 |
InspectAPedia® |
InspectAPedia
|
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair | Ask a Question or Search InspectAPedia |
 Mobile ViewINSULATION IDENTIFICATION GUIDE INSULATION INSPECTION & IMPROVEMENT AIR BYPASS LEAKS AIR LEAK DETECTION TOOLS AIR LEAK MINIMIZATION AIR LEAK SEALING PROCEDURE AIR SEALING STRATEGIES ASBESTOS FLOORING HAZARD REDUCTION ASBESTOS-FREE INSULATION MATERIALS ASBESTOS IDENTIFICATION IN buildings ATTIC LEAKS, CONDENSATION & MOLD BASEMENT WATERPROOFING BATHROOM VENTILATION BASEMENT LEAKS, INSPECT FOR BASEMENT CEILING VAPOR BARRIER BASEMENT HEAT LOSS BIOGAS PRODUCTION & USE BLOWER DOORS & AIR INFILTRATION BLOWER FAN CONTINUOUS OPERATION BLOWER FAN OPERATION & TESTING BLOWN-IN INSULATION BRICK LINED WALLS BRICK VENEER WALL INSULATION BRICK VENEER WALL Loose, Bulged BRICK WALL DRAINAGE WEEP HOLES BUCKLED FOUNDATIONS due to INSULATION? BUILDING NOISE DIAGNOSIS & CURE CATHEDRAL CEILING INSULATION CATHEDRAL CEILING VENTILATION CRAWL SPACES CRAWL SPACE DRYOUT PROCEDURES CRAWL SPACE GROUND COVERS CRAWL SPACE INSULATION RETROFIT CRAWL SPACE SAFETY ADVICE CRAWL SPACE VAPOR BARRIER CRAWLSPACE MOLD ADVICE MOLD CLEANUP by MEDIA BLASTING MOLD ON DIRT FLOORS SUMP PUMPS CONDENSATION or SWEATING PIPES, TANKS COOLING LOAD REDUCTION by ROOF VENTS DEHUMIDIFICATION PROBLEMS DEW POINT CALCULATION for WALLS DEW POINT TABLE - CONDENSATION POINT GUIDE ENERGY SAVINGS in buildings AFUE DEFINITION, RATINGS AIR BYPASS LEAKS AIR CHANGE RATE ACH HEAT SAVINGS AIR CONDITIONING HEAT PUMP SAVINGS AIR LEAK SEALING PROCEDURE APPLIANCE EFFICIENCY RATINGS ATTIC LEAKS, CONDENSATION & MOLD COOLING LOAD REDUCTION by ROOF VENTS DRYER VENTING ENERGY AUDIT - How to Use a Free One ENERGY SAVINGS MAXIMIZE RETURNS ON ENERGY SAVINGS PRIORITIES ENERGY SAVINGS RETROFIT CASE STUDY ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE ENERGY SAVINGS RETROFIT OPTIONS ENERGY STAR PROGRAM ENERGY USE MONITORING GLASS vs HEAT MIRROR SOLAR GAIN/Loss HEAT LOSS in buildings HEATING COST SAVINGS METHODS HIGH MASS TRADEOFFS, HEATING vs COOLING HOUSE DOCTOR, how-to be INSULATION INSPECTION & IMPROVEMENT INSULATION LOCATION - WHERE TO PUT IT RADIANT BARRIERS REFLECTIVE INSULATION ROOF COLOR RECOMMENDATIONS Skylight Energy Efficiency SOLAR ENERGY SYSTEMS THERMAL MASS in buildings TIMERS for ELECTRIC WATER HEATERS VENTILATION, BALANCED HEAT COST SAVINGS WIND ENERGY SYSTEMS WINDOW EFFICIENCY Features & Ratings WOOD, COAL STOVES & FIREPLACES ENVIRONMENTAL HAZARDS - INSPECT, TEST, REMEDY EXTERIOR WALL SIDING TRIM & FINISHES FIBERGLASS INSULATION FIBERGLASS HAZARDS FIBERGLASS PARTICLE CONTAMINATION TEST FIBERGLASS INSULATION MOLD FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP FOOTING & FOUNDATION DRAINS FOUNDATION CRACKS & DAMAGE GUIDE FOUNDATION WATERPROOFING FRENCH DRAINS FRAMING DETAILS for BETTER INSULATION FRAMING DETAILS for DOUBLE WALL HOUSES FRAMING METAL STUD PERFORMANCE FREEZE-PROOF A BUILDING FROST HEAVES, FOUNDATION, SLAB GREEN BUILDING CONSTRUCTION CODES GUIDES GREENHOUSE DESIGN for SOLAR HEATING GREENHOUSE / SUNSPACE GLARE HEAT LOSS in BUILDINGS HEAT LOSS DETECTION TOOLS HEAT LOSS INDICATORS HEAT LOSS PREVENTION PRIORITIES HEAT LOSS R U & K VALUE CALCULATION HEAT LOSS RATE CALCULATIONS HOT ROOF DESIGNS: Un-Vented Roof Solutions HOUSEWRAP INSTALLATION DETAILS HUMIDITY LEVEL TARGET HUMIDITY CONTROL TO PREVENT MOLD Humidity Encourages Mold Dust Mites & Bacteria What humidity level? Humidity in Basements Humidity in Attics Target Humidity Reaching the Target Measuring Humidity Tools for Measuring Humidity ICE DAM PREVENTION INDOOR AIR QUALITY & HOUSE TIGHTNESS ICE DAM PREVENTION INSULATION IDENTIFICATION GUIDE INSULATION INSPECTION & IMPROVEMENT INSULATION CHOICES Insulation Air & Heat Leaks INSULATION FACT SHEET- DOE INSULATION for GREENHOUSE or SOLARIUM INSULATION IDENTIFICATION GUIDE INSULATION INSPECTION & IMPROVEMENT INSULATION LOCATION - WHERE TO PUT IT INSULATION MOLD INSULATION R-Values & Properties ICE DAM PREVENTION INSULATION IDENTIFICATION GUIDE INSULATION INSPECTION & IMPROVEMENT LEAD POISONING HAZARDS GUIDE LEED GREEN BUILDING CERTIFICATION LOG HOME ENERGY EFFICIENCY LOG HOME GUIDE MOISTURE CONTROL in BUILDINGS ATTIC CONDENSATION CAUSE & CURE Attic Moisture or Mold Sources BASEMENT CEILING VAPOR BARRIER BASEMENT De-Watering Systems BASEMENT HEAT LOSS BASEMENT LEAKS, INSPECT FOR Chimney Leaks CONDENSATION or SWEATING PIPES, TANKS CONDENSATION on WINDOWS & SKYLIGHTS CORROSION in ELECTRICAL PANELS CRAWL SPACE DRYOUT PROCEDURES DEW POINT CALCULATION for WALLS DEW POINT TABLE - CONDENSATION POINT GUIDE DRYER VENTING DRYWELLS, FRENCH DRAINS for FLAT SITES DRYWELL DESIGN & USES EFFLORESCENCE, Salts & White / Brown Deposits ELECTRIC PANEL MOISTURE EXTERIOR WATER SOURCE ELIMINATION FLOOD Damage Assessment & Repairs FLOOD DAMAGED FOUNDATIONS FLOOD VENTS & FLOOD PORTS FLOODS IN buildings-priorities FOUNDATION DRAINS, INTERIOR FOUNDATION WATERPROOFING GEOTEXTILES & DRAINAGE MATS GRADING & SITE WORK, EXTERIOR GUTTERS & DOWNSPOUTS HUMIDITY CONTROL & TARGETS INDOORS HOUSEWRAP AIR & VAPOR BARRIERS LOG HOME Leak Diagnosis & Cure LOG HOME Condensation &Moisture MOISTURE in BUILDING WALLS, EFFECTS MOISTURE in CELLULOSE INSULATION MOISTURE CALCULATIONS MOISTURE CONTROL in BUILDINGS MOISTURE METER STUDY MOISTURE PROBLEMS: CAUSE & CURE MOLD in buildings Mold in Fiberglass Insulation Mold on Books, Book Conservation Mold on Fiberboard Insulating Sheathing MOLD PREVENTION AFTER FLOODING MOLD RESISTANT CONSTRUCTION ROOF VENTILATION SPECIFICATIONS Soffit Intake Vents & Attic Condensation WATER ENTRY in buildings VENTILATION, WHOLE HOUSE STRATEGIES MOLD INFORMATION CENTER MSDS Material Safety Data Sheets MVOCs & MOLDY MUSTY ODORS MYCOPHOBIA, STAINS MISTAKEN for MOLD MYCOTOXIN EFFECTS of MOLD EXPOSURE Nanomaterials Hazards NOISE / SOUND DIAGNOSIS & CURE NOISE CONTROL for HEATING SYSTEMS NOISE CONTROL for FLOORS NOISE CONTROL for PLUMBING NOISE CONTROL for ROOFS ODORS & SMELLS DIAGNOSIS & CURE PASCAL CALCULATIONS ROT, FUNGUS, TERMITES ROT, TIMBER FRAME SOLAR ENERGY SYSTEMS SOUND CONTROL in buildings STAIN DIAGNOSIS on BUILDING INTERIORS THERMAL MASS in buildings THERMAL TRACKING & HEAT LOSS VAPOR BARRIERS & CONDENSATION in buildings ATTIC CONDENSATION CAUSE & CURE BASEMENT CEILING VAPOR BARRIER CONDENSATION or SWEATING PIPES, TANKS CRAWL SPACE VAPOR BARRIER DEW POINT CALCULATION for WALLS DEW POINT TABLE - CONDENSATION POINT GUIDE FELT 15# ROOFING, as HOUSEWRAP/VAPOR BARRIER HOUSEWRAP AIR & VAPOR BARRIERS HOUSEWRAP INSTALLATION DETAILS HOUSEWRAP PRODUCT CHOICES HOUSEWRAP at SILLS, SOLES, TOP PLATES HUMIDITY LEVEL TARGET MOISTURE CONTROL in BUILDINGS RAIN SPLASH-UP SIDING DAMAGE VAPOR BARRIERS & AIR SEALING at BAND JOISTS VAPOR BARRIERS & CONDENSATION in buildings VAPOR BARRIERS & HOUSEWRAP VAPOR CONDENSATION & BUILDING SHEATHING WATER BARRIERS, EXTERIOR WOOD SIDING FLASHING DETAILS VENTILATION in buildings VENTILATION in buildingS WALL FINISHES INTERIOR WATER BARRIERS, EXTERIOR BUILDING WATER ENTRY in buildings ATTIC LEAKS Moisture or Mold BASEMENT De-Watering Systems BASEMENT CEILING VAPOR BARRIER BASEMENT De-Watering Systems BASEMENT HEAT LOSS INSULATION for GREENHOUSE or SOLARIUM INSULATION CHOICES BASEMENT LEAKS, INSPECT FOR BASEMENT WATERPROOFING BASEMENT WATER ENTRY PREVENTION CATCH BASINS Chimney Leaks CONDENSATION or SWEATING PIPES, TANKS CONDENSATION on WINDOWS & SKYLIGHTS CORROSION in ELECTRICAL PANELS CRAWL SPACE DRYOUT PROCEDURES DEW POINT CALCULATION for WALLS DEW POINT TABLE - CONDENSATION POINT GUIDE DRYER VENTING DRYWELLS, FRENCH DRAINS for FLAT SITES DRYWELL DESIGN & USES EFFLORESCENCE, Salts & White / Brown Deposits ELECTRIC PANEL MOISTURE EXTERIOR WATER SOURCE ELIMINATION FLOOD Damage Assessment & Repairs FLOOD DAMAGED FOUNDATIONS FLOOD VENTS & FLOOD PORTS FLOODS IN buildings-priorities FOUNDATION DRAINS, INTERIOR FOUNDATION WATERPROOFING GEOTEXTILES & DRAINAGE MATS GRADING & SITE WORK, EXTERIOR GUTTERS & DOWNSPOUTS HUMIDITY CONTROL & TARGETS INDOORS HOUSEWRAP AIR & VAPOR BARRIERS LOG HOME Leak Diagnosis & Cure LOG HOME Condensation &Moisture MOISTURE in BUILDING WALLS, EFFECTS MOISTURE in CELLULOSE INSULATION MOISTURE CALCULATIONS MOISTURE CONTROL in BUILDINGS MOISTURE METER STUDY MOISTURE PROBLEMS: CAUSE & CURE MOLD in buildings Mold in Fiberglass Insulation Mold on Books, Book Conservation Mold on Fiberboard Insulating Sheathing MOLD PREVENTION AFTER FLOODING MOLD RESISTANT CONSTRUCTION ROOF VENTILATION SPECIFICATIONS Soffit Intake Vents & Attic Condensation SEALERS, Basement Floor & Wall Moisture SEWAGE CONTAMINATION SITE & SURFACE DRAINAGE SUMP PUMPS GUIDE SWEATING (CONDENSATION) on PIPES, TANKS VENTILATION in BUILDINGS VENTILATION DESIGN PROBLEMS & SOLUTIONS VENTILATION, WHOLE HOUSE STRATEGIES WATER ENTRY in BUILDINGS WET BASEMENT PREVENTION WINDOW / DOOR AIR LEAK SEALING HOW TO Window Flashing & Sealing Guide WINDOW LEAKS INTO BASEMENT SUMP PUMPS GUIDE SWEATING (CONDENSATION) on PIPES, TANKS WATER ENTRY in buildings WIND ENERGY SYSTEMS WIND TURBINES & LIGHTNING WIND WASHING INSULATION At EAVES WINDOWS & DOORS WINTERIZE A BUILDING More Information |
Here we explain how the dew point, the point at which moisture condenses out of air onto a surface, is obtained for a building cavity or building surface. InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers nor with topics or services discussed at this website.This article includes excerpts or adaptations from "A Rotting Timber Frame", by Steven Bliss, adapted by permission, courtesy of the Journal of Light Construction. Readers should review DEW POINT CALCULATION for WALLS for a bit more theory on calculating the point in walls at which moisture will condense. Also see MOISTURE PROBLEMS: CAUSE & CURE and HOUSEWRAP AIR & VAPOR BARRIERS and HUMIDITY CONTROL TO PREVENT MOLD also VENTILATION in buildings. Our page top photo shows severe moisture condensation on a basement window and window frame. © Copyright 2012 InspectAPedia.com, Daniel Friedman, Steve Bliss, Wiley & Sons, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website. How to Find the Dew Point for Any Temperature & Relative Humidity in a BuildingInsulating foam sheathing is becoming fairly common in cold climates. But in most cases, it violates the rule of thumb that the exterior of a building should be five to ten times more permeable than the interior. This is particularly true with foil-faced sheathings. Some people argue that it’s all right to use insulating sheathing, because it warms up the wall cavity enough to prevent condensation problems. This is supported by tests (based on 40 percent relative humidity indoors) done at the U.S. Forest Products Laboratory in Madison, Wisc.
As shown in our illustration above, To find the dew point for any temperature and relative humidity: Start with the temperature, move up to the current relative-humidity line. Move left to the saturation curve, and down to find the dew point, as shown. How can you gauge the dew point for yourself? First, you need to calculate the dew point of the interior air during the winter. To be conservative, assume an indoor relative humidity of 50 percent at 70°F. That gives you a dew point of about 50°F, according to the chart at above left. Next, calculate how cold it must be outside before the inside surface of the sheathing will fall below the dew point. The inside surface of the sheathing is where condensation is most likely to occur. You can calculate the temperature at any point in the wall if you know the total R-value inside and outside that point. The temperature rise through the wall and the R-values are in direct proportion. For example, in Wall A, when it’s 32°F outside, the temperature at the sheathing surface is 7/18 of the way from 32 to 70, or 47°F, which is below the dew point of the interior air. In Wall B, with two inches of foam, the temperature at the sheathing surface is 53°F — safely above the dew point. Since the average winter temperature in most of central and northern New England is at or below 32°F, Wall A appears risky unless you use one of the more permeable rigid insulations (headboard or rigid fiberglass), or have a perfect air/vapor barrier. That’s not a bet you should make. How to Read a Psychometric Chart to Find the Building Surface Dew PointSeveral detailed psychrometric charts useful for determining the dew point are shown and linked-to just below. There moisture is expressed in vapor pressure in mmHg (millimeters of mercury). The curved relative-humidity lines intersect with the diagonal lines to show the dew point for various temperatures and levels of relative humidity.
Relative humidity (RH) is the amount of water in the air. We use the term "relative" humidity because the amount of water that a given volume of air can hold decreases as temperatures drop. (Think of it as cold air being more dense, squeezing water molecules out of suspension in the air.) So RH is the ratio of the actual water vapor pressure in the air to the water vapor pressure in air that is fully saturated (can't hold any more water) at that same temperature. Wet bulb temperature (Twb) measures the amount of water that can be taken out of the air (by evaporation) - the old "sling psychrometer" used a simple mercury thermometer with a wet cloth over the sensing bulb to measure Twb by swinging the thermometer around at the end of a string - or properly: a sling psychrometer. Really. Also see Tools for Measuring Humidity This article describes alternative methods for measuring indoor temperature, humidity, relative humidity. See the left-most curve, the 100% relative humidity line for a simple case - that's air that is 100% saturated. So on the chart below, notice that on the left-most curve, the wet bulb temperature equals the dry bulb temperature - that is, when the air is fully saturated at 100% RH, no more air water can be evaporated out of the air. Define Dew Point: Now the good part: the dew point (Tdp) is the temperature at which water vapor just starts to condense out of air that is cooling - for example when warm moisture-laden air contacts a cool surface inside of a wall cavity. Above the dew point the moisture stays in the air. At or below the dew point moisture leaves the air and in buildings, condenses on the cooler surface that the air is contacting. This also means that if you are measuring the relative humidity in a room, the RH number only has meaning if you measure the room temperature at the same time and location. That's why, for example, when measuring basement humidity we will get different RH measurements in the center of the room than we will find right against a cool foundation wall - we discuss this in more detail at HUMIDITY LEVEL TARGET. Dew Point Example: in the chart below, if the room temperature (Tdb) is 43 degC and the relative humidity (RH) is 20% (the curved line reading up from 43 degC) then the dew point is 15 degC (reading horizontally across to the left-most curved line and noting the dew point temperature scale set along that curve). Dew Point Example 2: in the psychrometric chart given below, read up from 50 degC dry bulb temperature to the 20% RH curve, then follow the horizontal line from that point to the left to the outermost curve on the chart. If you don't go blind following this chart (click to enlarge it) you'll see that the Dew Point (Tdp) and also the wet bulb temperature (Twb) at the end of that line is 21 degC. Lots of variations of psychometric charts are available; they will all work about like this. To convert temperatures from Fahrenheit to Celsius use: Tc = (5/9)*(Tf-32) where Tc= the Celsius temperature and Tf= the Fahrenheit temperature. A nicely detailed free psychrometric chart is provided by Linric who also provide professional psychrometric software and other tools. A simple chart and additional explanation of moisture, mold, and the dew point can be seen in this NIOSH psychometric chart article. -- Adapted with permission, from material by Steven Bliss and appearing originally in the February 1987 issue of The Journal of Light Construction Questions & Answers regarding this article. Ask a Question or Search InspectAPediaHTML Comment Box is loading comments...
Recommend / Share this Article
... Technical Reviewers & References
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website. AIR SEALING STRATEGIES
... Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
|
