A CHEAT SHEET FOR ENERGY MODELLING OLD HOMES

Sustainable Energi Solutions

The biggest errors occur in modeling estimates of energy use in older homes; usually post-retrofit energy use is pretty close to modeled estimates, but pre-retrofit use is dramatically overestimated because of poor assumptions, biased inputs, and bad algorithms.”

Poor assumptions. Models and auditors underestimate the efficiency of existing heating equipment, they often assume 60% efficiency for old furnaces.

Low R-value estimates for existing walls (R-3.5) and attics. Many defaults are biased; they assume R-3.5 for an old wall when many old walls actually perform at R-5 or R-6. Energy models often underestimate the effects of a high framing factor, thick sheathing, and multiple layers of old siding, all of which impr
ove a wall’s R-value.


Low R-value estimates for existing single-pane windows. We assume that old single-pane windows are R-1, when they are probably closer to R-1.35 or R-1.4. When calculating the outside surface film coefficient, they assume worst-case conditions — in other words, that the wind is always blowing away heat from the window. They do it that way because the design load is always calculated for the coldest, windiest day of the year (even though the coldest day usually isn’t windy). If an auditor calculates single-pane windows at R-1, he’s assuming that the wind is blowing continuously nonstop all winter long. But in a real house, the wind speed is often close to zero up against the window. 

Low or absent estimates for thermal regain. Energy models underestimate thermal regain from basements and crawlspaces. Most models get big things wrong, like how basements and crawlspaces work. Vented crawl spaces usually aren’t at the outdoor temperature. When the outdoor temperature is 10 degrees, a vented crawl space can be at 50 degrees. Why is it that when we insulate a basement ceiling, we get minimal savings — maybe zero savings, or maybe $20 a year? 

Well, if you have a furnace and ductwork in the basement, you are regaining a lot of the heat given off by the furnace and ducts, due to the directional nature of air leakage in the wintertime. The stack effect brings basement air upstairs. The basement is pretty warm, so the air leaking into the house is warmer than the models predict. A similar effect happens in attics: because of the stack effect, most of the air leaving the house leaves through the attic. In a leaky house, you might have 200 cfm of air flow being dumped into the attic. That makes the attic warmer than the models predict. If the attic is 50 degrees, the heat loss through the ceiling insulation is less than the model assumes.

Also check for foundation heat loss, infiltration, wall heat loss, attic heat loss, framing factors, edge effects, window heat loss, window heat gain, exterior shading, interior shading, the effect of insect screens, air films, HVAC equipment performance, duct efficiency and regain, AC refrigerant charge, and air flows over HVAC coils. There are many unknowns: soil conductivity and ground temperatures are unknown. Wind speed is unknown. Leak locations are unknown. 

Experience says that better results can be predicated by asking the right set of questions, than running complex computer models. 

(the questions posed above are by no means exclusive, comments from readers of this blog are appreciated)

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Tags: Energy, Modelling, efficiency, energy, net, zero

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Comment by Bijou Lulla on July 17, 2015 at 8:41am

There is usually no right or wrong answers for these things, modelling and delivering energy savings is much an effort of 'art' and 'science'.

Every house is different, and climate patterns are becoming more unpredictable going forward. 

A good template for action is designing right measurement protocols (a blend of the 'experience' and 'science'), measuring what is important (few relevant metrics and not 100's of data points), disciplined project execution process, and tracking these results over time which involves re-visiting the client out in the future (one year is a good moratorium) to verify our efforts. 

How many contractors re-visit their clients only to validate their assumptions? 

Comment by tedkidd on July 16, 2015 at 9:16am

Nice post Jeff. Some high level insights. 

Are you tracking your project savings? Are you comparing them to projected savings? Can I see those numbers? 

Comment by Jeff Martin on July 16, 2015 at 9:07am

I'm in Syracuse,NY.  Our HPwES program here require models to be "trued" up to actual usage we must obtain from the customer.  We use TREAT which calculates it's own usage, you can't enter it.  When the model calculates you have to adjust R values & efficiency to get Base & heating usage to be true.

I do have a differing opinion on most of your statements as from our personal experience and work practices.

In NY our walls & ceilings are always modeled at 4 or higher.

The window calc's you mentioned would have a negligible effect on use or savings, with a glazing area of 10-15% the R.3-.4 over such a small area won't amount to much, and is typically offset by other savings not calculated.

Thermal Regain at least here is dependent on the blower door number. A home with a reading 3-7 times BAS which is most common with our  housing stock, full basements are very cold.  Crawlspaces WITHOUT vents just leakage often have water lines freeze & customers have added either heat tape, pipe wrap, additional heat source or a combo of the 3 to keep the lines from bursting.  Here you can't leave a crawl space vented if there are water lines present, unless you have properly installed insulation much like a doublewide mobile home. 85%+ of our customers that have a room over a crawl space instantly tell us during the initial interview the floors in that room are FREEZING.

Here we have additional savings that often far outweigh a few that may be overstated.  Our Cooling Degree Days are very few, so modeling software only calculates small usage.  However most use A/C 30-50 days a year if not more.  The humidity plays a big role, & customers are just dropping the temp a couple degrees & using the AC as a glorified dehumidifier.  The local weathermen include their versions of a "muggy meter" in their forecasts & often tell people they will need AC just to sleep comfortably when the temp is near 70.

The second is the thermostat set points. Since our housing stock is typically very leaky, customers often have the temp set a couple degrees higher than needed in an attempt to compensate for the drafts.  In probably 30-40% of homes this effect is worsened by the forced air ducts.  These homes still have the supplies on interior walls as they are from the original gravity feed Octopus furnace that was installed when the home was built.  The temperature disparity from one side of the room to the other can be several degrees.  This leads to an additional couple degrees warmer or the Tstat, plus obvious comfort issues.   Once the distribution is fixed, & the home is properly air sealed the Tstat can often be set a couple to several degrees cooler.  How much we don't know until after the work so we can't calculate those savings. 

Comment by tedkidd on July 16, 2015 at 8:16am
Wrong. You nicely illustrate the where, but not the why.

The biggest errors occur because modeling accurately is more work, there is little to no incentive to model accurately, and in many cases there IS incentive to overstate savings.

To solve the problem of modelling accuracy you have to fix the why. Do that and the where will solve itself.
Comment by Nick Semon on July 16, 2015 at 7:58am

You bring up some really excellent points.  Many of the homes in my area are very old and certainly tick most of the boxes in your post.  In particular, I'm curious about how you deal with low estimates for thermal regain.  I can modify HVAC efficiencies and R-values pretty readily, but if I'm not tinkering with the model algorithm, what do you suggest for that thermal regain estimate?

Incidentally -- does anyone have experience with Optimiser's ability to swing the model to match actual historical energy use?  I'm playing around with it now, and I'd love to hear about any experiences with that regarding older homes.

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