Comments on: Hidden Air Leakage Sites in Your Attic

By: Allison Bailes

Allison Bailes — Tue, 30 Nov 2010 00:07:25 +0000

Thanks, David. I thought you
Thanks, David. I thought you might step in and explain those tradeoffs between envelope improvements and PV further. It's a good point that you need to understand if you want to determine the best measures for making a home as efficient and cost-effective as possible.

By: Allison Bailes

Allison Bailes — Tue, 30 Nov 2010 00:00:09 +0000

Sam, good point about
Sam, good point about effective R-value. I think ENERGY STAR Version 3 takes a big step forward by requiring reduced thermal bridging, and thus higher effective R-values.

By: Allison Bailes

Allison Bailes — Mon, 29 Nov 2010 23:53:52 +0000

Green Inspector, Building
Green Inspector, Building Science Corporation is one of the best sources of info on insulated rooflines. Here’s an article you can download from their site about this topic:

http://bit.ly/eJwcIw

By: David Butler

David Butler — Mon, 29 Nov 2010 23:52:20 +0000

Les, REM does a pretty good
Les, REM does a pretty good job with ceilings under vented attics. Not so good with encapsulated attics. Unless AE added support for encapsulated attics in latest version, you have to model attic as a vaulted ceiling for the rooms below (per Dave Roberts when he worked at AE) and then make a judgment call as to how much this over or understates the actual load.

Depending on how you model the duct location, I believe a vaulted ceiling understates the load when used as a proxy for an encapsulated attic, especially if you have lots of uninsulated knee walls. In the case of a bonus room (over a garage), I always specify the thermal boundary at the kneewalls since the space below is unconditioned.

By: David Butler

David Butler — Mon, 29 Nov 2010 23:29:54 +0000

Since my name was invoked, I
Since my name was invoked, I'll chime in. Actually, I'm somewhat skeptical of the PassivHaus concept. What Allison is referring to is my concern that super high performance envelopes and HVAC gear are often justified based on some rate of energy inflation. That's fine as far as it goes, but in markets where air or ground source heat pumps are cost effective, it's important to recognize that renewable energy effectively caps energy inflation in a lifecycle cost analysis. For example, if electricity is $0.10/kwh (and rising) and the subsidized cost of PV is currently $0.18/kwh (and falling, at least until 2016), then you won't be far off if you predict the two will intersect at around 13 to 15 cents. In that scenario, it makes no sense to justify (or should I say rationalize) the PassivHaus approach based on electricity costs rising to 20 cents. The analysis is more speculative when fossil fuel prices must be considered.

In parts of AZ, the amortized cost of a PV system per kWh is already lower than grid power. In that case, the cost of any efficiency upgrade must be less than the equivalent in PV capacity at current prices.

By: Allison Bailes

Allison Bailes — Mon, 29 Nov 2010 21:15:19 +0000

Les, Jason is definitely on
Les, Jason is definitely on the leading edge in the world of EEMs. He knows more about them than anyone else I know. In fact, he's even going to do some training in how to do EEMs, and he's going to have a booth adjacent to ours at the RESNET conference.

The energy code allows for tradeoffs, so R-20 in the roofline isn't necessarily a code violation if the rest of the envelope allows the overall U-value to meet or beat the code.

And yes, energy modeling is a great way to make decisions about which way to go. Absolute numbers don't always come close to actual usage, but when you look at the difference in usage between various scenarios, I think REM does a pretty good job.

Finally, the diminishing returns of extra insulation are real, but you have to look at everything. PassivHaus standards are really tough to meet, but if you get to the point where you can eliminate the big expensive heating system in a cold climate, extra insulation pays for itself. But, as David Butler would say, you also have to look at the cost effectiveness of superinsulation relative to putting photovoltaics on the house. There are a lot of variables.

By: Sam Young

Sam Young — Mon, 29 Nov 2010 19:50:07 +0000

Asking a production builder
Asking a production builder to spray over the rafters is not an argument I’m going to win easily. Why? As soon as I go to energy modeling, R-19, not R-30 becomes the minimum! Let’s not forget R-30 rated is not R-30 effective. By the time I get a foam depth equal to R-25, I may be lucky to have an R-19 effective — depending on whether I have 2×6 or 2×8 rafters. I’d like the building code to state what effective R-value they want so I can help a builder achieve. The code is written backwards!

By: The Green Inspector

The Green Inspector — Mon, 29 Nov 2010 19:06:37 +0000

Allison, what are the proper
Allison, what are the proper standards for spray foam for the underside of the roof when changing the envelope/thermal barrier with regard to type of spray foam and depth (here in the northeast vs the southeast and west)?

By: Les Watts

Les Watts — Mon, 29 Nov 2010 19:06:30 +0000

Good article Allison.&
Good article Allison.

Jason is on the leading edge by using what has been around for more than a decade, EEM's!! I look forward to seeing the proliferation of this financing and the opportunities that it 'lends'.

The R-30 debate is one that will be ongoing!! Inspectors have a lot to do and very little to work with and cannot test and verify spray foam like they can fiberglass. A blind eye would be the fact that they are not enforcing the air leakage code, but that is something Martin Holladay has written about in his GBA blog.

In Rem/Rate though, the diminishing returns is evident and reproducible. I want to know if we should be pushing for more thickness and suffer setbacks in tightening the envelope due to a decrease in demand due via an increase in cost by 35+%, or should we be paying attention to ROI and the info that our energy modeling software supplies us and pushing for air leakage testing instead? Two wrongs does not make a right, but is Rem/Rate lying to us? Just take one of your files that has a tight envelope and change the R-value only for the ceiling and see what you get. I may be putting to much faith in Rem/Rate, but the numbers are what we live by...RIGHT?

By: Allison Bailes

Allison Bailes — Mon, 29 Nov 2010 18:41:00 +0000

Paul, I agree it would've
Paul, I agree it would've been best to take the fiberglass out completely, but they did blow it all to the edges, so that most of the ceiling sheetrock is showing.