Re: Passive Houses
They had detailed explanation for why they didn’t use insulated shutters for the Riverdale Net Zero project in Canada. I don’t think they included all appropriate numbers. I wonder if they adjust nightime heatloss to decreased temperatures instead of avg.?
I have a book on underground construction funded by the state of MN? A study to encourage energy efficienct building during the first energy crises in the 70’s. They offer a formulae that will give you savings from earth bermed effect which correlates to mass effect in relation to extreme winter temp swings that will add efficiency thru making up temp difference in time lag during less extreme temps, where with less mass, when the temp drops to –10, the heater needs to respond more immediately.
Everything you mentioned about thermal mass sounds right. I think the above situation accounts for marginal better performance.
On 12/29/08 3:22 PM, "Robert Riversong" <[log in to unmask]> wrote:
--- On Mon, 12/29/08, Ben Graham <[log in to unmask]> wrote:
the fact that insulated shutters are considered not cost effective to install based on energy analysis defined as cost ratios, yet quadruple paned windows are, bewilders me.
I don't know why insulated shutters wouldn't be cost-effective, but as an efficiency strategy they rely completely on occupant intervention. If they are not used at night, efficiency would not improve. If they are left closed during a sunny day, efficiency would likely decrease. Efficient windows, if they also have reasonable SHGC, work to improve efficiency 24/7 regardless of occupant behavior.
Are you willing to share your analysis methods?
I share them in some detail in my Efficiency by Design class at Yestermorrow. I don't give out my spreadsheet both because it was designed to be comprehensible only to me and not for others, because I am not going to guarantee its accuracy or be responsible for the way others use it, and because I encourage others to create their own spreadsheets to meet their particular needs. If you use your own, then you'll understand how its inputs become its outputs.
Ive never seen one that reflects thermal mass lag advantages.
This is a controversial subject. Follows is some of what I've posted in a Taunton Press Breaktime discussion on the "greenness" of ICF homes.
A Canadian monitoring study concluded was "No thermal mass impact or higher effective insulation value was observed."
A US comparative study concluded: "In general, the ICF homes were approximately 20% more energy efficient than the wood frame house. Given the three homes similarities in air tightness, the increased energy-efficiency is largely due to the higher effective R-value of the walls and continuous insulation at the slab...No dramatic comfort differences were noted between ICF and wood-framed construction, but the ICF home had several thermal comfort measures showing a slight improvement."
A further analysis of this study concluded: "In the final report, NHAB researches concluded that this 20% difference was caused by the R-7 difference in wall R-values (ICF wall R-value was about R-20, conventional 2x4 wood stud wall R-value was about R-13). However, simulation data developed by ORNL for a similar 1300 ft2 one story house suggests that for the same climate a difference between R-20 and R-13 should yield a maximum 8 to 9% difference in annual whole building energy consumption. This suggests that most likely thermal mass related energy savings during the NAHB ICF study were in the neighborhood of 11%."
This conclusion is speculative, but if correct would indicate a marginal improvement in energy-efficiency, not the 50%-100% improvement often ascribed to high-mass walls. And any improvement is highly climate-dependent (see below).
Additionally, a 2001 National Renewable Energy Laboratory study concluded: "In this climate [Pueblo CO], incorporating massive building materials is an effective strategy for ensuring smaller diurnal indoor temperature swings in low-energy residential building designs...Massive building construction most effectively improves comfort during the cooling season only when the mass can be pre-cooled at night. If the mass cannot be pre-cooled, then it is likely that the indoor temperatures will be higher than a comfortable level during the summer. It is also important that the mass be located within the conditioned space and insulated on the exterior."
To prove that last point, Oak Ridge National Laboratory compared four building mass strategies:
- Exterior thermal insulation, interior mass (Intmass)
- Exterior mass, interior thermal insulation (Extmass)
- Exterior mass, core thermal insulation, interior mass, and (CIC)
- Exterior thermal insulation, core mass, interior thermal insulation (ICI).
They concluded: "The thermal mass benefit is a function of wall material configuration, climate, building size, configuration, and orientation. From ten analyzed U.S. locations, the most beneficial for application of thermal mass are Phoenix, AZ and Bakersfield, CA."
The Phoenix house earned a DBMS (dynamic benefit for massive systems) of 1.43 (times nominal R-value). Houses in Atlanta and Denver earned 1.24 -1.25 and in Miami and Minneapolis 1.06 -1.07 (virtually the same as a low-mass house).
They also concluded: "Comparative analysis of sixteen different material configurations showed that the most effective wall assembly was the wall with thermal mass applied in good contact with the interior of the building. Walls where the insulation material was concentrated on the interior side, performed much worse. Wall configurations with the concrete wall core and insulation placed on both sides of the wall performed slightly better, however, their performance was significantly worse than walls containing foam core and concrete shells on both sides."
In every case, ICF walls performed noticeably worse in their mass effect than mass walls with exterior insulation.
Additionally, they concluded: "For high R-value walls, up to 8% of the whole building energy could be saved in Minneapolis and 18% in Bakersfield when wood-framed walls were replaced by massive wall systems. Thermal mass layers must be in good contact with the interior of the building in these walls.
Whole building possible energy savings in houses built with ICF walls were estimated as well. Three houses with 800-3000 ft² of floor area were simulated for this purpose. It was found that for ten U.S. locations, ICF walls of R15 and R20, the average potential whole building energy savings (ICF house vs. conventional wood-framed house) can be between 6 and 8%."
An article about "Thermal Mass and R-Value" at BuildingGreen.com, states that: "The mass effect is real. High-mass walls really can significantly outperform low-mass walls of comparable steady-state R-value – i.e., they can achieve a higher “mass-enhanced R-value.” BUT (and this is an important “but”), this mass-enhanced R-value is only significant when the outdoor temperatures cycle above and below indoor temperatures within a 24-hour period. Thus, high-mass walls are most beneficial in moderate climates that have high diurnal (daily) temperature swings around the desired indoor setpoint."
So the evidence to date indicates that high-mass walls can improve thermal efficiency by 6% to 8% in a heating-predominant climate (potentially more in a cooling climate), but good contact with the interior space is required for optimum performance. This means that ICF wall systems are - from a thermal perspective - a poor application of mass and insulation, and only marginally better than conventional wood-frame walls.
Given that it's very simple to improve wood-frame walls to outperform ICF walls, the thermal benefits alone are not a justification.
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