Conducting a Mini Manual-J
Is there a rough way to estimate the energy savings in BTU from installing 4″ of rigid polyiso insulation on top of the roof sheathing for a room with a cathedral ceiling, where the only existing insulation is 5.5″ batts between the 2×6 rafters?
I know the official way is Manual J, but most software seems to be Windows only (I have a Mac), another one costs $200, and hiring an HVAC pro seems like overkill, but that might be what I’m stuck with.
The 240V wall-unit AC can cool the room adequately but it seems like a big waste, since the room gets so hot. On the other hand, I estimate that adding the insulation will cost at least $3600 in labor+materials, so payback time could be lengthy.
Details:
* Climate Zone 2
* Room is the entire 2nd story, over a conditioned room on the 1F
* No shading over the roof
* Room is 23′ x 24′
* Double-pane 2×3 window on the East wall
* 2 double-pane 2×3 windows on the South wall
* 1 double pane 2×3 window on the West wall
* 1 exterior door
* Walls are 2×4 framing with fiberglass batts
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Replies
Yes, comparing the impact of changing just one envelope component can be done without calculating the whole load. The r value now is probably about R 18, and you would be adding maybe r24, so the total would be R-42.
The additional information we would need would be the total number of cooling degree days in your climate, which you can get from degreedays.net, among other places, and the area of your roof. If you've got a lot of sun on your roof and heating from that, using the cooling degree days would be an underestimate of the benefit, but calculating a lower limit on the benefit can be useful.
Thank you, Charlie. Austin's Cooling Degree Days is about 3420. Roof area is about 754sf. There's no shading so there's indeed a lot of sun on the roof.
So ignoring the sun, the load is area/R-value = 17.95 or 41.9 BTU/h per degree. The difference is 23.9 BTU/h per degree, or, multiply by 24 to get 574.5 BTU per CDD. Multiply by 3420 to get 2 million BTU/year difference.
At SEER 14, that's 2 million /14 = 143,000 Wh = 143 kWh/year.
Considering the sun, a wild guess would be that that might be multiplied by 1.5 to 2, so we could call it 200 or 300 kWh/year.
So yes, payback time will be lengthy.
Thank you, and wow, that's a 100-year payback time!
A wild estimate is saving 4 hours of AC time per day on a 2600-watt AC for 4 months = 10.4 kWh/d x 30.5d x 4mo. = 1269 kWh, which is 4x your estimate, but even saving 1269 kWh, @ 12¢ kWh is $152/yr., or a 24-year payback time with my original estimate of $3600 for labor+materials. (In the meantime, I found an error and realize that the true cost is likely to be closer to $4468, for payback time of 29 years).
This isn't necessarily a deal-killer, though. Besides some modest energy savings, there's also a comfort/noise benefit, and the new owners (I'm likely selling) will likely be in the house for decades.
Okay, I found the formula for hourly heat gain: BTU/h = ft2 x (∆°F ÷ R-value)
This probably isn't so helpful in my case because the radiant heat from the sun is forcing heat into the house, it's not just a matter of the outside temperature being hotter. Still, let's work through some numbers.
Aged polyiso is R5.5 for 1 inch and 32sf.
I'll assume the metal roof is about 100°F even when the outside ambient temperature isn't nearly that, for a 20°F difference.
So, 32sf x (20°F∆ ÷ R5.5) = 116 BTU/h per 1" sheet.
My project would use 96 sheets, so 116 BTU/h per sheet = 11,136 BTU/h.
Roof should be hot for only about 8 hours a day, or 89,088 BTU/d.
Four hot months each year would be 89,088 BTU x 30.5 x 4 = 10,868,736 BTU.
An Energy Star AC I'm looking at is 18,000 BTU/hr for 1500 watts.
10,868,736 BTU ÷ 18,000 BTU/hr = 604 hours of operation saved per year (which is pretty close to my 488-hour wild estimate in my previous post, BTW).
604 hours of operation saved per year @ 1500W = 906,000 wh = 906 kWh, x 12¢ = $109/yr., or a 41-year payback time.
While Austin TX is in cooling dominated climate zone 2, it's not a cooling-ONLY climate (with rare exceptions for better than code homes designed specifically for higher wintertime solar gain.)
Austin typically sees about half the HDD of CDD, but half is not zero, and the number of hours per day when there is a heating load (however small) is more than the daily hours of cooling load. Don't forget to include heating season savings when looking at ROI.
There are a couple of web-based freebie Manual-J(-ish) calculators worth considering that work with nearly any web-browser, namely LoadCalc.net and CoolCalc.com . Even though they are not as tweakable or accurate as a full on professional tool they aren't terrible. I've found that by discounting the air infiltration and ventilation to zero LoadCalc's numbers typically overshoot by less than 25% compared to a pro tool. I have much less experience with CoolCalc, but it's a true Manual-J. As long as you are aggressive (per the Manual's instructions) on the input assumptions it should be pretty good, but in the hands of nervous newbies (and nervous HVAC "pros" with heavy thumbs) erring to the conservative at every turn (like any Manual-J tool) it can deliver truly ridiculous numbers.