Slides on Thermal Design - Building Envelope Systems | AE 544, Study notes of Architecture

Download Slides on Thermal Design - Building Envelope Systems | AE 544 and more Study notes Architecture in PDF only on Docsity! Thermal Design  Original Material by A. Hamid  Modified by J. Mitchell 10/03 Purpose of Thermal Design?  Why do we undertake thermal design?  Write your own reasons  We’ll discuss them. How we Control Heat Transfer  Roof – Insulate & Reflect  Walls – Insulate, Reflective, seal gaps, control vapor movement  Windows – Insulate, Emissivity, seal gaps, control vapor  Doors – Insulate, Seal gaps  Foundations – Insulate, avoid water The Mechanisms of Heat Transfer  Now we’ll look at how heat actually moves.  This is almost all in Beall – it should be a review. How Heat Moves Cooler area Heat flow Temperature High Pressure Low Pressure Warmer area Mass flow Pressure Convection  Convection: transfer of heat by the movement of air or water  Heat moves with the mass of fluid. Warmer or colder replaces original.  Complex - Simplify Radiation Non-Linear •Highly Complex •We must simplify •Do so with “equivalent temperatures” – empirically derived 4TQ   Radiation Behavior Absorptance 0.25 ---> 0.95 Reflectance 0.1 ---> 0.95 Emittance 0.08 ---> 0.95 Transmittance (calculated) Note that Reflection + emittance + transmittance is equal to 100% More Complex  3-D complexity – usually try to ignore it  Beall does deal with it slightly with ties analysis  Time effects  Daily  Sun motion  Yearly  Sun Path variation  Shading variation from vegetation  Material Properties  Thermal Mass  Time variation – degradation  Temperature & Humidity variation Calculations  Wall Gradient Calculator Spreadsheet  Thermal gradient across a wall  Saturation vapor pressure across a wall  Actual vapor pressure across a wall  Joint width necessary to address component movements and construction tolerances.  Thermal Bridging  Wind pressure on a wall in both PSF and inches of water Factors affecting thermal performance - continued Mass  Heat migrates through solid materials from the hot side to the cooler side. The time of delay involving absorption of the heat is called thermal log.  The amount of energy necessary to raise material temp is proportional to the wt of the material.  Heavy materials like concrete and masonry absorb and store a significant amount of heat and substantially retard its migration. This characteristic is called thermal storage capacity. It affects the rate of conductive heat transfer and is a critical consideration in passive solar heating and cooling strategies. Effect of Moisture Content on Thermal resistance wr 100 T 1 g 4 2 80 = a « L. - a é s 60 4 4 | Cork | 3 40-—- Perlite = EOL 4 é oe BOF = Fibarboard = LL | i 0 200 400 Moisture Content (percent dry wt) ic} Figure 3.12 Thermal resistance and moisture content of insulation. (From Tobiasson, New Wetting Curves for Common Roof Insulations.) Loss of Thermal Resistance It is recommended to have TRR be greater than 80%. Less than 80% insulation is considered wet. See table 3.13 in text. Thermal Resistance Ratio: TRR = wet thermal resistivity dry thermal resistivity Recommended Minimum Thermal 4 resistance of BE in the US