BUILDING ENVELOPE HEAT FLOW. Building Envelope

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1 BUILDING ENVELOPE HEAT FLOW stuff lots of stuff happens when a building meets a climate Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 1 Building Envelope A general term that describes the various enclosure elements of a building (walls, floor, roof including fenestration). A broad collective term for the construction assemblies that separate the inside environment from the outside environment. Fenestration refers to the design and/or disposition of openings in a building or wall envelope. Fenestration products typically include: windows, doors, louvres, vents, wall panels, skylights, storefronts, curtain walls, and slope glazed systems. Wikipedia Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 2 1

2 Building Envelope Intentions We may also study the physical control as an exchange of energies. To permit this, we will introduce the concepts filter, connector, barrier and switch. An opaque wall thus serves as a filter to heat and cold, and as a barrier to light. Christian Norberg-Schulz Intentions in Architecture Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 3 Envelope Design Intents Barrier permits no (or very little) flow between the exterior and interior Norberg-Schulz Connector allows free flow between the exterior and interior Filter regulates/meters flow (is intermediate to barrier and connector) Switch changes from a barrier to a connector over time or with a change in conditions A flow may involve heat, light, sound, insects, air, people, liquid water, water vapor Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 4 2

3 Envelope Design Intents Grondzik Transformer accepts one energy form and converts it to another, more useful, form of energy (a PV module for example: accepts solar radiation and converts it to electricity) In summary, an envelope element may act as a barrier, a connector, a filter, a switch, or as a transformer Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 5 Envelope Design Criteria CRITERIA are used to benchmark design intent Intent: I want to block does this mean an absolute barrier? or a very good barrier? or an OK barrier? or just more barrier than connector? Intent: I want to accept how much free flow makes for a connector on your project? The term vapor barrier has been replaced with the term vapor retarder which probably has more to do with truth in advertising than change of intent. Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 6 3

4 Heat Flow A recap of the fundamentals: Heat flow is always from higher temperature to lower temperature (thereby increasing entropy) Heat may flow via the potential mechanisms of: Conduction Convection Radiation Evaporation sensible processes (heat that impacts temperature) latent process (heat that impacts moisture) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 7 Why Worry About Building Envelope Heat Flow? For energy code compliance (legal) For rational design decision making It affects occupant thermal comfort It affects assembly durability (condensation) For climate control system sizing It can seriously affect energy use and carbon production It is a prerequisite to passive systems design (insulate then insolate*) To retain architectural control of design decisions *first reduce loads, then supply solar heat Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 8 4

5 Victor Olgyay s Design Sequence from Design with Climate intent = thermal comfort criterion after site design methods after active systems ambient environment after envelope design and passive systems Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 9 An Outline of Upcoming Issues Sensible heat flow review Opaque elements and sensible heat flow The big picture Components of the big picture Opaque elements and latent heat flow Transparent/translucent elements why and how they are different from opaque Infiltration where there are no elements (gaps, holes, leaks) Complexities (confounding considerations) Time lag Sol-air temperature Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 10 5

6 Sensible Heat Flow The driving force is a temperature difference (Δt) the greater the Δt the greater the potential for heat flow The potential mechanisms for sensible heat flow are Conduction Convection Radiation Sensible heat flow can be impeded by thermal resistance (R) provided by insulation the higher the R the lower the heat flow Heat flow paths may be in: Series (flow is through A, then through B, then C) Parallel (flow may be through A or through B) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 11 Opaque Envelope Assembly Typical Real Situation example: a wall assembly with several materials; each material has a thermal resistance (R); heat flows from high to low temperature heat flow (q) exterior temp (say 30 deg) interior temp (say 70 deg) R R R R R i R R R s red and green arrows represent parallel flow paths through the same overall assembly; these paths involve different materials and different heat flow rates Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 12 6

7 Opaque A Simplified Picture q exterior temp (say 30 deg) interior temp (say 70 deg) U the many constituent materials of the wall can be conceptually collapsed into one equivalent material ; the many Rs (previous) can be resolved to one U simplifying life for the designer NOTE: there will be a different U for each parallel path; these can be combined using an area-weighted averaging process Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 13 Quantifying Heat Flow through Opaque Envelope Elements q = (U) (A) (Δt) where: q = heat flow (Btu/hr) [result] and U = overall coefficient of heat transfer (Btu/h ft 2 deg F) [heat flow impedance] A = surface area of element (ft 2 ) [element size] Δt = temperature difference across element (deg F) [driving force] Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 14 7

8 U (U-factor) U stands for overall coefficient of heat transfer Overall All sensible modes of heat flow Overall All materials in an assembly Applies ONLY to assemblies (not to materials) (a brick wall has a U-factor, a brick does not) U-factor is a measure of the ability of an assembly to conduct heat (a higher U-factor means more heat flow) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 15 U (U-factor) U-factors for some common assemblies have been precalculated and tabulated Rarely, though, will a U-factor be found in references for your specific construction assembly (wall, roof, floor) You will usually calculate a U- factor from the R-values of the constituent components of the assembly as follows: U = 1 / (R1 + R2 + R3 + ) U = 1 / R total Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 16 8

9 U-factors in ASHRAE Standard 90.1 Muncie is in Climate Zone 5 three different building types are indicated (nonresidential, residential, semiheated) minimum thermal requirements (including U-factors) are listed for a variety of envelope assemblies and variations ASHRAE Standard : Energy Standard for Buildings Except Low-Rise Residential Buildings Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 17 Climate Zone 2: Standard 90.1 The US Gulf Coast is in Climate Zone 2 compare some of these values to comparable values for Climate Zone 5 climate conditions affect code requirements (for some envelope elements) ASHRAE Standard : Energy Standard for Buildings Except Low-Rise Residential Buildings Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 18 9

10 Climate Zone 5: 50% AEDG Muncie s climate zone compare some of these values to comparable values from ASHRAE Standard 90.1 The AEDG is a look-up guide to high-performance buildings the 50% refers to 50% better than code minimums 50% Advanced Energy Design Guide for Small to Medium Office Buildings Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 19 Some Typical Window U-factors all these products (options) are commercially available to you as a designer LSG = light to solar gain ratio Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 20 10

11 R-value (thermal resistance) A measure of a material s ability to impede heat flow (a higher R means less heat flow) Can usually be found for building materials (but not always, particularly for unconventional materials) Look to architectural reference books Look to product literature (catalogs, Internet) A key selling point for insulation materials Quirky units deg F ft 2 h / Btu If not found, R must be calculated using k or C unconventional materials Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 21 Some Typical Insulation R-Values R-value per inch these values are generally temperature and moisture dependent Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 22 11

12 k (thermal conductivity) A measure of the ease with which heat is transmitted Applicable to homogeneous materials (such as concrete, wood, steel, glass, brick) Tabulated per inch of thickness A basic thermophysical property of a material R = (thick) (1/k) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 23 C (thermal conductance) A measure of the ease with which heat is transmitted Conductance is applicable to non-homogeneous materials (such as concrete block, air spaces, air films, composites) Tabulated per specified thickness (usually a unit size) A basic thermo-physical property of a material unit R = (1/C) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 24 12

13 Roadmap to Opaque Heat Flow q = (U) (A) (Δt) To solve, need to find or calculate U U = 1 / (R1 + R2 + R3 + R4 + ) May need to find or calculate R R = (thick) (1/k) R = 1/C Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 25 Latent Heat Flow (water vapor movement) temperature difference vapor difference The driving force is a vapor pressure difference (a difference in absolute humidity or humidity ratio) The means (mechanism behind the flow) is Permeance (space within a material s structure that allows for water molecule migration) Flow is impeded by impermeance Vapor retarders provide impermeance Rated by their permeability (in perms) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 26 13

14 Opaque Assembly Moisture Flow M = (μ) (A) (ΔW) vapor resistance heat resistance Moisture flow (M) is a function of permeance [μ], area (A), and vapor pressure difference [ΔW] Low flow requires low permeance a good vapor retarder that is also properly located Vapor flow is fairly independent of heat flow, and may often be in the opposite direction Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 27 Transparent & Translucent Envelope Assemblies The Big Picture radiation (from sun at several million deg) q exterior temp (say 30 deg) interior temp (say 70 deg) U short-wave radiation heat flow is independent of convection/conduction heat flow (q) and is independent of U Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 28 14

15 Transparent Heat Flow Conduction and convection (temperaturedriven) heat flow are addressed by the equation q = (U) (A) (Δt) U-factor is still a property of concern with transparent materials But radiation is not affected by thermal resistance (by R, and thus U) by definition, radiation freely passes through a transparent (or translucent) material We need a way to address radiation flow Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 29 Transparent Radiation Flow q = (A) (SHGC) (SHGF) q = radiation heat flow (Btuh) A = surface area of element (ft 2 ) SHGC = solar heat gain coefficient; a measure of impedance to radiation flow SHGF = solar heat gain factor (Btuh/ft 2 ) the driving force (magnitude of radiation) Note: ASHRAE methodology and terminology has changed with updates to the Handbook of Fundamentals; nevertheless, the underlying concepts remain the same Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 30 15

16 Infiltration The Big Picture wind air bathroom fan air exterior temp (say 30 deg) air interior temp (say 70 deg) U infiltration = air leakage; which bypasses U by flowing through gaps in construction; may involve an air temperature and/or moisture difference Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 31 Infiltration There is no thermal resistance to infiltration, since air leaks through a gap in a material or assembly Air flow is driven by an air pressure difference [caused by wind, stack effect, or a fan] Flow rate is a function of the size of the gap(s) and magnitude of the driving force Infiltration is expressed in cfm (ft 3 / min) Infiltration rate can be estimated during design and measured after construction Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 32 16

17 Infiltration Heat Flow: Sensible q = (cfm) (1.1) (Δt) where air may leak in q = heat flow (Btu/h) cfm = air flow rate (leakage) 1.1 = units conversion factor Δt = temperature difference (in deg F) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 33 Infiltration Heat Flow: Latent q = (cfm) (4840) (ΔW) q = heat flow (Btu/h) cfm = air flow rate (leakage) 4840 = units conversion factor ΔW = humidity ratio difference (in lb/lb) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 34 17

18 Opaque Complications Mass Effects time lag decrement time lag = delay in heat flow; decrement = reduction in heat flow (due to mass) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 35 Opaque Complications Sol-Air Sol-Air Temperature Where t sol-air (temperature of a building surface exposed to the sun) includes the effects of air temperature and the temperature equivalent of absorbed solar radiation (heat) air temp + no solar effect air temp + solar effect higher sol-air temperatures result in higher heat flows (see N versus S above) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 36 18

19 the architect is like a composer making sure the right instrument is in the right location to ensure the desired effect airmoisturecontrol.com/energy-efficiency/building-enclosure-design-guide/ Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 37 facplan.typepad.com/fpablog/2010/02/ stuff lots of stuff happens when a building meets a climate Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 38 19