-op to reprogram the meter so I can flip the switch to start making power. Still have so much work left to do on this house.