R value "value" calculator

[RatLabGuy]

I'm looking at various reasonable ways to improve the HVAC loss with the windows in my sunroom, which is 9x 4' tall windows covering probably 80% of the 3 outside walls + a glass pane door. The 9k mini split I added does a fine job keeping it warm, assuming summer will as well. Probably going to cover them w/ cellular shades.

Just for S&Gs, How would I go about estimating the current heat load and what the proportional change would be as I were to change the average R of the walls? E.g. I can guestimate a mean R per foot for the walls and ceiling, but then what? I know the kWh cost and the alleged wattage of the mini split.

95% sure the answer is that most things will never payout (I'm sure window replacements wouldn't) but I figure its a fun exercise.

 

[Mac5005]

Nebraska Energy Quarterly, September 2017

Newsletter of the Nebraska Energy Office

neo.ne.gov

Start there. That at least gives you the basis of the formulas for r and u values.

You should be able to then compare btu difference between current and hypothetical change.

Then change btu into your desired rate whether it be watts ( cooling) or leave in btu for heating.

You know the current setup and what it costs to heat & cool, but that would be for the entire residence, you’d really need to know what it costs energy wise for that sunroom to evaluate any potential change to that specific room.

This is based off, I assume, you are trying to determine cost of increasing effective rvalue of the wall will net xyz reduction in utility cost to maintain same temperatures, with similar T inside vs out.

Then it gets even muddier bc I assume you aren’t increasing r value across entire house.

But also remember the hierarchy for any wall, roof, or floor system is water>air>vapor>thermal.

So you can add to the thermal control layer but that’s pointless if there are compromises in the air control layer.

 

[Mac5005]

Also just bc the wall insulation may be r-13 you need to compare the actual wall r value which is diminished by the studs and windows.

Typical 2x4 on 16” center typically reduces effective r-value of the wall by 17-18%.

Single pane windows are r-1,

Double pane r-2/3 depending on age and details,

Some triple pane approach r-7.

 

[Mac5005]

This is screenshot from that link

 

[RatLabGuy]

Nebraska Energy Quarterly, September 2017

Newsletter of the Nebraska Energy Office

neo.ne.gov

Start there. That at least gives you the basis of the formulas for r and u values.

You should be able to then compare btu difference between current and hypothetical change.

Then change btu into your desired rate whether it be watts ( cooling) or leave in btu for heating.

You know the current setup and what it costs to heat & cool, but that would be for the entire residence, you’d really need to know what it costs energy wise for that sunroom to evaluate any potential change to that specific room.

This is based off, I assume, you are trying to determine cost of increasing effective rvalue of the wall will net xyz reduction in utility cost to maintain same temperatures, with similar T inside vs out.

Then it gets even muddier bc I assume you aren’t increasing r value across entire house.

But also remember the hierarchy for any wall, roof, or floor system is water>air>vapor>thermal.

So you can add to the thermal control layer but that’s pointless if there are compromises in the air control layer.

Thanks!
This is mostly a thought texperiment. Shopping for cellular shades I found myself asking, "Is the $500 difference for double-cell over standard single cell, which supposedly nets an additional ~0.8-1 R, on top of the ~2 from the shades really worth it?"

The sunroom is attached by the original sliding glass doors, which have an opening of only ~32". There's definitely some heat transfer but it isn't a lot, thats why we added the mini split.

The floor is just tile put right over the original cement slab (it used to be a porch) so there's a lot of cold coming through it too.

 

[shawn]

Don't waste your time. Plug it into Rescheck if you want, but it's GIGO. You can fix air leaks (even with curtains) and improve how comfortable the space is, but the payback on improving R value is measured in decades.