Authored By:Scott Bowe, Patrick Molzahn, Brian Bond,Richard Bergman, Terry Mace, and Steve Hubbard

PUB-FR-396 2007

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IntroductionIn recent years, we have received hundreds ofrequests for small scale lumber dry kiln plans fromwoodworking hobbyists. People were looking foran inexpensive and simple kiln design that could costeffectively dry as little as a few hundred board feet oflumber or as much as several thousand board feetof lumber each year. There are many different kilndesigns including solar, dehumidification, direct orindirect fired, and vacuum kilns. For small operations,there is a large amount of information available onsolar kiln design and operation. However, solarlumber kilns are not effective for large portions ofthe calendar year in northern climates (due to coldwinter temperatures).

Our goal was to develop a small scale kiln thatproduced quality kiln dried lumber, was functionalthroughout the year, was reasonably inexpensiveto build, and was constructed with common buildingmaterials from the local lumber yard and hardwarestore. With these constraints in mind, we chose simple2” x 6” construction to build the drying chamber andpowered it with a common basement dehumidifier,electric heater, and four attic fans.

This publication outlines the basic construction of asmall scale dehumidification dry kiln. When properlyloaded, this kiln can dry approximately 600 board feetof 4/4” lumber. This plan allows for flexibility in thedesign of the kiln. The kiln volume can be modified tocustom fit the individual’s needs. As with any valueadding opportunity (including wood drying), outcomescan never be guaranteed. It is highly recommendedthat users of this publication seek basic training inlumber dry kiln operation. Mistakes can be both costlyand frustrating. Sources of additional information arelisted at the end of this publication.

Materials List andBuilding PlansThe following section includes a materials list, the basickiln plans, and the basic electrical considerations forbuilding the dehumidification dry kiln.

Please refer to Appendix A which shows the LumberDry Kiln Bill of Materials. All of these products werepurchased at a local home center, except the remote

hygrometer/thermometer which was purchased at alocal electronics store. Total costs for the materialswere $2,704 based on 2006 prices.

Appendix B illustrates the kiln construction layoutincluding a plan view, section view, front elevation, backelevation, and detailed illustrations of the door sections.These drawings are not to scale (see Appendix B1-B6).

The floor system was framed using 2” x 8” construction16 inches on center (see plan Appendix B1). Three-quarter inch CDX plywood was used for the inside floor.One-half inch pressure treated plywood was used toclose the underside of the floor system. Pressure treatedplywood is recommended since the bottom of the kilnmay be in moist conditions if stored outside. If storedoutside, place on skids to raise the kiln off of the ground.The floor system was insulated with rigid foam insulationcut to fit between the 2” x 8” joists. A plastic vaporbarrier could be used under the three-quarter plywoodfloor; however, rigid foam acts as it own vapor barrier.

The side and rear wall systems were framed using 2” x 6”construction 16 inches on center (see section and backelevation Appendix A). One-half inch CDX plywood wasused for the inside wall sheathing and three-quarter inchT-111 plywood siding was used on the exterior walls.The walls were insulated with R-19 fiberglass batts.A plastic vapor barrier was used between the insidewall sheathing and wall studs (see Figure 1).

Figure 1: A plastic vapor barrier was used between theinside wall sheathing and wall studs.

The door side of the kiln was framed using 2” x 4”construction (see Appendix B3: Front Elevation andAppendix B5: Door Details). The doors were designedto be removed in a three-panel system. Doors of thissize are difficult to hinge without sagging, so removablepanels were a suitable alternative (see Figure 2).The door system was insulated with rigid foam insulationcut to fit between the 2” x 4” members. One-half inchCDX plywood was used for the inside door sheathingand one-half inch T-111 plywood siding was used onthe exterior of the door panels. An access door wasadded on the back kiln wall to allow access to thedehumidifier and electronic equipment (see AppendixB4: Back Elevation). This prevented the need for openingthe main door section and crawling over the lumberpile when access was needed to the dehumidifier.

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Figure 2: Removable panel doors.

The roof system was framed using 2” x 6” rafters16 inches on center (see Appendix B2-B4). One-halfinch CDX plywood was used for both the interior andexterior sheathing. The roof system was insulated withR-19 fiberglass batts. A plastic vapor barrier was usedbetween the inside sheathing and roof rafters. Theroof system was covered with standard roofing feltand fiberglass shingles (see Figure 3). Aluminum fasciaand plywood soffit was used to finish the roof system.

Figure 4: The hinged baffle houses the four fans andcontrols the airflow.

A baffle system was hung from the roof system down tothe top level of the lumber charge. The baffle consisted of2” x 4” construction with one-half inch CDX sheathingand was hinged at the top using standard door hinges.The baffle served as a platform to mount the four wholehouse attic fan units which circulate air (see Figure 4).In addition, the baffle controlled the airflow within the kilnwhich forced air through the lumber charge. Initially,two attic fans were installed with resulting air speedsof 220 cubic feet per minute which was not sufficientfor white woods such as maple or basswood. Two morefans were added in the final design which produced airspeeds in excess of 400 cubic feet per minute. These fanswere wired to allow either two or four fan operation.

Figure 3: The kiln roof system was designed foroutdoor use.

Standard hardware used for the framing included 16dand 8d nails. Stainless steel screws should be usedto fasten the interior sheathing given the extremelycorrosive nature of a dry kiln environment. In addition,it is important to use stainless steel fasteners whichcome in contact with pressure treated wood. ACQpressure treated wood is extremely corrosive onnon-coated and galvanized fasteners. These fastenerscan fail in as little as one year.

Other hardware included slide bolts, handles, andweather stripping for the doors. Again, stainless stealwould be preferred in any applications that come intoclose contact with the inside kiln environment.

Miscellaneous materials included an exterior gradestain for the kiln siding and paint for the interior wallsand floor sheathing.

Electronic EquipmentRelative humidity, air speed, and temperature are themain factors to consider for successful lumber drying.Each of these factors and the appropriate monitoringequipment is discussed in the following paragraphs.

A standard household dehumidifier was used to controlthe relative humidity within the kiln. Water was carriedthrough a drain hose from the dehumidifier through ahole in the rear wall into a measuring bucket. Measuringthe amount of water removed over a given period oftime is useful in determining the rate of moisture lossin the lumber charge. Early in the drying process, thedehumidifier controls did not allow for a high enoughrelative humidity setting and too much water wasremoved from the charge. To alleviate this problem, the dehumidifier was plugged into a simple lamp timerwhich cycled the dehumidifier on and off to slow therate of moisture removal. Different timing sequencesare required for different species and thicknesses toachieve a safe allowable moisture removal rate (seeFigure 7). Alternatively, the dehumidifier could havebeen plugged into and controlled by a simplehumidistat. These humidistats have a wider relativehumidity control range.

A hinged baffle system was hung from the roof systemdown to the top level of the lumber charge. This systemhoused the attic fan units and was hinged to swing upand out of the way while loading the kiln.

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An electric space heater was added to the kiln to boostthe operating temperature later in the drying process.During test runs, the space heater was not turnedon since the heat generated by the dehumidifier’scompressor combined with the heat from the four fanmotors was sufficient to reach the required dryingtemperatures. If the kiln were located outside of aheated building in the winter months, the electricheater would likely be needed to reach desiredtemperatures (see Figure 5).

A simple remote hygrometer/thermometer purchased ata local electronics store allows the kiln operator to readthe interior relative humidity and temperature withoutentering the kiln. This device consists of a measuringprobe and a wireless output screen which can be

mounted ontothe outsideof the kiln oranywherewithin rangeof the kilnprobe. It isrecommendedthat a Lithium-ion batterybe usedin exterior,cold-weatherapplications(see Figure 6).

Figure 5: Heater, measuring bucket, and dehumidifierwith lamp timer.

Figure 6: Hygrometer/thermometer.

PREPARING THE LUMBEREnd coat the green lumber with a commercial endcoating immediately after sawing to prevent largelosses from end checks and splits. End coating is lesseffective when applied after the lumber has begun todry. The end coated lumber should then be stacked inthe kiln with approximately 12 inches of clearance onthe front side of the stack and approximately 30 inchesof clearance on the rear side of the stack (dehumidifierside) to permit adequate air circulation. These spacesare known as plenum spaces. Arrange the lumber inneat layers, separated by stickers or spacers runningperpendicular to the lumber’s length. The lumber ineach layer must be uniform in thickness. If the sawnlumber is quite variable in thickness, one or both facesof the boards should be planed before drying to obtaina uniform thickness. This will reduce warping duringdrying. Particularly in warmer months, green lumbershould be stacked with stickers quickly after sawingor it may begin to develop stain – an undesirable andhighly visual defect.

Stickers used between each layer of wood are typicallythree-quarter inch thick, one-and-one-quarter incheswide, and as long as the lumber pile is wide. It isimportant that stickers are uniform in thickness andkept dry. Stickers are placed perpendicular to theboards’ length, every 12 to 18 inches along the pile’slength. The stickers are placed directly above thestickers in the lower layers. If the lumber is of varyinglengths, shorter boards are staggered by alternatingthem from one end to the other with the longest piecesalways on the outside edges. The ends of every boardshould be supported with a sticker. Stickers hold thelumber flat, preventing warp, while also allowing airto circulate through the pile and dry the lumber.

While the lumber is being stacked or prior to loadingthe kiln with pre-stacked lumber, several sampleboards must be cut (see Appendix C: Proceduresfor Cutting and Using Sample Boards). Periodicallychecking these sample boards will allow you todetermine the moisture content of the lumber in thekiln and to observe possible drying defects.

Lessons LearnedAs mentioned earlier, stainless steel fasteners andhardware should be used. Failure to use stainless steelwill result in premature failure of the hardware and thepotential for stained or discolored lumber. Alternativeconstruction materials for some portions of the currentdesign may be more desirable. One example wouldbe to use impermeable wall insulation such as aspray applied closed cell foam system to limit in-wallmoisture penetration. Moisture loss through the kilnstructure was noticed at the end of the drying cycle asthe lumber moisture content continued to decrease butno condensate was collected from the dehumidifier.

As constructed, the kiln doors are large and heavy. Onealternative is to buy or build a structurally insulated panel.These panels consist of rigid foam sandwiched betweentwo sheets of a wood panel product such as plywood ororiented strand board. This design would make a muchlighter door panel and provide superior insulation.

A combination of the rapid moisture loss driven by thedehumidifier and the high air speeds driven by the fourfan units can overwhelm the novice dry kiln operator.Limiting the run time of the dehumidifier and reducingthe fan operation to two fans will make the kiln moreforgiving for the novice operator.

Basic Operation of aLumber Dry KilnWARNINGAlthough lumber drying is a very simple process, itrequires constant monitoring and detailed record keepingto produce a quality product. Expensive grade lumbercan quickly be destroyed beyond use by extensivedrying defects (e.g., checking, honeycomb, end split,etc.). We strongly recommend that you read additionalkiln operation materials such as the Dry Kiln Operator’sManual listed at the end of this publication. In addition,several kiln operator training courses are offered in theeastern, western, and central United States.

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For information on drying schedules and schedulesfor specific species and thicknesses, see the referenceto the Dry Kiln Operator’s Manual and the DryKiln Schedules for Commercial Woods: Temperateand Tropical.

In measuring the rates of moisture loss, it is impracticalto weigh every board in the kiln and electric metersare not accurate above 30 percent moisture content.Therefore, the sample board method should be usedto determine the moisture content of lumber in thekiln. This method uses short, carefully chosen sampleboards sawn from larger pieces of lumber and thesesample boards are weighed periodically and estimatedmoistures are calculated. Since the wettest or slowestdrying lumber in the kiln has the highest risk of splittingand cracking, the sample boards should represent thiskind of lumber. Use samples from the most recently cutwood, the widest and thickest boards, lumber with themost heartwood, and boards that are quartersawnwhenever possible (see Appendix C: Procedures forCutting and Using Sample Boards).

MONITORING THE DRYINGPROCESSWhile the kiln described in this publication is designedfor simple operation, you will still need to closely monitorthe drying process. Monitoring the moisture loss andquality of the wood during the drying process will helpyou maximize the efficiency of the kiln. Simply explained,when drying lumber you want to dry fast enough toprevent stain and long drying times, yet slow enough toprevent checks and other drying defects. The prevalenceof certain defects is influenced by the species, thickness,moisture content, and drying conditions.

Monitoring the moisture content during the dryingprocess is important to avoid drying too rapidly (whichresults in quality loss) and to know when the load hasreached the desired moisture content. Measure thesample boards daily to determine the daily rate ofmoisture loss. This loss should be compared with thesafe drying rate for that species (see Figure 7 forhardwoods). If the drying rate is too fast, then it maybe necessary to increase the relative humidity setpointof the dehumidifier or turn two of the four fans off.

Lumber should be dried to the maximum safe drying ratewithout exceeding the daily loss. The safe drying raterefers to the moisture loss in one day, not the averageloss over several days. Figure 7 lists the safe drying ratefor common one inch thick North American hardwoods.Softwoods are typically able to dry at a much faster rateand moisture content losses above 10 percent per dayare quite common.

When lumber is at its highest moisture content, itis important to adhere to the safe drying rate. Mostunwanted checking and splitting in drying lumber occursas the wood loses the first one-third of its moisturecontent. Once the lumber dries below 22 percentmoisture content, the risk of creating new cracks andsplits is low. Dry lumber for interior use to a moisturecontent of six to eight percent for most areas of thecountry. This will limit the amount of dimensional changesthat would occur when the wood is processed and inservice. Lumber used for exterior purposes is usuallydried to 12 to 15 percent moisture content. Softwoodlumber used for framing is commercially dried to19 percent moisture content. However, fewer buildingdefects would result if softwood framing lumber weredried down to 12 percent moisture content.5

Walnut

Tupelo (Black Gum)

Poplar, Yellow

Oak, White Upland

Oak, Red Upland

Oak, Red Lowland

Maple, Soft (Sapwood)

Maple, Hard

Gum, Red

Elm, American

Cherry

Birch, Yellow

Beech

Ash, White 10.4 4.1

1” Thick

MAXIMUM RATE OF MCLOSS PER DAY (%)

2” Thick

4.5 1.8

6.1 2.4

5.8 2.3

10.4 4.1

5.3 2.1

6.5 2.6

13.8 5.5

1.0 - 2.5 -

3.8 1.5

2.5 1.0

13.8 5.5

10.9 4.3

8.2 5.5

SPECIES

Figure 7: Safe Drying Rates for North AmericanHardwoods.

AcknowledgementsThis project would not have been possible withoutthe cooperation and support of Mr. Patrick Molzahn,Cabinetmaking and Millwork Program Director atthe Madison Area Technical College. Mr. Molzahnand his students were responsible for the constructionand testing of the kiln. This project was funded bythe United States Department of Agriculture ForestService State and Private Forestry Economic ActionProgram administered by Mr. Lew McCreery.

DisclaimerCommercial products named in this publication arefor informational purposes only. The authors do notendorse these products and do not intend discriminationagainst other products which may also be suitable.

AuthorsSCOTT BOWEAssociate Professor and Wood SpecialistUniversity of Wisconsin-Madison, Madison WI

PATRICK MOLZAHNCabinetmaking and Millwork Program DirectorMadison Area Technical College, Madison WI

BRIAN BONDAssociate ProfessorDepartment of Wood Science & Forest Products,Virginia Tech, Blacksburg VA

RICHARD BERGMANDrying SpecialistUnited States Department of Agriculture Forest ServiceForest Products Laboratory, Madison WI

TERRY MACEForest Products Utilization and Marketing SpecialistWisconsin Department of Natural Resources - Divisionof Forestry, Madison WI

STEVE HUBBARDWood ConsultantHubbard Forest Solutions, LLC, Madison WI

Additional Web-basedReferencesLUMBER DRYING MANUALSThe following publications can be downloaded freeof charge from www.fpl.fs.fed.us:

Simpson, W. T., ed. 1991. Dry Kiln Operator’s Manual.United States Department of Agriculture. ForestService. Forest Products Laboratory. Madison, WI.www.fpl.fs.fed.us/documnts/usda/ah188/ah188.htm

Denig, J., E. M. Wengert, and W. T. Simpson. 2000.Drying Hardwood Lumber. United States Department ofAgriculture. Forest Service. Forest Products Laboratory.General Technical Report. FPL-GTR-118.www.fpl.fs.fed.us/documnts/fplgtr/fplgtr118.pdf

Boone, R., C. J. Kozlik, P. J. Bois, and E. M. Wengert.1993. Dry Kiln Schedules for Commercial Woods:Temperate and Tropical. United States Department ofAgriculture. Forest Service. Forest Products Laboratory.Madison, WI.www.fpl.fs.fed.us/documnts/fplgtr/fplgtr57.pdf

Forest Products Laboratory. Air Drying of Lumber.United States Department of Agriculture. ForestService. Forest Products Laboratory. General TechnicalReport. FPL-GTR-117.www.fpl.fs.fed.us/documnts/fplgtr/fplgtr117.pdf

WEB RESOURCESDepartment of Forest Ecology and Management,University of Wisconsin, Madison, WIhttp://forest.wisc.edu/

Department of Wood Science and Forest Products,Virginia Tech, Blacksburg, VAwww.woodscience.vt.edu/

United States Department of Agriculture Forest ServiceForest Products Laboratory, Madison, WIwww.fpl.fs.fed.us/

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ITEM MATERIAL COST/UNITQUANTITY2

TOTAL COSTRunners 4” x 4” x 12’ TTD $11.16 $22.32

3Floor 1/2” x 4” x 8’ TTD Plywood $32.84 $98.523Floor 3/4” x 4” x 8’ CDX $28.86 $86.042Rim Joists 2” x 8” x 12’ $11.77 $23.541Center Blocking 2” x 8” x 8’ $7.96 $7.962Rim Joists 2” x 8” x 8’ $7.96 $15.927Joists 2” x 8” x 8’ $7.96 $55.728Insulation 2” x 4” x 8’ Rigid Foam $19.89 $159.123Insulation 1” x 4” x 8’ Rigid Foam $9.45 $28.3530Studs 2” x 6” x 92-5/8” $5.92 $177.606Plates 2” x 6” x 92-5/8” $5.92 $35.525Plates 2” x 6” x 12’ $8.81 $44.0520Header 9-1/2” LVL (LF) $3.09 $61.803Insulation R-19 Fiberglass Batt (150 SF) $14.99 $44.978Interior Sheathing 1/2” x 4” x 8” CDX $21.01 $168.0810Siding 1/2” x 4” x 8” T-111 $37.53 $375.301Sill Sealer 50’ Roll $3.60 $3.608Corners 1” x 4” x 8’ $3.36 $26.885“Z” Flashing 1-1/4” x 10’ $2.26 $11.304Water Table 2” x 4” x 10’ $3.73 $14.921Vapor Barrier Roll $12.99 $12.999Frames 2” x 4” x 8’ SPF $2.98 $26.8210Face Moulding 1” x 4” x 8’ SPF $3.36 $33.604Insulation Rigid Foam $14.65 $58.608Casing 1” x 4” x 8’ SPF $3.36 $26.887Rafters 2” x 6” x 10’ $7.93 $55.512Sub-fascia 2” x 6” x 12’ $8.81 $17.622Fascia 1” x 8” x 10’ SPF $6.65 $13.302Fascia 1” x 8” x 12’ SPF $7.98 $15.961Soffit 1/4” x 4” x 8’ $14.50 $14.504Interior Sheathing 1/2” x 4” x 8’ CDX $21.01 $84.042Exterior Sheathing 1/2” x 4” x 8’ CDX $21.01 $42.022Insulation R-19 Fiberglass Batt (100 SF) $14.99 $29.981Felt 15# Asphalt Felt $13.94 $13.944Shingles Fiberglass $11.99 $47.965Drip Edge “D” Style 10’ Lengths $2.78 $13.9012Slide Bolts for Doors - $5.47 $65.648Handles for Doors - $6.24 $49.927Weather-stripping 70 LF (10’/Package) $2.56 $17.924Whole House Fans - $37.99 $151.961Dehumidifier - $229.00 $229.001Drain Hose - $5.74 $5.741Electric Space Heater - $49.98 $49.981Remote Hygrometer/Thermometer - $54.68 $54.682Exterior Stain Gallons $20.96 $41.922Paint for Interior Gallons $15.98 $31.962Anchor Seal End Coating Quarts $18.00 $36.00

TOTAL COST OF BUILDING MATERIALS $2,703.85

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PROCEDURES FOR CUTTING AND USING SAMPLE BOARDS1. Select a few boards from the stack of lumber that represent the slowest drying material. These would be

the widest, thickest, and highest moisture content boards. Boards that contain the most heartwood or arequartersawn are also suitable.

2. Cut a 24 to 30 inch sample that isfree of knots and at least 12 inchesfrom the end of the board. Thencut two one inch sections from thesample board as shown (seeFigure A). Make sure to numberthe sections and sample board.

3. Immediately weigh the one inch sections (accuracy of about one gram required) and record the weight.Measure immediately after cutting since it is important that they do not gain or lose weight before weighing.

4. End coat the sample board with a roofing cement or commercial end coating (see Figure B). Now, weigh the sample board and record the weight on the sample (accuracy of 0.1 pounds).

5. Place the sample board in the lumber stack in a location where it will dry at the same rate as the rest of thelumber in the dryer (see Figure C).

6. Place the one inch sections in an oven 215ºF to 218ºF (102ºC to 103ºC) and dry – usually 18 to 36 hoursdepending on the oven. Reweigh and obtain the oven dry weight.

7. Calculate the moisture content of each section and average the moisture content (MC) of two sections toobtain the moisture content of the sample board.

8. Estimate the oven dry weight of the sample board using average moisture content and weight from step #4,and record this so it can be used for future moisture content calculations.

9. To determine the moisture content at any time, reweigh the sample board and calculate.

%MC = - 1 x 100[ ]Wet WeightOven Dry Weight

Estimated OvenDry Weight x 100Wet Weight (Step #4)

(100 + %MC)=

Current %MC = - 1 x 100[ ]Current WeightEstimated Oven Dry Weight

Figure A: Dimensions of a sample board for controlling the drying process.

Figure B: Roofing cement and commercial end coating. Figure C: Stack of lumber with sample boards.

PUB-FR-396 2007