Replies

1. Expert Member
   Peter Engle | Feb 27, 2021 09:07am | #1

   Your math looks to be about right. R value is calculated as (square feet)*(temp.diff.)/(heat loss). If these are the real outputs from a Manual J or similar analysis, I would go back and check the inputs. Make sure the square foot measurements are accurate - it's easy to forget some areas or make mistakes adding wall areas for instance.

   The window total R-value may be nearly correct. Window manufacturers often advertise R-values (or U-values_ through the glass. The total R-value of the assembly is significantly less. It is also possible that the Manual J inputs assume significant air leakage through the windows. If your windows actually have a whole-window R-value of 3.4, they probably also have nearly zero air leakage. Many Manual J programs have presets for air leakage that may not be accurate in high-performance houses. That will skew the results.

2. 
   josephny | Feb 27, 2021 12:36pm | #2

   I didn't do this Manual J, but inputs (R valvues for walls, roof and windows) see off.

   Even through the spray foam between the studs on the wall totals R25, the whole wall assembly after factoring in bridging through studs is probably half that.

   Does that mean that this Manual J is grossly inaccurate -- and that my heat load is in reality far higher?

1. 
   charlie_sullivan | Feb 27, 2021 01:22pm | #3

   In reality, I think your heat load is overall smaller. The window and roof transmission numbers are too high, and the wall too low.

   Sometimes HVAC contractors do a manual J really sloppily, because they plan on oversizing anyway, and they just do something sloppy to say they did it. I'm hoping you didn't pay for this to be done. You might want to pay for someone to do it better. Or do it yourself.

1. 
   josephny | Feb 27, 2021 03:28pm | #4

   That's interesting because I thought the derived overall r values or the walls was way too high. While I thought the derived overall r value for the roof was way too low. And it feels like the heat loss through the windows seems large.

   But I'm trying to figure out how this happened. The square footage for everything is about right (house is 1500 sqft 1st floor; 1500 sqft 2nd floor; 1000 sqft 3d floor. overall 45'x34', 11' 1st floor; 10' 2nd floor; 3rd floor height goes from 0 where rafters meet the 45' wall to 14' at peak).

   Now what?

3. 
   josephny | Feb 28, 2021 02:08pm | #5

   I just get more and more confused the more I study this analysis.

   Other than the questions around how the walls, windows and roof total 53,247 btu/h, I am totally in the dark how adding the ERV adds another 26,278 BTU/H in heat load. All the lay-person's explanations tout ERV's/HRV's as reducing the heating load. Is this in comparison to leaving a window open for the same amount of fresh air CFM?

1. 
   charlie_sullivan | Feb 28, 2021 02:33pm | #6

   Yes, it's in comparison to leaving a window open that it saves energy, but still, 26 kBTU/h is huge for an ERV. I think the analysis is junk and it's time to stop trying to figure it out and to either to a better analysis or hire someone to do one.

1. 
   josephny | Feb 28, 2021 03:12pm | #7

   I did hire someone -- I did not do this one myself. I hired someone who claims to be an expert at this.

   I can't go back to him and say 'the experts online say you did this all wrong' without pointing out specific errors.