Overhead Door Insulation Calculations

[Ashful]

Rather than derailing another thread [1], I'm posting separately on R-value calculations on garage doors here, thinking it might be of interest to several readers. I had taken a few basic equations provided by woodgeek, and applied them to a few different garage doors I was considering. Total surface area is 140 sq.ft., for two 9.5 x 7.25 foot doors.

Doors under consideration are urethane-foamed steel panel doors (typ.R16), styrene insulated PVC frame and panel doors (typ.R9), styrene insulated wood overlay doors (typ.R4), and uninsulated wood frame and panel doors (typ.R1).

This is for a shop, where I'll have auto change-over thermostat's set to 55F or 60F heat and maybe 85F or 90F cool, the primary goal being keeping things dry and preserving paints and chemicals. Thus, I plotted heating and cooling based on our local (SE PA) HDD's and CDD's, with heating base temperature = 55F and cooling base temperature = 85F:

The cost (Dollars/year) was derived using my proposed cooling system's HSPF = 10 and SEER = 20.5 (conservative, given temps I will be running), and our net PECO billing of $16.75/kWh.

I was trying to determine if the much more cosmetically appealing PVC frame and panel door (R9) was going to cost me enormously in heating and cooling, versus a more efficient (but less attractive) urethane on steel panel door (R16). The total cost difference PER YEAR was only $8, and in fact I could even go down to a wood overlay door (R4) with a penalty of only $32/year. The radiation penalty for going to a lower R-value on a pair of garage doors, given how I plan to heat and cool, is surely way below the dollars that will be lost to drafty door seals.

Knowing someone might have a problem with my chosen base temps, I also re-did this using more typical base temps for heating and cooling a house (65F / 80F). This might be of interest to those who finish a garage as a spare den or exercise room, and keep it heated closer to house temps:

As woodgeek had already stated qualitatively in the other thread, increasing R-values yield diminishing return. The difference between R1 (uninsulated wood frame and panel door) and R9 (PVC/styrene frame and panel door) is enormous, at $250 per year, but the difference between R9 and R16 is not that significant.


[1] - https://www.hearth.com/talk/threads/insulation-calculations.136769/page-2#post-1861889

 

[Swedishchef]

Did you pick a company from which you're buying your doors?

Andrew

 

[DickRussell]

Be careful in using advertised R values of manufactured garage doors. Effective R of one more typically is around a third of advertised. Here is a good read on the subject:
http://www.greenbuildingadvisor.com/blogs/dept/musings/energy-efficient-garage-doors

Granted, the blog is five years old, but I suspect little has changed in the advertising of center-of-panel R value as though that were whole-door R value. Here is a more recent blog from GBA on the more general subject of heating a garage:
http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/how-heat-garage

 

[Ashful]

Hi guys. Yep... well aware of the trickery in door ratings. What'cha gonna do, tho? You're at the mercy of industry "standards".

Doors will be Fimbel RT-11S.

 

[Swedishchef]

Niiiice!
I find door insulation values are like car mileage ratings. It seems like gas mileage ratings are taken while doing down a slight slope with a wind in the back and a drive that weighs 50 lbs! However, it is a good guide. Lol.

Andrew

 

[ewdudley]

Let's not get ahead of ourselve's with all the numbers. If you don't want to loose all you're heat just buy all the insulation you can afford, you'll never regret it!

 

[maple1]

This appears to be a question of $ spent vs. appearance.

Beauty is in the eye of the beholder & I don't know of any math that can measure that.

 

[Ashful]

Let's not get ahead of ourselve's with all the numbers. If you don't want to loose all you're heat just buy all the insulation you can afford, you'll never regret it!

Well, I could afford to just spray foam the whole barn 2 feet thick... or just heat with no insulation, but I wouldn't do that. It's always a compromise between up front vs. extended cost, and cosmetics. The numbers are just one of several factors in the overall decision, but still something you want to consider.

 

[esengi]

This is the way to go. New garage door that is insulated and airtight. The make very good aluminum doors that are insulated as well. No way to ever get a sh***y door as airtight - I've tried and failed. I have a sh***y door that came with a small amount of white styfoam insulation https://mechanicguides.com/best-garage-door-insulation-kit/ and while I should have replaced it because that would have been much easier I've spent way to much time insulating it - but it is kind of fun. Over the top of the oem vinyl backed styrofoam I added 1/2 of reflective foam cut to size and glued in with insulation glue. I taped it as best I could but because of moisture building up at the seams not all of the tape has held - still it does create a seal albeit imperfect. I used rubber seals with glue backing between all the sections - they are easy to install with the door halfway open and they make a huge difference. Lastly I am currently putting reflective fiberglass over the top of it all to give it a chance at meeting firecode and to cover the seams. It is not airtight at the edges of course but the glass lays down pretty good and I'll add some expansion insulation to tighten it up the best I can. I'm using self taping sheet metal screws with big rubber washers on them plus tape to make sure I don't have a problem with this stuff falling off. It is r6.7 when properly installed so I assume I'll grab at least a portion of that on top of the foamboard on top of the styrofoam + walls blown full of cellulose + attic insulated to between r-30 and r-40 depending on the spot = pretty darn warm in my attached garage even without running the heater.

 

[firefighterjake]

Zombie thread!