BUILDING ENERGY NEEDS with particular attention paid to THERMAL INSULATION

Transcription

1 BILDING ENERGY NEEDS with particular attention paid to keep the heat in keep the heat in keep the heat in keep the heat in keep the heat in keep the heat in keep the heat in HERMAL INSLAION keep the heat in keep the heat in keep the heat in keep the heat in keep the heat in keep the heat in keep the heat in ENGS Sustainable Design Benoit Cushman-Roisin 9 April 05 Environmental impacts caused by buildings occur During their construction (6% of related economic activity) - procurement of raw resources - processing of materials - transportation to site - assembly + wastes at every step During their use (5% of related economic activity) - heating & cooling - lighting (electricity) - water (sinks, toilets, kitchens + energy going into hot water) - paper, food, etc. (indirect energy consumption solid waste) During their demolition (poorly known) - demolition proper (incl. dust generation) - hauling materials away from old site - landfilling (leachate to groundwater) Statistics: In the context of the entire.s. economy, residential buildings account for 0% of the overall energy consumption 50% of greenhouse gas emissions 8% of electricity consumption 6% of water consumption 0% of material use (incl. 5% of timber consumption) 6% of hazardous waste generation % of toxic air emissions.

2 Among the life-cycle stages of a building, the greatest impact is caused during use (lifetime of 0 to00 or more years): 9% of energy going into a building is consumed during use. For electricity, the percentage is 95%. (Blanchard & Reppe, niversity of Michigan, 998) And, during use, the most significant environmental impact is due to heating and cooling (% + 8% = 50% of energy consumption in SA in 009). Energy consumption is environmentally impacting for three reasons:. Heating oil and natural gas are non-renewable resources;. Products of combustion (fumes) are polluting the air;. Greenhouse gas emissions affect the climate. Statistics: SA: 05.7 x 0 6 Bs/year =,000 kwh/year for average dwelling Norway: 7,50 kwh/year for a detached house,000 kwh/year for an apartment news.bbc.co.uk//hi/science/nature/7677.stm Statistics in the nited Kingdom

3 A better thermal envelope is one of the best ways of achieving environmental benefits. he financial picture is very much positive! Heating (or cooling) of buildings is necessary because heat has the nasty habit of moving away from where you want to keep it. In other words: It leaks! here are three physical ways by which heat moves from one place to another: - conduction (molecular collisions in the material) - convection (movement of carrying fluid, like air in/out building) - radiation (electromagnetic wave). In designing buildings, we are concerned with all three.

4 For the moment, let us consider movement of heat by conduction. heat flux conductivity (temperature gradient) Q K K L Q in out in L out he heat flux Q is expressed in Bs per ft per hour (or W/m ). he -value characterizes the ability of the wall to conduct heat from one side to the other. Rewrite the equation for : Q in out hus, -values are expressed in Bs per ft per hour per degree F (in Europe and most of the rest of the world: W per m per o C).

5 5 Now, consider a wall consisting of several layers, such as - indoor sheetrock - wood frame - outside plywood - shingles 0 Q Q R R R R Q Q Q Q Q out in 0 0 where R i = / i Since R-values are reciprocal of -values, they are expressed in (ft x hour x o F) / B (in metric: m x o C/ W) he R-value of a material indicates its Resistance to transferring heat from one side to the other. he higher the R-value of a material, the better insulation it provides. nlike -values, R-values are additive for materials arranged sequentially. R =

6 R-values of building materials: ½-inch sheetrock drywall in outside plywood 0.6 Rough sawn cedar siding.5 Aluminum, steel or vinyl siding (hollow backing) 0.6 Asphalt roof shingles 0. Concrete blocks.0 Dual-pane windows.9 riple-pane windows with argon gas 5.88 Brick (common size) 0.0/inch Pine wood 0.95/inch Concrete 0.09/inch.5-in fiberglass batt insulation.00 Biobased Insulation 70.0 inches.00.5 inches inches 8.00 Cellulose spray./inch Icynene spray.6/inch Icynene pour formula.0/inch rethane foam 5./inch Expanded polyurethane 6./inch Polyiso foam 7.6/inch Not good! In general, stone, brick and concrete are poor insulators. (Way to remember: marble feels cold at the touch.) Air 5.5/inch Not bad, but as long as it remains still! Additional values can be obtained at: In medieval stone castles, tapestries were hung on the stone walls not just for decoration. hey were an essential item in keeping the château warm. his tapestry is a small one, only a few feet wide. Some tapestries could be much bigger and cover entire walls. 6

7 Air is an excellent insulator (R = 5.5/inch), as long as it does not move. It has poor conduction but good convection properties. his is why most types of insulation consist of air trapped in fiber or bubbles to prevent air from moving. Examples: fiberglass, cellulosic spray, and hay bales. Slow or absent air motion near a wall, either inside or outside, creates a thermal boundary layer, with an R-value of its own: Still air R = 0.68 (good approximation for inside) 5 mph wind outside R = 0.7 (typical for outside in winter used for heating calculations) 7.5 mph wind outside R = 0.5 (typical for outside in summer used for air-conditioning calculations) Such small R-values do not make a big difference along walls but do help along windows. Example: he R-value of a traditional wall. Still air inside thermal layer 0.68 Paint negligible ½-inch sheetrock mil plastic moisture barrier negligible.5-inch batt insulation.00 ½-inch exterior plywood 0.6 -inch foamboard 5.00 Rough sawn cedar shingles.5 5 mph wind outside thermal layer 0.7 OAL R-value.6 Corresponding -value = / R-value = Bs/(ft.h. o F) Dartmouth College has adopted R = 6 for walls in all its new buildings. For windows, values range between and 6.5 depending on operability. 7

8 Other example: he R-value of a traditional roof. Still air inside thermal layer mil plastic moisture barrier negligible 9-inch batt insulation 0.00 ½-inch exterior plywood 0.6 Felt paper 0.06 Asphalt roof shingles 0. 5 mph wind outside thermal layer 0.7 OAL R-value.97 Corresponding -value = / R-value = 0.0 Bs/(ft.h. o F) Dartmouth College has adopted R = 0 for roofs in all its new buildings. he overall -value of a wall is the reciprocal of its R-value: R R R R etc. he envelope of a building is composed of solid walls, windows, doors, roof, etc. he total heat lost by a building is the sum of the heat losses through all these surfaces: Q ( in out ) [in Bs per ft and per hour] otal Heat Loss ( wall window door roof etc.) ( wall area window area door area roof area in out ) common factor [in Bs per hour] 8

9 Quick remark in passing: o reduce the heat load, there is something that one can do besides using material with higher R-values and that is to reduce the surface area through which heat is lost (A factor in preceding equation). Examples: he sphere is the geometric shape that minimizes the ratio of surface to volume. If right angles are required for ease of construction, then the cube is the form that has the least area per volume. Attached barn and garage to maximize common walls he quantity HL ( wall window door roof wall area window door area roof area area etc.) hours day is called the Heat-Loss. It is expressed in Bs per day per o F. Knowing how the building is constructed leads to the determination of its Heat-Loss value. he heat lost by a building in Bs per day is thus: otal Q HL ( in out ) 9

10 Example: Salt-box conventional house otal external wall surface =,898 ft R =.7 = otal window/door surface = 7 ft R =.9 = Roof area = 8ft x 0 ft =,50 ft R =.97 = 0.0 HL = (,898 ft )(0.068) + (7 ft )(0.508) + (,50 ft )(0.0) = 77.5 Bs/(hour x o F) then convert from hours to days = 6,660 Bs/(day x o F) A slight complication: Air infiltration Exchange of air between indoors and outdoors also takes place, primarily through doors being opened and closed, and through cracks where walls connect to foundation and to roof, around window frames, etc. Warm inside air is lost, and incoming air from outside taking its place needs to be heated to inside temperature. his adds to the heating requirement of a house. It is not negligible. Procedure: I = H air x Volume x Rate of air exchange where H air = heat required to raise temperature of ft of air by o F = 0.08 Bs/(ft x o F) Volume = indoor volume affected (usually all interior minus attic) Rate of exchange = / per hour meaning that the inside air is replaced twice every three hours I = Infiltration loss (in Bs per hour and per o F) 0

11 in addition to opening and closing the entrance door. Example of infiltration calculation: Same salt-box house, with inside air volume of Volume =,9 ft I = (0.08 Bs/(ft x o F)) (,9 ft ) (0.667 / hour) = 7.8 Bs/(hour x o F) =,0 Bs/(day x o F) Adding the pieces: Add this heat requirement to the heat leakage by conduction through walls, windows and roof: HL = 6,660 +,0 = 0,880 Bs/(day x o F) Overall, the heat requirement splits as follows: - Walls, Bs/(day x o F) 0% - Windows,87 Bs/(day x o F) % - Roof, Bs/(day x o F) 0% - Infiltration,0 Bs/(day x o F) 9% OAL0,880 Bs/(day x o F) 00%

12 Degree-Days Degree-day is a quantitative index measuring the energy demand to heat or cool buildings. A mean daily temperature (average of the daily maximum and minimum temperatures) of 65 F is the base for both heating and cooling degree-day computations. Heating degree-days are summations of negative differences between the mean daily temperature and the 65 F base. Cooling degree-days are summations of positive differences from the same base. he assumption is that the desirable indoor temperature is 7 o F, of which 7 degrees will be derived from other sources than the heating system, such as appliances, light bulbs and moving people. For example, heating degree-days for a station with daily mean temperatures during a seven-day period of 6, 65, 60, 56, 5, 65 and 6 o F, are, 0, 5, 9,, 0 and, for a total for the week of heating degreedays. Degree-days for Lebanon, New Hampshire: Climatological data & actual values for the last few years Average Year 00 Year 0 Year 0 Year 0 Year 0 Year 05 January February March April May June July 5 0 August September October November December OAL 7,8 6,0 7,0 6,76 7,5 7,69 Above data from: See also:

13 From the Heat-Loss value of the building and the degree-days of the climate in which it sits, one can determine the heating requirement of the building, called the Heat Load: Heat Load HL (degree- days) Example: Salt-box house in Lebanon, NH: climatological value Heat Load = (0,880 Bs/(day x o F))x(7,8 degree-days) = 79,96,0 Bs for the average year = 8,8 MJ/year =,0 kwh/year he questions are now:. How can we reduce this heat load? And, for a given heat-load requirement:. How can we provide this heat from the sun or other clean source of energy? Answers in forthcoming lectures!

14 Proof that it can be done in Hanover, New Hampshire! And in Norwich, Vermont: he Landau House