U-Values

11 posts / 0 new
Last post

To All,

I seem to be having some issues with my baseline design and my first
parametric run. (I am working in detail mode with INP file) When I make my
baseline wall value R-13 and change it in my run to R-27 my heating (MBTU)
load goes down by 4% however my electric rate (kWh) goes up 1%. I don't
really understand because I thought both would have gone down. Am I doing
something wrong?

Thanks,

PETER HILLERMANN

Peter Hillermann's picture
Joined: 2011-09-30
Reputation: 0

Peter,

We'd need to know more about building location, climate, load profile, etc.
When does the cooling electrical consumption increase (time of year and/or
time of day) vs baseline?

I have seen this happen when there's a lot of internal load and with the
right climate.

Steven

Steven Savich's picture
Offline
Joined: 2011-09-30
Reputation: 200

I would say that's pretty much to be expected. As R value decreases, more heat is lost though the envelope, so cooling loads decrease.

Typically when you increase R-value, the heating savings will outweigh the additional cooling cost - however I'm sure there are specific climates where this is not the case.

Alex Krickx

John Dossmith's picture
Offline
Joined: 2011-09-30
Reputation: 0

Peter,

This is the exact reason I've come to really enjoy energy modeling.
Anytime you get results that you don't expect, whether your
preconceptions are right or wrong, you're bound to learn something -
sometime profound!

You must make the call on whether this makes sense ultimately, but there
are probably reasonable explanations for what you're seeing. It would
help us help you if you shared more, like what climate the building is
in, what systems you're using... etc. You will want to come up with
explanations, then investigate the model results to see if that syncs
with the behavior modeled.

As a start: R-27 is a high wall value - Note that walls/roofs can be
both over- and under-insulated. Likely, you're seeing gas savings from
losing less heat in the winter and thus having less heating load, while
simultaneously trapping heat inside your building during the cooling
months, increasing your cooling equipment loads. Hard to make more
specific guesses without more information.

Sidenote - take care that you're not skewing results by wiping out the
effects of wall studs when comparing a new baseline insulation value
(Re: Table A9.2.B, 90.1-2004/2007).

~Nick

NICK CATON, E.I.T.

Nick-Caton's picture
Offline
Joined: 2011-09-30
Reputation: 805

To my friends on the list,

Those explanations have made perfect sense. Nick this too is the reason I am
absolutely enjoying it. I was just telling my boss how your perception of
design changes when you introduce what could actually be going on. I love
the analogy Nick posted about trapping heat in the summer. It makes perfect
sense because the more you insulate a building the better it is at NOT
transmitting heat energy. So you put 1 piece of storefront on your building
that introduces radiant heat to your floor and you're cooking the inside or
like you said one piece of equipment generating enough heat and it won't let
it escape.

Thanks Nick light bulb just went off.

Thanks,

PETER HILLERMANN

Peter Hillermann's picture
Joined: 2011-09-30
Reputation: 0

I think it is expected if your internal heat loads (during cooling) are
usually higher than external heat gain which means the building is
"trapping heat" . This happened to one of our models where we have a
constant 120 kW electrical heat generation from an industrial
control/electrical room (Southern Ontario climate ). The increased
insulation was actually increasing the cooling load significantly and we
had to find an optimum insulation value.

Bernardo Majano, B.Eng.

Bernardo Majano's picture
Offline
Joined: 2011-09-30
Reputation: 0

Peter et al,

Nick said what I was going to say! I'm guessing at this point that your
climate is fairly warm (i.e. Georgia climate) and that the increased insul
is like Nick stated (over-insulating) for the cooling aspect of your model.

I had a project once on a college campus in Canada where the design team did
such a good job at thier ECM design stratagies that they over-designed the
building and moved the building characteristics from being a heating
dominated cimate building to being cooing dominated. We (as the sim team)
made the recommendations that they actually remove thier ventilation heat
recovery systems (completely) and lessen the "thightness" of the envelope
design so that the building could experience some losses through the skin
and balance out the energy use of the building operations. This was the
first time I've had to recommend to "scale back" on the design strategies.
(FYI-the HVAC was tied to a geothermal loop that needed to have more
balanced energy use for it's proper operation. With too much cooling to the
ground the geothermal system was becoming unbalanced. For the real
simulation-techies, we then integrated our simulations with TRNSYS to
replicate the increasing ground temps that would be resultant if the
building energy was not better balanced in the design to work with the
geothermal system & well field.)

Just like for so many of us when this light bulb goes on we can really see
the value of energy simulation for aiding design choices and this is when
energy modeling is actually FUN! :)

pasha

Pasha Korber-Gonzalez's picture
Joined: 2011-09-30
Reputation: 600

I've experienced the same issue with wall insulation in a mult-family
model, and a high-rise office model, in coastal southern California.
The design team just didn't understand why more insulation didn't equate
to more energy savings. The cooling penalty attributed to less heat
escaping through the walls during summer outweighed the natural gas
heating savings. Addition of more insulation beyond code requirements
in moderate climates is not necessarily more efficient was the lesson
learned.

JEFF STALLER

Jeff Staller2's picture
Offline
Joined: 2011-09-30
Reputation: 0

Pasha,

You are absolutely correct. When I started this journey I was trying to use
"common sense design," which turns out not to be as common sense as one
would think.

As a side note for the project I'm working on does anyone know the ASHRAE
watt/ft2 count for a car dealership? Is it the 1.5 or 0.9, I hear it all
kinds of ways and I'd like to get some other opinions.

Thanks,

PETER HILLERMANN

Peter Hillermann's picture
Joined: 2011-09-30
Reputation: 0

I feel like such a nerd...

Off the top of my head:

1.5 is the figure for interior lighting of retail, whole building method

0.5 is the figure for tradable car lot surface, exterior lighting

~Nick

Nick-Caton's picture
Offline
Joined: 2011-09-30
Reputation: 805

Thank you Nick, and sometimes it pays to be the nerd. I see Bill Gates
smiling everyday.

Peter Hillermann's picture
Joined: 2011-09-30
Reputation: 0