ASHRAE Journal - August 2008 - (Page 60)

Energy Flow Across Enclosures By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE B uildings don’t work they way they once did.* Folks are always saying that. But, let me make the case regarding only one factor, and you decide. Other factors exist, but I want to focus on only one for the moment. This one Higher Levels of Thermal Resistance overarching factor is the amount of energy exchange across the How do we dry wet wood? We kiln dry it. We heat the wood building enclosure. so that the water in the wood is warmer than its surroundings. I think that higher levels of thermal resistance and reduced We add energy to the water and it evaporates. There is a huge heat gain across building enclosures have forever changed the exchange of energy. When we put warm wet coffee beans in performance of buildings and not necessarily in a good way. a room with very cold surfaces, we call it freeze-drying. The And, it will get worse before it moisture leaves the coffee 50 gets better. Sound familiar? beans and accumulates on The lens I am going to use to the cold surfaces. There is Wood Plywood look at this factor is moisture. a huge exchange of energy. 40 Stucco Why? It is one of the principle The greater the rate of energy Concrete damage functions acting on exchange, the greater the rate Brick 30 materials along with heat and of moisture movement. Dryultraviolet radiation.† I think ing cannot happen without an 20 moisture is the key to underexchange of energy. standing the performance of When my Mom and Dad buildings in general, as well as bought their first house in 10 in this particular case. Toronto, Canada, in 1957, The consequences of this there was no insulation in 0 reduced energy exchange can the walls, and the house was 0 10 20 30 40 50 60 70 80 90 100 Mean Relative Humidity, % be seen everywhere: mold, leaky to air because it had a part-load humidity problems, Figure 1: Sorption Curve for Common Building Materials. Note high air change driven by a rot, and corrosion. Highly that moisture content goes up as relative humidity goes up. No traditional chimney. We lived insulated building enclosures temperature dependence or vapor pressure dependence exist ex- in a 1,200 ft2 (111 m2) house with reduced heat gain have cept where temperature affects relative humidity or where vapor and in January, when the outlow drying potentials and in- pressure affects relative humidity. side temperature dropped to creased interior moisture loads. 0°F (–18°C), Mom cranked When they get wet, they don’t dry. Stuff is beginning to stink, up the 300,000 Btu (317,000 kJ) oil furnace to maintain an rot, break, and annoy. interior temperature of 70°F (21°C). The energy ﬂow across the Two things need to be looked at here: levels of insulation building enclosure was enormous, but oil was cheap, and we and airtight assemblies, and heat gain through glazing, roof were comfortable and happy. The energy ﬂow was so enormous assemblies, and interior loads. Adding insulation and providing that the building enclosure was simultaneously kiln dried and airtight assemblies reduces energy ﬂow. Which is good, right? freeze-dried. In fact, the drying potential was so high, we were Well, not always. Less energy ﬂow from the inside to the outside uncomfortably dry. As a result Dad insisted that the furnace means the materials on the outside of the building are colder have a newfangled gadget attached to it—a humidiﬁer. How in the winter. The colder the materials on the outside of the things have changed. building become, the wetter they become and the wetter they Well, what changed? We’ve begun to insulate, and insulate stay. This is not good. Less energy ﬂow from the outside to the exceptionally well, making the assemblies tighter to air change Equilibrium Moisture Content (%) inside in the summer means less heat that is generated inside in the summer, which means the air-conditioning system runs less. This is a good thing right? Well, not always. We create something called the part-load humidity problem. *Yes, I know, pretty arrogant for a relative youngster to say. I am in my 50s, but when you get to this age, you spend as much time looking back as you do looking forward. My mentors tell me it helps with perspective. It has been said that to understand the future you have to understand the past. I am not so ambitious. I am only thinking of today. I think to understand the present you have to understand the past. †Someone once said that 80% of all building-related durability problems are due to three principal damage functions (water, heat, and UV radiation) with 80% of the 80% being water. I don’t know who said this. Can someone help me out? I might have just made it up, but I have been saying it so long that I no longer remember if I said it or not. Aren’t footnotes great? 60 ASHRAE Journal ashrae.org August 2008 http://ashrae.org

Table of Contents Feed for the Digital Edition of ASHRAE Journal - August 2008

ASHRAE Journal - August 2008 Contents Commentary Industry News Letters Meetings and Shows Maintain to Sustain—Delivering ASHRAE’s Sustainability Promise Ultraviolet Germicidal Irradiation: Current Best Practices Improving Humidity Control With Energy Recovery Ventilation Single- or Two-Stage Compression Data Center Cooling: Using Wet-Bulb Economizers Building Sciences InfoCenter Practical Pointers Products Emerging Technologies Washington Report People Special Products Classified Advertising Advertising Index

ASHRAE Journal - August 2008

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