Reminder: Above-Ground Envelope Heat Loss

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1 good envelope DESIGN HEAT LOSS (part 2) bad envelope Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 1 Reminder: Above-Ground Envelope Heat Loss Heat loss through any assembly with air on both sides (walls, roofs, fenestration) is dealt with through U-factors. Infiltration through such assemblies is dealt with by estimating the air leakage rate (although methods for doing so have not yet been fully discussed). There are other assemblies that don t fit the above-ground category which will now be looked at. Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 2 1

2 Slab-on-Grade Heat Loss q = (F)(P)(Δt) sensible loss where F is an empirical heat loss factor (used with correlation tables) and P is the slab perimeter any design action that will reduce F, P, or Δt will reduce design heat loss, reduce heating system size (first cost), and reduce energy usage (mitigating carbon emissions if an active system is used) U-factor does not apply to this situation as U assumes air-to-air heat flow (and this condition involves mainly air-to-dirt heat flow) q = heat flow; F = empirical heat transfer factor (for a very complex situation); P = perimeter of slab; Δt = temperature difference across assembly Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 3 Slab-on-Grade F values 8 slab-on-grade construction options are available (via data from laboratory research) 4 basic types, either insulated at R 5.4 or uninsulated ASHRAE Handbook 2013 Fundamentals F Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 4 2

3 Slab-on-Grade Constructions for Correlations these are the only configurations for which research data are available a designer must select the best match ASHRAE Handbook 2001 Fundamentals Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 5 Basement Heat Loss q = (U avg )(A)(Δt) sensible loss where U avg is an empirical heat loss factor (estimated by correlation) and A is wall or floor area; floors and walls are analyzed separately any design action that will reduce U avg, A, or Δt will reduce design heat loss, reduce heating system size (first cost), and reduce energy usage (both resources and operating costs) A conventional U-factor does not apply to this situation as U assumes air-to-air heat flow q = heat flow; U avg = empirical heat transfer factor (for a very complex situation); A = surface area of assembly; Δt = temperature difference with average soil Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 6 3

4 need to identify: depth below surface and insulation condition Basement U avg Values need to identify: depth below surface and floor dimensions ASHRAE Handbook 2013 Fundamentals Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 7 Recap: Sensible Design Heat Loss The total sensible envelope heat loss equals: Loss through opaque walls/doors Loss through roofs (U)(A)(Δt) Loss through windows/skylights Loss through cantilevered floors (if any) Loss through slab-on-grade floor (if any) (F)(P)(Δt) Loss through basement walls (if any) (U avg )(A)(Δt) Loss through basement floor (if any) Infiltration loss (cfm)(1.1)(δt) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 8 4

5 Sensible Design Heat Loss roof example residential room heat loss distribution % the distribution of losses for any given room or building depends upon design decisions; but note that windows are substantial (high U) and roof loss is relatively low (good insulation); foundation wall losses are exceptionally high and an aberration due to poor design; infiltration is not included in this example, but might be (1 ach)(2000 ft3)(1.1)(61) = 2,062 (equal in magnitude to the window loss) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 9 Design Sensible Heat Loss: Summary Several simplifying assumptions make analysis of design heat loss fairly easy Steps that can reduce heat loss should come intuitively from the equations that define heat loss relationships follow the numbers (and variables) All losses involve the building envelope which is the sole focus for reducing design heat loss AND the architect s responsibility Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 10 5

6 Heat Loss Details Thermal Bridging A thermal bridge is a component (or a combination of components) in a building envelope assembly through which heat is transferred at a substantially higher rate than through the surrounding assembly area. A steel stud can act as a thermal bridge, as can any structural (or metal) element that spans continuously from exterior to interior. Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 11 What s Going on Here? question and photos courtesy of Prof. Alison Kwok, University of Oregon Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 12 6

7 Heat Loss Details Condensation Condensation of water vapor moving through a wall or roof will occur at the dew point (if water vapor can get there) The temperature drop through each material in an assembly is proportional to its R (the greater the R of a material the greater the slope of the temperature plot) Temperature (and thus dew point) at any point in an assembly can be easily predicted during design. out WINTER wall section in water vapor heat Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 13 Condensation forensic-applications.com/moulds/mouldphotos/ nothing good happens when water vapor condenses within an envelope Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 14 7

8 Condensation blueskyhs.com/askhomepedia/uploads/ nothing good happens when condensation forms on a window or skylight Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 15 Heat Loss Details Thermal Gradient T point x = T exterior + (R to point x / R total ) * t A small R results in a small difference in temperature across a material A large R results in a substantial difference in temperature across a material The total temperature difference ( t) is proportionally allocated across all the materials (including air films) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 16 8

9 Free Software: OPAQUE take a look: Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 17 Design Latent Heat Loss Latent heat loss is seldom calculated during the design process (unless humidity control is critical to building success); but analysis methods are available Total latent loss is comprised of: Loss through opaque walls/doors Loss through roofs Loss through cantilevered floors (if any) Infiltration loss Glazings are impermeable (no latent loss) Slab-on-grade floors and basements are often vapor-proofed (no latent loss) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 18 9

10 Above-Ground Latent Loss Through opaque elements: ql = (M)(A)(Δp) latent loss Analyzed assembly-by-assembly (since M and A differ for walls, doors, roofs) Any design action that will reduce M, A, or Δp will reduce latent heat loss, reduce humidifier system size (first cost), and reduce energy usage (saving resources and reducing operating costs) ql = latent heat flow; M = permeance of assembly (resistance to vapor flow); A = surface area of assembly; Δp = vapor pressure difference across assembly Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 19 Infiltration Latent Loss As a result of air leakage: ql = (cfm)(4840)(δw) latent loss Estimating infiltration is the trick Any design action that will reduce cfm (infiltration) or ΔW will reduce latent heat loss, reduce humidifier system size (first cost), and reduce energy usage (saving resources and reducing operating costs) ql = latent heat flow (Btuh); cfm = air leakage rate; 4840 = unit conversion factor; ΔW = absolute humidity difference across assembly Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 20 10

11 Transparent Material Latent Loss ql = 0 latent loss is zero Transparent/translucent materials (glass, plastics) are impermeable (they inherently block water vapor flow) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 21 Slab-on-Grade Latent Loss ql 0 it is assumed that latent loss is nil A slab-on-grade will normally be waterproofed in such a way that vapor flow is blocked (for example, with plastic sheeting) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 22 11

12 Basement Heat Loss ql 0 it is assumed that latent loss is nil Basement walls and floors will normally be waterproofed in such a way that vapor flow is blocked Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 23 Envelope Barriers products.construction.com/swts_content_files/ HEAT AND VAPOR note difference in thickness between vapor retarder (a film); sensible heat retarder (fibers with obvious thickness); and air retarder (film) AIR providing appropriate barriers in construction details has architectural impacts Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 24 12

13 Side Issue: Degree Days Heating Degree Days are a means of characterizing climates (essentially, the persistence of cold weather during the underheated period) HDD 65 = heating degree days (using a 65 F base) daily HDD 65 = (65 F average daily outdoor temperature) [65 F is a balance point temperature; other base temperatures are available and reflect a sense of what an appropriate balance point temperature would be] Cooling Degree Days (and Hours) are also used (but with less success, since they ignore latent loads) CDD 65 = cooling degree days CDD 65 = (average daily temperature 65 F) CDH = cooling degree hours CDH 75 = (average hourly temperature 75 F) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 25 Muncie = apx HDD Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 26 13

14 Building Energy Performance is Now Being Publicly Rated Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 27 US (building) Ireland (building) US (appliance) ASHRAE Building Energy Quotient rating system Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 28 14