Many of our clients had been using the “Prescriptive R-Value” method of compliance for reroofing projects. This is the simplest of the compliance paths, but also the most conservative. With the adoption of the new 2016 New York State Energy Code on 3 October, it may be time to consider an alternative – namely, the Prescriptive U-Factor, since the new code changed the required prescriptive R-value of above-deck insulation in Climate Zones 5 and 6 from 20 to 30 – a 50% increase. The prescriptive U-factor method requirements, as well as the thresholds for COMcheck and REScheck, have changed similarly, but require less insulation for tapered insulation systems. Here’s why.
The R-value of insulation, which most everyone is familiar with, has a straightforward but subtly complex relationship with its inverted counterpart, the U-factor. Simply put, U=1/R and R=1/U. This means that while an assembly’s R-value increases linearly with insulation thickness, the corresponding U-factor decreases asymptotically. Many design professionals are not clear what the implications of this are, regarding variable or tapered insulation thicknesses. Are you?
What this means is that R-values cannot be averaged by area. In order to determine an effective R-value for varying amounts of insulation, you can invert the R-values to arrive at U-factors for each area, obtain an area-weighted average U-factor, then invert the result. See below.
In the world of roof insulation, tapered thicknesses to create positive pitches to drains are common. This makes the calculation rather complex, as shown below.
So how does a conscientious design professional determine the adequacy of a tapered insulation roof assembly to meet code? In order to come up with the effective U-factor for each roof area, either the natural log formula or charts for various roof insulation configurations can be used. These both require nothing more than the maximum and minimum R-values of the tapered insulation. We have simplified and summarized the analysis to present some common code-compliant configurations for both polyisocyanurate and rock wool insulation.
The above figures illustrate the different amounts of insulation required for the different energy code compliance paths: the prescriptive minimum R-value method, which mandates a minimum amount of insulation at all areas of roof, and the prescriptive U-factor method, as well as the envelope component tradeoff method, which both allow an averaging of the U-factors to compare to the maximum prescribed value. For the latter two methods, thicker areas of insulation can offset the thinner areas, such as around drains. COMcheck and REScheck both use the envelope component tradeoff method – you need to understand how to properly determine the effective average insulation values in order to correctly use these programs.
Another major difference between the R-value and U-factor methods is the ability to include the thermal resistance of the other, non-insulation components of an assembly, including the interior and exterior air films, in the calculations to determine an area’s U-factor. For opaque walls this can be significant, but less so for roof calculations – since the thermal requirements are so high, the contribution of the insulation is a much larger percentage of the overall thermal resistance of an assembly.
The fourth compliance path, total building performance, requires the use of energy modeling to show a reduction in the energy cost of a building compared to a base building. Using this method, tradeoffs between envelope and mechanical system performance can offset each other. However, with this method the baseline against which the proposed building must be compared must be modeled with an envelope performance meeting the prescriptive requirements, so this is not an easy way to reduce required insulation amounts.
Finally, with the prescriptive U-factor, envelope trade-off, and performance compliance paths, any thermal bridging conditions present must be taken into account by including an area of roof with a higher U-value in the calculations. This includes any grillage penetrations, roof edge angles, and clerestory wall support details. Consult a structural engineer with an understanding of how to quantify the thermal losses of thermal bridging details for advice.

I recently asked a group of about 20 structural engineers in Pittsburgh if they thought that structural engineers should have any obligation to address energy code requirements. Less than half of them responded affirmatively. We then explored the topic a bit – which was apparently the first time that many of them had considered such a notion.

  • Is compliance with Energy Codes any less important than compliance with other codes, such as the Building Code? Of course SE’s consider the structural portions of the IBC – Chapters 16, 17, and the others – of paramount importance to their work in providing assurance of structural safety and reliability. How can energy efficiency be considered in the same category of importance? Well, the truth is – noncompliance with any part of the applicable codes just as illegal as any other. Code requirements are code requirements.
  • For those SE’s who feel that the design and detailing of building envelopes is a separate task for others to address – how many SE’s show a vapor barrier under their slabs on grade? The purpose of this material – frequently included by rote in our details – is to mitigate vapor drive through the building envelope. It has absolutely nothing to do with the structural performance of the slab. In this way, structural engineers have been incorporating fairly sophisticated building science principles into our designs for years. The inescapable fact is that buildings are integrated systems, and structural components can have major effects on nonstructural system performance – especially envelopes.
  • Some structural engineers show foundation insulation (where the climate zones warrant it) and some do not. The problem develops when the insulation integrates with – or even interrupts – the structural foundation and perimeter slab edge detail. Building envelope professionals now realize that proper detailing of the insulation conditions at slab edges and continuity of insulation with minimal thermal breaks can greatly affect the amount of energy loss through the envelope – and can be an essential aspect of compliance with energy code requirements.
  • A serious problem developed with steel shelf angle and roof edge angle details while we were not paying attention. The prevalence of continuous wall insulation has obliged us to design these elements with thick horizontal projecting legs to span across the insulation. Ironically, these conditions represent tremendous building energy loss due to thermal steel bridging. There are alternatives to these thick, continuous conductive plates through the insulated envelope that should be illegal in my opinion, and in some places actually are.
  • Many other structural conditions at building perimeters warrant consideration of thermal transfer effects, including balconies, canopies, lintels, steel-framed roof overhangs, and CFMF conditions. These represent opportunities to actively engage with architects, owners, and other members of the design team to address these conditions, which can lead to very positive results.

As a Professional Engineer, I feel it is important to have a high level of control in what I design and what gets built under my stamp. If structural details need to be modified to improve the energy performance of the envelope of a building, it should be done by the Structural Engineer of Record – whether the purpose is to assist with code compliance, or to better coordinate with the architectural design, or because we know that proper attention to our structural details can improve the energy performance of our buildings.

These eight tenets form the basis of my perspective as a structural engineer in 2015.

  1. Anthropogenic Climate Change (ACC) – that is, the changing of the planet’s climate due human emissions of Global Warming Potential (GWP) gases into the atmosphere – is real and is occurring now.
  2. The short-term effects of ACC are presently happening, in the form of warming air and oceanic temperatures, increased intensity and frequency of storms and drought events, geographic shifts in plant and animal species concentrations, and reduction of polar and glacial ice. The long-term consequences of ACC are not clear, but may include catastrophic conditions.
  3. Mitigation of the most severe potential effects is possible, if enough action is taken. Effective action needs to be taken on several levels: Personal, Political, and Professional.
  4. Individuals need to take personal action to reduce their contribution to GWP gas emissions. Most important actions include reducing fossil fuel usage in home heating, cooling, and electrical usage, product awareness (including food), and fuel usage for travel.
  5. Political action needs to be taken to restrict high GWP gas emissions activities, especially for energy production, transportation, and manufacture of materials with high GWP gas emissions. Emissions of GWP gases in the form of carbon dioxide equivalent (CO2-e) by weight, must be appropriately factored into the costs of goods and services.
  6. Every professional must consider how their profession can facilitate the mitigation of CO2-e gas emissions, either by technological advances or implementation, education, or facilitating the transition to economic and social systems where CO2-e mitigation is the norm.
  7. Engineers must acknowledge their role as leaders in creating, maintaining, and advancing the various systems that society has become dependent upon, and incorporate an understanding of the need to reduce GWP gas emissions into every aspect of their work.
  8. Structural engineers must develop and implement strategies to reduce GWP gas emissions from the manufacture and construction of the structures they design, and acknowledge the role that a building structure can play in the ongoing heating and cooling operations of conditioned buildings.

Comments and suggestions are welcome!

Here is a radical, yet very doable, concept to greatly reduce the CO2 emitted from the construction of slabs-on-grade for most buildings, compared to conventional construction:

• Screed the slab ½” low, do not trowel finish it, and apply a self-leveling underlayment topping.
• Reduce the slab concrete’s design strength to, say, 100 psi. *
• Further reduce the Portland cement in the slab using fly ash and slag.
• Use a superplasticizer to achieve a 0.42 W/C ratio (including the weight of the fly ash and slag in C).
• Reduce the thickness of the slab from what is typically needed to minimize warping, since the less-strong concrete will have much less potential to warp.

* – The underlayment topping distributes point loads, so that the slab can be much less strong. If the base concrete is 100 psi, that’s still 14,400 psf, or over 200 times the required strength. A floor design load of 100 psf = less than 1 psi, yet we typically use (at least) 3000 psi concrete.

For example, say a 10,000 square foot, 5″ thick concrete slab-on-grade is specified at 3000 psf at 28 days. A standard mix design would have, perhaps, 400 lbs. of Portland cement per yard. This would be about 425 lbs. of CO2 per yard, with the majority of that emitted into the atmosphere during the manufacturing of the Portland cement.

Total lbs. of CO2 = 10,000 sf x 5/12 ft thick / 27 cf/cy x 425 lbs./cy = 65,600 lbs.

Yes, that’s nearly 33 TONS of CO2 for every 10,000 sf of slab, or 6.6 lbs for every square foot.

If we use a 3½” thick, low-strength, low-cement concrete, with 50 lbs. Portland cement (say 100. lbs CO2 per yard) and fly ash and slag with superplasticizers for a 0.42 W/C ratio, screeded level but not finished, and a ½” thick self-leveling underlayment topping of, say, 400 lbs CO2 per yard:

Slab CO2 = 10,000 sf x 3.5/12 ft thick / 27 cf/cy x 100 lbs./cy = 10,800 lbs.
Topping CO2 = 10,000 sf x 0.5/12 ft thick / 27 cf/cy x 400 lbs./cy = 6,200 lbs.
Total lbs. of CO2 = 17,000 lbs.

That’s 48,600 less lbs. of CO2, or a reduction of 74%.

Note that the CO2 emissions of the concrete and underlayment are estimated, for this exercise.

Other benefits:

• No need for a trowel finish on the concrete slab
• Slab finishes can be applied sooner, due to reduced levels of water vapor emissions
• Reduced potential for cracking and warping
• Cost neutral: Less expensive concrete, less slab placement labor offsets underlayment cost

To be clear: I’ve never seen such a slab. Concrete suppliers are reluctant to use low-strength concrete in a finished application, and contractors and engineers are reluctant to perform such “experiments.” But I can see no reason why this won’t work. The fact that such ideas, which could dramatically change the CO2 landscape of building construction, are not even being considered is testament to the fact that at this time in the U.S., the market value of reducing CO2 in building construction systems is zero – or less.

Two brand-new references provide much-needed veracity to this approach:

Palmer, William D., Jr. “Fast, Flat, Moisture-Free, Concrete Construction Magazine October 2014

National Ready-Mixed Concrete Association (NRMCA), NRMCA Member Industry-Wide EPD for Ready Mixed Concrete, October 2014

Lightly Loaded Foundations - Two Options

These two sections through two lightly loaded perimeter foundation walls provide the exact same function, yet the amount of concrete in the right section is about 1/3 the volume of concrete of the left section. Since the manufacture of Portland cement contributes 6 to 8% of the total global anthropogenic emissions of carbon dioxide, this is a way to significantly reduce the amount of CO2 emitted by the construction of a building. For a 50 X 100 foot building, this technique would eliminate about 30,000 lbs., or 15 TONS, of CO2. Other benefits include lower construction cost, less materials, shallower excavation, and faster construction time. So why isn’t this system being used more frequently? Two responses: (1) Change is slow in the construction industry; and (2) It takes a little more design effort to design and detail a frost-protected shallow foundation. This is just one technique that designers could use to make significant reductions in the carbon emissions of building structures – if such a thing were ever give a financial value in the market.

Climate Reality: An Engineering Challenge – at SUNY Institute of Technology in Utica, NY

I’ve been invited to talk about climate change on Earth Day – soo much greener than picking up plastic in a park!

Climate Reality talk Earth Day 2013

Reduce the amount of Portland cement in slabs on grade, and use cementitious topping for surface hardness. This can reduce the CO2 footprint for the slab by 75%, cutting the CO2 emissions by about 120 tons for a 50,000 square foot building.

The standard concrete slab on grade uses much, much more Portland cement than that which is required for performance purposes. Let’s get some experiments going to see if we can take advantage of this way to reduce CO2 emissions from buildings!

CO2 Calcs:
for simplicity, assume 1 lb. of Portland cement creates 1 lb. of CO2 emissions

A 5″ concrete slab V = (5/12)/27 (cf/cy) = 0.0154 cy/sf
with 400 lbs/cy cement: 6.17 lbs of CO2 /sf
with 50 lbs/cy cement: 0.77 lbs of CO2 /sf

A 3/8″ thickness of concrete topping V = (.375/12)/27 (cf/cy) = 0.0116 cy/sf
with 500 lbs/cy cement: 0.58 lbs of CO2 /sf

So, 5″ thick concrete slab (call it flowable fill) with a 3/8″ thickness of concrete topping
is 0.77 + 0.58 = 1.35 lbs CO2 /sf
This is a reduction of 78% of the CO2 of a normal slab, or 4.82 lbs CO2 /sf.
For a 50,000 sf building this is 241,000 lbs, or 120 tons of CO2 emissions avoided.

Assuming a firm, well-compacted subbase (which is the norm), the floor slab for a normal building needs to be no stronger than 100 pounds per square foot, plus point load resistance.
A common ultimate compression strength for concrete for slabs on grade is 3000 psi.
3000 psi X 144 (square inches/square foot) is 432,000 psf
This is well over 1000 times stronger than the service load of the slab:
432,000 / 1000 = 432 psf
Flowable fill with 50 lbs/cy Portland cement plus sand, fly ash, and water normally has an ultimate compressive strength of about 75 psi.
75 psi x 144 (square inches/square foot) = 10,800 psf ultimate strength capacity
High-strength cementitious topping would add significantly to the surface’s puncture resistance.

The topping should be delayed until most of the shrinkage has occurred. Since most of the shrinkage from flowable fill is due to water evaporation, 28 days might be sufficient. The floor surface should be carpeted, covered with highly resilient flooring, or the topping should be detailed and control joints laid out to allow for some degree of shrinkage movement.