Feb 2, 2011

We have Cool Roofs, why not Cool Walls?

If the 2006 International Energy Code, as adopted in Honolulu, allows Cool Roofs, why do they not allow Cool Walls? 

More heat gain occurs through a roof, which has a greater degree of exposure to the sun. However, simply providing a roof membrane (ie coating or material) and ventilation can satisfy Code requirements. Why is it then not acceptable to provide a similar coating for walls?

A further irony: the 2006 International Energy Code, as amended for Hawaii, requires wood framed walls to have a min R-13 insulation. Whereas Cool Roofs that have no R-value insulation are allowed. Instead, a min Thermal Emittance and Solar Reflectance are specified (see graphic). 

Insulation (such as fiberglass batts) is measured by an R-value. On the other hand, Thermal Emittance and Solar Reflectance do NOT have an R-Value and provide no insulation. For example, cooking with aluminum foil in the kitchen -- the foil does not provide insulative value. Place foil between your fingers and a pot roast and you'll get burned! Foil (similar to Astrofoil used in cool roofs) is a reflective barrier that works by reflecting radiated heat, but over time within an enclosed oven, foil can also reflect trapped heat into the covered dish, helping food cook faster. If you want to grab a hot item with your hands, use mittens to insulate your hands. Insulation slows, but does not stop the passage of heat or reflect it.

On the mainland, allowing heat gain through walls is essential in winter. So a heat reflective coating on walls would reduce heat gain into the home -- good in summer, but bad in winter, when you'd want to maximize passive heating opportunities. 

In our tropical Hawaiian climate, heat reflective coatings on walls -- Cool Walls -- would help to reduce the overall amount of heat gain. 

The argument for requiring wall insulation for all new homes homes is that insulation is required to trap air-conditioned cold air inside and keep the heat out. In practice though, R-13 wall insulation may only slow (but not necessarily stop) thermal energy. Also, we know from physics (2nd law of thermodynamics): energy naturally flows from hot to cold. So in theory, a one-way barrier that prevents heat from entering, but allows cold to escape, would would keep a cold house cool, forever.

In practice, this barrier can be provided by a heat-reflective coating (ie. paint). Paint is well suited to retofit applications of existing homes and can go a long way to bringing the thermal envelope of older single-wall (ie. board and batten) homes in Hawaii, closer towards compliance with current Energy Conservation Code requirements. Remember, the Energy Code was adopted in 2009 (verify), so every home built before 2009, even double wall homes, are non-compliant.

Although alternative methods of compliance (such as computer generated energy modeling) are allowed within the Energy Code, the language of the code is vague and approvals are therefore subject to the absolute discretion of the plan examiner. Such modeling is rather complex and not within the normal scope of architectural or engineering practice, making this alternative method of compliance out of reach for the overwhelming majority of homeowners. 

Most importantly for Hawaii, integrating heat-reflective coatings into the Energy Code helps legitimize their use and increases awareness of simple coatings as a means to save on cooling costs.

Painting is part of the normal maintenance cycle for a home or commercial bldg. Exterior paint is one of the few non-invasive Retrofit applications that can be done  easily and would offer a very cost effective way for all pre-2009 (the year the Energy Conservation Code was adopted in Honolulu) structures to make huge strides toward IECC compliance and reduce the state's overall energy dependence.

Policy Recommendation: Integrate Heat-Reflective coatings and other barriers into Hawaii's Energy Code to enhance opportunities for (all pre-2009) homes and commercial spaces to reduce their overall energy dependence.