The Low Spark of Raised-Heel Trusses

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raised heel truss, photo by APA - The Engineered Wood Association

A comfortable, energy efficient home begins with a good building enclosure. That means control layers. You've got to control the flows of moisture, air, and heat. Today, let's talk about controlling heat and further, let's focus on heat going through the top of the house. If you've read my Flat or Lumpy article, you know it's better to have a uniform coverage of insulation rather than a lot of insulation in some places and little insulation in others. But you may not know about one place where most homes just can't get enough insulation.

Yep. I'm talking about the space over the exterior walls, where the roof comes down and leaves little space for insulation. Take a look at this photo below. It's from the Building America page on minimum insulation at the attic eave.

Conventional roof trusses leave little space for insulation over the exterior wall (Building America photo)

The trusses seem to be made from 2x4s, so there's about 4" of space for insulation over the exterior wall (bottom of photo). The rest of the attic will have about 12" or more of blown insulation. Hmmmm. Is there some reason that the space over the exterior wall might have lower susceptibility to heat loss and heat gain? In summer, it could be a bit cooler there because of venting (not shown above). In winter, that spot is likely to be worse than the middle of the attic, again because of venting. Overall, though, there's not good reason to have less insulation there.

And speaking of venting, check out this Building America photo showing incorrect installation of the baffles.

Incorrect placement of the ventilation baffle here means no insulation over the exterior wall (Building America photo)

The problem here is that the cardboard baffles are stapled to the inside of the top plate. In this house, as a result, there's going to be absolutely no insulation directly over that exterior wall (unless it was corrected). That's the kind of stuff that causes not only higher energy bills but also comfort problems because of reduced mean radiant temperature, durability problems because of condensation (including frost on the ceiling and walls in cold climates), and indoor air quality problems because of mold growth due to the condensation.

But we know how to solve this problem. The pen test will help you spot problems like this. The solution for roofs framed with trusses is something called the raised-heel truss, also called an energy heel truss or energy truss. (But be sure to see the name I propose below!) The diagram below comes from a paper by APA - The Engineered Wood Association titled Raised-Heel Trusses for Efficient, Cost-Effective, Comfortable Homes.

conventional vs. raised-heel roof trusses

Note the extra height at the heel of the raised-heel truss (right) compared to the conventional truss (left). You can get about 15" of space for insulation in raised-heel trusses. That's a lot better than 4", and it makes the attic insulation overall much closer (if not equal) to the flat side of things than the lumpy side. Insulation, like gravy, should not be lumpy, you know.

The Building America photo below shows a raised-heel truss and you can see even with the baffle for venting installed, there's a lot of room for insulation.

A raised heel with room for much more insulation over the exterior wall (Building America photo)

Let me draw your attention to one more benefit of raised-heel trusses. You can run the exterior sheathing up from the wall onto the heel of the truss. That gives you a nice dam to prevent wind-washing, a common problem at the eaves of conventional roofs. 

Those venting baffles shown in a couple of the photos above are supposed to prevent wind-washing, but when they aren't installed, as is the case in many older homes, or when they're improperly installed, wind comes up through the soffit vents and blows the insulation back into the attic. Even if an attic started with some insulation over the exterior walls, there may not be any there now if the attic is subject to wind-washing. That sheathing on raised-heel trusses makes it nearly impossible for the insulation on the edge to be blown inward.

The same principles apply to roofs framed onsite. You need to adjust the framing at the edge to allow for more insulation over the exterior walls at the eaves. Here's a Building America diagram showing how to do it by putting another top plate on the top of the ceiling joists.

Raised top plate for adequate insulation over exterior walls

If you're building a house and not adjusting your construction techniques to allow for adequate insulation at the exterior walls, you're missing a big opportunity. My only problem with raised-heel trusses, however, is the name. Raised, to me, indicates that the whole thing is lifted up. The top chord is higher but so is the bottom chord. Really, though, the whole thing is taller, not raised. So I propose we give these energy-saving trusses a new name: high-heeled trusses. Yeah! I like it. That's got some serious low spark1.

 

Related Articles

Flat or Lumpy - How Would You Like Your Insulation?

The Thermal Bridge To Nowhere

Batts, Blown, or Sprayed - What’s the Best Attic Insulation?

The Pen Test — A Control Layers Tool for Architects and Contractors

 


 

1. I may be dating myself with this reference to the classic 1971 song by Traffic, The Low Spark of High-Heeled Boys, but let me state for the record that I was only 10 years old when it came out and didn't find out about it until much later. Now Joe Lstiburek, on the other hand, is an old guy who probably listened to the album in his bedroom after hockey practice and got yelled at by his parents for playing it too loudly.

 

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Comments

Aug 7 2017 - 10:50am

Your pictures with raised heel trusses show them installed on a double top plate with the trusses aligned with the studs. Why is there a double top plate instead of a single one? Do we just like the thermal bridging? Is there any structural reason to justify the double plate?

Allison
Bailes
Aug 7 2017 - 10:55am

Good catch, Roy. First, those aren't my images. They're from the Building America page on attic insulation at the eaves. Second, no, thermal bridging is a bad thing. We don't like it at all. Not one bit. Ideally, there's continuous insulation outboard of the sheathing, but that's not the norm...not yet anway. Third, getting builders to move away from double top plates is difficult. They tie the exterior and interior walls together, and single top plates require stacking the trusses over the studs. It can be done but sometimes you gotta pick your battles.

Aug 7 2017 - 12:52pm

I find it interesting that builders want a double top plate so that they can tie walls together. Metal nail plates or even truss connector plates could be used to tie walls together, right? Surely if these connector plates are suitable for trusses, they would work for walls. I worked residential construction in the 60's and 70's. Even with 2x4 studs on 16" centers and double top plates, we still aligned the roof framing with the studs whenever we could.

Aug 7 2017 - 1:14pm

Theoretically it would be best to build the entire structure out of insulation but that just can't be done with this equipment. As a builder I have had to find the balance bw structure and thermal performance. Insulation doesn't hold the walls together nor can you nail moulding (and keep it straight) to max spacing studs, I've tried it and there are always complaints. Its easy to get tunnel vision on efficiency but there are many other factors involved. Exterior insulation or (my fav) a double stud wall give you the best of both worlds. Great thermal performance and it makes the lipstick look good too! If you forget the lipstick, trust me, someone will remind you and its usually the one paying the bill.

Aug 8 2017 - 9:36am

A few years ago, I was involved with the TxAIRE homes project at the University of Texas at Tyler. We built two research homes. They were both quite conventional architecture for that area. One house used conventional construction techniques (2x4 studs, 16" centers, insulated ceilings, etc.) and the other used a lot of advanced framing techniques (2x6 studs, 24" centers, minimal header sizing, no cripples, single top plate, insulated roof deck, drywall clips instead of backing, etc.). We counted every stick of lumber in both houses and showed that the advanced framing house actually used slightly less wood (board feet) than the conventionally framed house with at least a 50% increase in wall R-value. I suspect that the framing labor was also less, but we didn't track that. I was involved with these houses for several years after construction was completed and saw no problems structurally or aesthetically with the framing or drywall (1/2" everywhere). These were starter home designs (1500 sq.ft., one-story) and in this area, no one uses ceilings less than 9' high any more, so pre-cut studs and standard drywall sizes were not an issue. We did have some issues during construction such as improperly nailing on the header nail plates, but these were corrected. On the advanced house with 2' stud spacing, the sheathing was still nailed on 16" centers using the lines on the outside. Luckily I caught that problem before it was too late (The framers never noticed it). I realize that any changes to standard practices can cause problems, but I think that those can be overcome through training and inspections.

Aug 8 2017 - 3:54pm

Any energy data available on the two houses?

Aug 8 2017 - 4:59pm

Not much ever got published. I was involved with the design, construction, and instrumentation, but then our research funding ran out and I had to find another job. The houses are still there, but there isn't much research going on that I can see.

Aug 7 2017 - 11:09am

I love when you dabble in heavy Traffic.

Aug 7 2017 - 11:17am

I have always liked the design of raised-heel trusses to add insulation above the outer wall. One thing I just don't understand though, why do engineers/planners still specify more insulation in the uppermost ceiling area VS walls? I know that eons ago the stack effect was used as a rationale for additional ceiling insulation because that was where the largest delta T existed. But now every high efficiency home also includes either a mechanical ventilation system or a filtered/dehumidified mechanical air recirculation system - so it eliminates the stack effect by de-stratification. Now the largest delta T is either at the floor or in the basement walls. In fact, since attic air is always hotter than ambient air, the ceiling delta T now is the lowest. And yet the prescribed insulation values persist.

Aug 7 2017 - 1:28pm

I think mostly because it's cheap and easy to add more insulation over the ceiling. Adding 6" more of blown cellulose insulation in the attic is simple. Adding 4" more of foam insulation over the exterior is more difficult and expensive.

Aug 7 2017 - 2:10pm

Prescriptive R-values for ceilings are higher than walls because (a) ceiling insulation, typically fibrous, is not encapsulated on six sides, and (b) ceilings (under vented attics) are exposed to MUCH higher delta-T's.

It's useful to note that prescriptive R-values in the commercial energy code (IECC Section C402) are lower for roofs where there's no attic vs. ceilings under a vented attic. The residential code doesn't make such a distinction, probably because virtually all homes, until recently, had vented attics.

In homes with encapsulated attics with foam roof insulation, I typically specify 1.5 x wall insulation for the roof. A house with R13+R5 walls would get R28 roof. This often requires additional paperwork to satisfy building inspectors. Or use the performance compliance path.

Aug 7 2017 - 6:13pm

David - is this true for all USA climate zones, or just 1-3 (the hottest) due to AC loads? I'm in Zone 5 (on the border with 4) and my (semi-vented, 1932) attic is usually a little warmer than ambient but not terribly so, and we are considered a "heating-dominant" area - where slightly warmer attics in the Winter would be a good thing. It would seem like having a properly vented attic area (with air chutes running from soffit to peak so as to better cool off the roof deck and minimize re-radiation) in hotter zones would also minimize AC loads and so preclude higher R-values. Just looks to me like the DOE just used a non-mechanical ventilation/de-stratification/marginally vented attic situation to give prescriptive R-value info. I do remember one blog where the builder used the same R-value in walls and ceiling because of this (although it was a very high, "super insulated" value). I'm also guessing that this would all be moot if you use one of the building design modelling software packages that would take ALL characteristics into consideration.

Aug 7 2017 - 11:49pm

The prescriptive codes are indeed a sledge hammer. Even in CZ6 where cooling is not much of a factor, the commercial code requires R49 for a ceiling under a vented attic versus R30 for a non-attic roof. There is some logic to that: You can't count on atticT being higher than ambientT in cold weather. Think just before sunrise after a day or two of sub-freezing overcast skies. Also, it's well known that fiberglass is subject to a reduction in R-value when house-attic delta-T exceeds more than about 35F. And then there's the possibility of wind-wash.

You wrote: "It would seem like having a properly vented attic area (with air chutes running from soffit to peak so as to better cool off the roof deck and minimize re-radiation) in hotter zones would also minimize AC loads and so preclude higher R-values."

Not so much. even with proper venting, it's not unusual to see a 30F or 40F bump over outside ambient in an attic. Still, as long as HVAC can be located inside conditioned space, it generally cost less to build a vented attic with similar or better performance compared to an encapsulated attic.

BTW, encapsulated attics (or cathedralized ceilings) still need more R-value than a wall (I typically specify 1.5:1) since roof surfaces tend to get much hotter than walls due to more direct sun angles and (typically) dark roof coverings.

Aug 7 2017 - 2:44pm

@Allison, it's worth noting that raised heel trusses can be designed so that the bottom chord extends past the plate to form the bottom of the soffit, simplifying and potentially lowering the cost of building the soffits. A scissor truss can also have a raised heel.

Allison
Bailes
Aug 7 2017 - 2:51pm

Good points, David. Trusses can take a lot of different forms. Extending the bottom chord out for the soffit makes more work for installing the wall sheathing over the truss heel, since now the framers have to cut slots for the truss chords.

Aug 8 2017 - 9:49am

Are raised heel trusses not used because of aesthetic reasons or is it because interior space restraints continue to dominate the placement of HVAC equipment in the attic.

Now that HVAC system is in the attic the builder must use ocSPF to encapsulate the attic so the dwelling can meet blower door requirements and not have to worry too much about duct leakage.

Aug 8 2017 - 3:16pm

@JC, I think it's mostly a lack of familiarity. Good for Allison for shining a light on this.

As for your other comment -- I think most multi-story and slab-on-grade homes are still being built with HVAC in vented attics. Although I certainly don't condone that practice, most builders would rather pay a bit extra for duct mastic than for encapsulating the attic with SPF.

Allison
Bailes
Aug 8 2017 - 3:58pm

JC, I agree with David. I think a lot of builders just aren't aware of raised-heel trusses. (That's not true for the ones who read this blog, though!)

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