Windows with Covered Porches
I am planning my first house in North Carolina (climate zones 4/3) and will have a wrap around porch on three sides of the house. The porch is 8′ wide with a roof on the 2nd floor (deck on second floor is not waterproof).
A while ago there was a podcast interview with either Allison Bailes or Joseph Lstiburek (been too long to remember) where they talked about windows. They mentioned that it was a waste to go for expensive windows with low SHGC on covered porches because there was no direct sunlight and almost no exposure to rain.
This makes sense to me but I can’t figure out what parts I can save money on and which ones are still important. Obviously, I still want a relatively low U-Factor and airtightness but what else is important?
Do I still need triple pane?
Should I still use a more expensive frame or is a high quality vinyl sufficient due to the low sun exposure?
And after all that, is there a window brand and series out there that would fit for this application?
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Replies
Double vs triple pane will show up in the installed overall U-value. If a double-pane can get you the U-value you want, then go for it. But on average the triple pane would have lower U-value.
Frame material will show up a little bit in the U-value as well as long term durability and temperature expansion/contraction. Your decision of U-value and frame material has very little to do with sun exposure. Not sure why being under a porch would make you approach the decision differently.
SHGC and maybe visible transmittance (VT) are the only two knobs you may want to turn in this application. You probably also want them to loosely match other windows in your home so they don't stand out too much, especially if you have other windows in this same room.
I can't give any brand recommendations because you have not adequately described what type of performance values you are looking for, if they are operable/non-operable, tilt-turn vs casement vs double hung, true divided light vs simulated, budget for the windows, color, etc.
From what I have read and my own experience, not all frame materials are created equal. Some vinyl windows will degrade quicker due to sun exposure. This is more a quality issue but it isn't something they advertise as part of the window specs.
I am basically looking for the magical best-value window. Most of the covered windows will be fixed. The few operable windows will be casement but those are under a 2' overhang so no excessive exposure either.
Budget is not set yet and part of why I'm asking this question. I am trying to see how much I should put aside for windows and if there is any savings or if I should just go with fiberglass, triple pane, Marvin windows everywhere.
I think Marvin makes a decent window but if you look at price and specs (mainly U-value) I think you will find other options which better performance at the same or lower cost. Search here for some recent price quotes from some people.
Marvin's prices and lead-times have come down quite a bit since the first of the year so it is worth a good luck.
Shaded windows under 8' porch roof are protected from the sun and direct rain, so you could relax your specs if needed. Triple pane windows in CZ3 offer no advantages, and in CZ4, could depend on your location and microclimate. I’m assuming you are in western NC, to be in CZ3-4. You could be close to CZ5 or with a similar microclimate, check 2021 IRC Chapter 11. An energy analysis can provide better results.
When it comes to frames, there are good and bad vinyl, wood and clad windows. Over the years, we’ve installed good vinyl windows (Sierra Pacific, Anderson and Pella) with great results. Aluminum clad windows are very good, but they cost more. Wood windows are good as well but come with more maintenance.
I recommend you follow Energy Star window specifications. See:
https://www.energystar.gov/products/residential_windows_doors_and_skylights/key_product_criteria
https://www.energystar.gov/products/res_windows_doors_skylights
We are in eastern NC, South Central zone in the link you attached.
Are you saying the benefits of triple pane wouldn't be worth the additional cost?
I always look at the 2021 IRC N1101.7(R301.1) Climate zones map. It shows a more detailed climate zones. https://basc.pnnl.gov/images/climate-zone-map-iecc-2021
There are no benefits to use triple-pane windows in CZ3... zero, zip, zilch, nada, nil!
Could you expand on this a bit? It seems a bit counter intuitive to say there is no benefit, as they generally have a higher u-value, you would think they would provide greater performance. Are you saying that there is no cost benefit or that there is simply no benefit at all? Also the noise dampening benefits are often extolled, is this also not true?
Do a simple BEopt model with each window type and try to see what energy usage difference there is. Multiply the difference by your nominal electricity cost. See what the payout is.
Ok, so, I did a very simple BEopt and here is what I came up with, let me know why this is wrong or doesn't add up.
Here is what i modeled:
2 story 2000 sf house simple box
window area #7: 18% f25 b25 L25 R25
ach 50 1
23 seer heat pump split
r21 2x6 walls
r38 roof
100% LED light
windows:
double pane .26 u .3 shgc
triple pane .17 u .3 shgc
location
Auburn CA
suburban
energy site usage:
8787 kwh with double pane
8564 kwh with triple
223 kwh difference
at $.34 cents kwh = $75.82/year
using that pnl study the range of increased cost of triple pane is 700-2400 for 2400 sf house.
Say its 10-30 year payback, or about 20.
I'm not saying that is particularly good but should be within the life of a window, especially a mid to upper end window.
What did I do wrong?
A few studies that showcase the benefits of triple-pane windows are done with a very high participation of Builders in northern climate zones. PNL did a great study 4 or 5 years ago, and the main focus was costs.
A couple other studies for R5 windows were done by the National Glass Association and the Northwest Energy Efficiency Alliance. Unfortunately, they are both northern climate studies.
I’ve done energy analysis in CZ3, and the cost benefits are big time on the negative side. The cost analysis I’ve done for CZ4 were in NM, AZ and CA, where temperatures are milder, no humidity and rural to semi-rural homes (ie, Albuquerque, Scottsdale, SoCal). Cost analysis in CZ4 is less than even, where most clients decide based on window costs.
Yes, there can be some benefits other than costs.
Noise reduction: It is much higher is urban areas, higher traffic, higher densities and minority neighborhoods in segregated areas. In general, down south, our cities have lower densities, flatter ground and lower structures that allow our cities noise levels to be less.
Although I have no studies to rely on, all our homes with very good building envelopes and low ±1ACH50, and those envelopes reduce to almost negligeable noise pollution with double-pane, argon filled, low-e windows. I design most all homes with 13% to 15% glazing.
Comfort and condensation: Lower temperatures for most of the year provide us with advantages that northern climate zones won’t have. Most CZ3 locations have winter temps in the 50s, and certainly above condensation issues, as long as whole house humidity is controlled.
Most of our CZ3 homes have high-performing HVAC systems, with U=.25 and SHGC=.30 windows, and in a 5k sf house, the increase is around 3k Btu’s. That can easily be checked with a good Manual J.
“ I am trying to see how much I should put aside for windows and if there is any savings or if I should just go with fiberglass, triple pane, Marvin windows everywhere.”
Windows are often the biggest line item in the budget. 20%?
From a dollars and cents point of view triple pane windows are crazy CZ3.
I do not recall the solar gain and other coating being the big factor in the price and when I modeled my house the differences were very small.
I did splurge and bought the Marvin fiberglass windows for my great room while the rest of the house got Anderson Silver Line windows. Yes I could detect air leakage around the cheap windows when the blower door was running but I am happy with my 1.6 ACH50 score.
Walta
Are you saying the cheaper windows were leaking around the frame? Would this not be an installation issue more than the product?
Did you consider doing something like AeroBarrier to catch any leaks?
Yanniktk, My air leak goes into the balancing springs of the windows Any goop in their would likely make operating the window impossible. I am not going to try to make a silk purse out of the sow’s ear I bought. Air movement is only detectable with smoke when a blower door is in place and operating, otherwise unnoticeable.
Like with most things you get what you pay for. When something costs less it is probably not as good.
If I had spent more for better window frames. Yes, my ACH50 number would be lower but not by much and I doubt I would notice any small comfort difference. Yes, I am disappointed that they have a small leaks. When you build a house, you must make thousands of choices. You must make the best choices you can with the info you have at the time. Once the choice is made and the product is installed one must let go of the roads not taken or you will make yourself crazy. If I had known what I know now I doubt I would chouse differently.
Walta