Handcrafted Traditional Timber Frames & Dovetail Log Cabins
Common Wall Enclosure Systems
intensive approach by adding Cedar Breather over the felt paper and eliminate all the strapping. Cedar Breather is a plastic three dimensional mesh fabric that functions in the same capacity as the strapping and will provide the needed air space between the felt paper and roofing material.
If you are considering using blown-in cellulose for roof insulation, most manufacturers advise that you do not vent the roof when using their product. Two very informative web sites detailing many aspects of cellulose insulation are www.applegateinsulation.com and www.regalind.com. Not to be overlooked is the fact that an additional 2x framing system will need to be constructed on top of the timber rafters to create the needed cavities to hold the loose-fill cellulose.
For those that live in a cold weather climate similar to mine, every winter brings with it “ice-dam-season”. Ice dams are massive, potentially destructive amounts of accumulated ice/icicles that build up in the gutters and eaves of under-insulated, poorly ventilated roofs. As the heated interior air within your house rises, it will pass through insufficient levels of roof insulation and warm the underside of the roof sheathing. Snow also acts as an insulator from above. This combination will raise the surface temperature of the roof/snow interface, creating many hours of snow melting. The water runs down the roof slope, and upon contact with the freezing surface of any roof overhang (eaves), or a porch roof, both of which do not have heated space below it, the water then reverts back to ice. The ice dams first gather in the gutters, then begin to back up the roof slope, possibly pushing itself under shingles and then re-melting as the dam climbs upward, making contact once again with the warm roof surface that melted it originally. Once the ice-melt penetrates your shingles and reaches the sheathing, it can leak (unseen) into wall cavities and into the interior of your house. Other issues that can cause roof temperature differential (cause of ice dams) can be heat loss through poorly sealed roof penetrations (vent pipes), thermal bridging at rafter locations, and a variation in roof snow thickness, which will allow the sun to heat exposed roofing and melt nearby snow.
Most problems can be avoided with careful planning and attention to detail at the design stage, and in combination with excellent workmanship and quality materials, they can be prevented during the construction phase. Preventing structural failures during the design stage by addressing potential problem areas is relatively inexpensive; fixing these failures after your house is finished and occupied can be stressful and very expensive. Review in detail your architectural plans with your designer and a respected general contractor, and make sure that all major and minor components and all related matters concerning the building envelope are addressed. Seemingly small issues that can lead to big problems if left unaddressed include: air-sealing/air-barrier procedures and materials, proper ventilation techniques, and the correct vapor control measures.
The Building Envelolope
Enclosing & Insulating your Timber Frame
Historically, barn frames and other out-buildings of the past featured only an exterior wood cladding, and the entire framework was visible from the inside for all to see, but in house frames it was not considered “fashionable” to have the timber framework exposed on the interior. The accepted practice in Colonial “dwelling-houses” called for the concealment of as much of the framing timbers as was practical. This was accomplished by an “infill” wall system comprised of smaller scantlings, usually a 4x4 stud-wall constructed in-between the posts and covered with a lath & plaster mix. Any portion of the now reduced-in-depth timber that remained to be seen was soon painted over with a home-made “white-wash” type of paint. Rafters and second floor timbers were treated the same way. This long-held view of the timber framework as a feature that should be hidden from view has given way to a more practical realization and a true appreciation that the massive timbers and finely crafted wood-to-wood joinery that defines a timber frame be at the forefront in one’s mind when choosing an insulation/enclosure system.
On the plus side is the relative ease with which windows, doors, mechanical systems, and electrical wiring can be installed within conventionally framed exterior walls and also interior partition walls. While this method of enclosure may be the most labor-intensive system of the three, it does hold appeal for those that may be doing the work themselves.
The choice of drywall as an interior wall surface for timber frames is quite common, and one effective method of attachment involves the use of plywood spacer strips nailed to the exterior face of the timbers. These strips can be 1/16” to 1/8” thicker than the drywall, (5/8” spacers to allow for ½” drywall). This allows you to slide the drywall sheets behind the posts and install them from the inside (avoiding possible rain) and after the frame is enclosed, with the added benefit of another seam that you do not have to tape. Also, a slightly thicker spacer will allow for any dimensional irregularities on the exterior face of the timbers. With your drywall on the
1) Wrap & Strap - While this first method may be the most cost-effective, it can also be quite labor-intensive. In what's referred to as a built-up enclosure, or wrap & strap, as the enclosure/insulation system is built up on the exterior of the frame. The building components that comprise this system include the interior finish, single or multiple layers of rigid foam, vertical strapping (strips of wood to create an air space or vent path) and then the exterior siding. There are a few different ways to sheathe and insulate your frame walls using this technique, and here I’ll describe one common method: 2 x 6 tongue & groove, V-jointed pine planking is fastened vertically to the outside face of the frame. This sheathing is the interior finish and nailing base for the exterior insulation and strapping. Along with the interlocking T&G joint, a horizontal timber girt spanning between posts serves to stiffen these floor-to-ceiling boards. Exterior walls can have electrical wiring chases on the interior, and hidden from view by creating a built-up baseboard around the perimeter. Additional wiring and fixtures can be located in stud-framed interior partition walls.
At this point in the construction process a vapor control layer may be required by your building code official, but perhaps your situation calls for using only a single thick layer of rigid foam to meet code R-values and create an unbroken thermal envelope. Due to the vapor impermeability characteristics inherent in a single thick layer of rigid foam insulation, the standard vapor barrier as we know it may possibly be eliminated in this type of assembly, although some building codes may still require one, and if that’s the case #15 builders felt is recommended instead of the sheet polyethylene commonly used. (The topic of vapor barriers and the characteristics of the three foam insulation types will be addressed in-depth a bit later). The 4’ x 8’ sheets of rigid foam are then applied in the thickness necessary and can be secured by using 1”diameter plastic cap nails that will penetrate the T&G a minimum of 1”. Since R-values vary among the three types of rigid foam, your choice of insulation material will dictate whether you can use a single layer or whether multiple layers will be required in the wall system in order to meet code R-values. If using double layers of the standard 2” sheets common in most building supply outlets, stagger all seams, both horizontally and vertically. It also may be advantageous if these insulation sheets have a T&G or shiplap type of interlocking joint around their perimeter. Butt the sheets tightly together and seal these seams with housewrap tape. The rigid foam sheets will be supported by a shelf created by the protruding edge of a 2 x 12 sub-sill.
Vertical strapping in the form of 1x3’s are laid on-the-flat @ 16”o.c. using long panel screws designed for this purpose and screwed through the foam to the frame and/or to the 2x6 T&G. If using rigid insulation with various facing materials, these facers also function as a drainage plane behind the strapping (assuming all seams are adequately sealed). If using rigid foam without facing materials, some building professionals recommend the use of #15 felt paper behind the strapping as a water resistive barrier, (drainage plane) and as a part of the wall systems exterior line of defense. It is vapor permeable, allowing air and moisture to pass through, and in the case that moisture happens to reach it from the exterior, the absorption capacity of the felt will help speed up the drying out process, unlike some common housewrap products currently on the market. When placed vertically to accommodate horizontal siding (clapboard, shiplap, etc.), the cavity created by this strapping acts as a vent path to allow trapped moisture to escape, which is a very important issue in regards to the longevity and performance of exterior wood siding, which is then applied.
I find it a good idea to anticipate that moisture will eventually find its way behind your wood siding. It could be in the form of wind blown rain being wicked through at overlap joints, nail holes, electrical outlets and at corner seams. On humid summer days, it may involve a vapor-drive process, where warm moisture-laden air is drawn behind the siding as it seeks a colder condensation surface. Whatever the source, that moisture needs a way out. A pressure-equalized air space created by vertical strapping provides an escape route for that moisture by neutralizing any air pressure differences between the front and back of the siding. In an un-vented wall unit, this air pressure imbalance is one of the forces responsible for drawing water into the wall assembly. Combined with Cor-A-Vent www.coravent.com at the top and bottom of the strapping (to keep out bees, wasps, birds), your wood siding will be allowed to breathe, thus keeping the entire wall assembly dry. This strapping, in conjunction with a drainage plane (facings on rigid foam) is an example of a rainscreen system. Careful installation of flashings at windows and doors are critical to the effectiveness of any rainscreen system.
Another point concerning wood siding, and especially if you plan to paint it, be sure to seal the backside of the boards as well as all end grain surfaces. When back coating your siding boards, it is advisable to use a water repellent preservative, instead of an oil based product. The water based preservative is vapor permeable, which will allow the wood to dry more quickly should the backside get wet. This recommendation only applies when there is a vented air space behind the siding, as peeling/cracking of the exterior surface paint is directly correlated to moisture absorption through the back side of unsealed, un-vented wood siding. If you plan to use vertical wood siding, such as board & batten or cedar shingles, it may be advisable to use a product such as Cedar Breather, which is explained further along.
2) SIPS- Another common method used to enclose a timber frame is the use of stress skin panels, now commonly referred to as SIPS, or structural insulated panel systems. The advantage to using SIPS is how very quickly and efficiently your frame can be enclosed and protected from the weather. An initial downside is they are probably the most expensive enclosure option, and as of late, some builders within the timber frame community that also act as GC's have stopped using them due to a variety of reported problems. Basically SIPS are pre-fabricated panels of rigid foam insulation sandwiched in between an interior and exterior skin of OSB. Some companies offer pre-cut openings for windows and doors, and wiring chases pre-routed within the panels. There are many SIP manufacturers throughout the country, and their products and options vary widely. A Google search will yield all the info you may need, both positive & negative. Of interest are the web sites of two panel manufacturers that I came across, www.foardpanel.com, and www.rtight.com. Both of these companies offer as a free download, a 50 page panel installation guide. While these installation guides are product-specific to each manufacturer, they do contain information and drawings concerning general construction issues such as window and door openings, properly installed flashings, HVAC systems, electrical wiring and plumbing, etc.
3) Stud Walls - The third enclosure option that virtually all carpenters would be familiar with is to build perimeter 2x stud walls fastened to the timbers exterior (outfill system), or between timber posts as an infill system and fill stud cavities with fiberglass batts, sprayed-in foam, or blown-in cellulose insulation. Some issues that come into play when adding secondary stud walls for enclosure: 1) the structural redundancy and the added cost of two framing systems for one structure, 2) the loss of depth at timber post locations if used as an infill system, 3) the heat loss through the “thermal bridges” at every stud location. These thermal bridges reduce the rated R-value of the cavity insulation by as much as 30%. However, this can be countered by adding a 1” sheet of rigid XPS foam (extruded polystyrene), and installed on the exterior of the studs.
The primary focal point of this guide pertains to methods & materials used in the completion of a timber frame structure. However, this information is also very applicable to any roof system you may choose for a log structure.
Material Properties of Rigid Foam Insulation
Since rigid foam insulation sheets are the most common material used to insulate timber frames, I’d like to talk a bit about the three types currently on the market, some of the characteristics of each, and hopefully sort out (and not add to) some of the confusion that may be involved in their selection. All three types come in 4’x8’ sheets, various thicknesses, and are available at building supply outlets.
1) Expanded Polystyrene (EPS). EPS rigid foam is the type most often seen in everyday items that we use such as convenience-store coffee cups, coolers, etc., and also mistakenly referred to in generic terms as Styrofoam. It is commonly known in the construction industry as beadboard due to its composition of many tiny polystyrene beads that are expanded with steam and pressure and filled with air during the molding process that bonds the beads together to form rigid sheets. It is typically white in color and you will find it stamped with an array of different trade names. Most commercially available EPS is made using 1 lb/pcf density foam, and as a result has a lower compressive strength than other foam types at this density rating. EPS has an R-value of about R-3.5 – R-4 per inch of material thickness @ 1 lb. density. Both the R-Value and the vapor transmission properties of EPS can be increased by increasing the density of the product, although higher density EPS is much less common in the market. At the product density commercially available, EPS foam will allow for slow vapor and air transmission through the cell walls.
2) Extruded Polystyrene (XPS). Both the above mentioned EPS and also XPS rigid foam are classified as thermoplastic foams, which have a definite melting range and will tend to soften at around 165 degrees F, and melt in the 200-210 degree F range, temperatures that can approach those found on southern exposure walls and roofs in the summer months. XPS is typically referred to as blue-board and manufactured by Dow Chemical Co. under the registered trademark name Styrofoam, which covers a wide range of extruded polystyrene building products used for foundation, wall, and roof insulation. Also marketed by Owens-Corning, it will be pink in color and labeled with the trade-name Foamular. XPS is a closed-cell foam and has a more constant, fixed cellular structure, thus allowing it to be stronger in compression, more dense (1.6 lb/pcf) and more water resistant than EPS. XPS has an R-value of approx. R-5 per inch of material thickness.
3) Polyisocyanurate, also referred to as urethane, or polyiso, is a closed-cell thermoset type of plastic generally used for higher temperature applications as it tends to remain stable over a wider temperature range. It does not usually exhibit a specific melting range but will instead char and burn when exposed to extremely high temperatures. Dow Chemical also manufactures this as a blue-board product under the Tuff-R trademark, with another recognizable name being Thermax. Polyiso rigid foam most always comes supplied with various facing materials front and back, with these bonded facers serving in a vapor-control capacity and also determining its placement whether it is in a wall or roof assembly, as illustrated in this tech. sheet from the National Roofing Contractors Association: www.nrca.net/rp/technical/manual/06pdfs/99_100.pdf. According to Dow Chemical, the facers on Thermax and Tuff-R also act as both a drainage plane and a weather-resistive barrier, eliminating the need for installing a housewrap or builders felt. This claim is contingent upon all seams being properly sealed according to manufacturers specifications. Facing materials can also perform a secondary function by improving the surface adhesion of the board for seam-taping and adhesives, as un-faced foam board may prove to be problematic concerning this task. Some common construction adhesives contain petroleum-based solvents that will dissolve un-faced EPS and XPS rigid foam on contact, with polyiso being unaffected by these solvents. Polyiso has a compressive strength at or above that of XPS, is usually manufactured in 2 lb densities, and has an (LTTR) R-value of R-6.5 – R-7 per inch of material thickness.
While the rated R-values for EPS and XPS rigid foam are generally stable and will remain constant for the life of the product, urethane is a bit different. Due to its propensity for out-gassing, the R-value of urethane slowly decreases over time, resulting in it being designated with a Long Term Thermal Resistance (LTTR) R-value representing a 15 year time-weighted average R-value that is listed above. Out-gassing, or thermal drift as the process is sometimes referred to, involves the high R-value gasses that are produced during manufacture (and are a part of the foam), slowly diffusing out of the foam and being replaced by air. This usually occurs within the first two years after manufacture, and then remains unchanged for the life of the product, resulting in a stabilized R-value. The facings do slow down this process somewhat, as the gasses are then forced out of the smaller perimeter edges of the board as opposed to the larger surface area of the board face.
Of the three types of rigid insulation that have been discussed, it seems that both XPS and polyisocyanurate are consistently categorized in various building science publications as closed-cell rigid foams, while I’ve seen contradictory classifications where EPS foam is referred to as both closed-cell foam and occasionally as open-cell foam. While this characteristic may seem a bit too nit-picky, it can have relevance as to whether or not a vapor diffusion layer will be necessary within your wall/roof system, which in turn will impact the performance of the entire assembly. The clearest explanation that I’ve come across so far that characterizes the different foam types as either closed-cell or open-cell involves a spec. sheet from the NAHB Research Center, and I’ve listed it below for you, your general contractor, or any code officials to look over.
CLOSED-CELL AND OPEN-CELL RIGID FOAM
Foam plastics are composed of millions of tiny cells or bubbles. When the cells are completely intact with no holes or spaces between the cells – it is called closed cell foam. When the cells have holes connecting one cell to another – it is called open cell foam.
Open-cell foam is usually made of many smaller foam beads that are expanded by temperature within the confines of a mold to shape the material. This process leaves open pathways or channels between the expanded beads that permit moisture and air migration through the material as a function of its density.
Closed-cell foam is made by including a “blowing agent” in melted foam plastic that causes tiny air bubbles (cells) to form within the plastic material as it is extruded into a desired shape. Thus, there are no open pathways through the material and it is resistant to moisture and air migration regardless of its density. All other factors equal, closed-cell foam also tends to be stronger (in bending and compressive strength) than open-cell foam.
I realize all this talk of using man-made synthetic foams on such a traditional/historic-based structure such as a timber frame may seem a bit contradictory to some, including myself. For those who are interested in insulating their frame using a more natural, “green” approach, the use of straw bales as an enclosure and insulating option for timber frames has gained a lot of attention in the last few years. The website www.strawbale.com can provide you with the necessary information you will need concerning that technique. Also google "green building techniques" will yield more natural alternatives.
Vapor Control & Rigid Foam in Walls & Roofs
Moisture, either in liquid or vapor form and generated from both the interior and exterior sides of a wall or roof, can be a threat to the components within the assembly unless properly accounted for. When constructing a wall/roof assembly with sheets of rigid foam, varying degrees of thickness will be necessary to achieve code R-values depending on the type of foam used. In the process, the vapor permeability (measure of materials resistance to water vapor) of the wall or roof insulation will be lowered to the point where a separate vapor control layer may not be required. According to www.buildingscience.com “The use of impermeable insulating sheathing materials of adequate thickness to ensure that the dew point temperature is not reached within the assembly, will allow for the interior vapor control layer to be eliminated from the assembly (installing a vapor retarder on the interior would actually be detrimental to the system)”.
As mentioned previously, some rigid foam insulations come supplied with various facing materials front and back, including aluminum foil, kraft paper, polyethylene, and glass-fiber. These facers can either be permeable or impermeable to water vapor and are tailored to match the end use of the product. They can function in several different capacities, chief among them being their vapor-control capabilities. These facings also govern the overall permeability of the rigid foam, as they are often much lower in permeance than the foam core itself. For rigid foam with no bonded facers (un-faced), its permeability to water vapor is correlated to the thickness of the material. For rigid foam insulations that have facings, their permeability does not change with increased thickness. Generally, the permeance of un-faced foam sheathing is based on a thickness of one inch. By increasing the thickness of the rigid foam in a wall or roof system, a corresponding decrease in the permeability of the collective assembly will result. For example, 1” of un-faced EPS has a perm rating of 5.00 with an R-value of about R-4 per inch. Using a single 4” sheet or double layers of the 2” sheets in a built-up wall unit will yield a cumulative R-value of about R-16 and lower the perm rating to 1.25, which is at the very low end of vapor semi-permeability and classifying the insulation as a Class III vapor retarder. A material thickness of 8” will be required in a built-up roof assembly to yield an R-value of about R-32 and lower the perm rating to .63, which is in the mid-range of vapor semi-impermeability and classifies the insulation as a Class II vapor retarder.
Un-faced XPS @ 1” thickness has a perm rating of 1.1 and an R-value of R-5 per inch. In a wall system incorporating 3” of XPS, whether using a single sheet or double layers, will yield an R-value of approx. R-15 and lower the perm rating to .37, which falls in the very low end of vapor semi-impermeable and classifies the insulation as a Class II vapor retarder. Layering several sheets in a roof assembly to achieve a cumulative R-value of R-30 will require approximately 6” of material thickness and further lower the perm rating to .18, which falls in the extremely low end of vapor semi-impermeable and classifies the insulation as a Class II vapor retarder.
Un-faced polyiso is very uncommon, possibly due to the issue of thermal drift. Most polyiso rigid foam is manufactured with various facing materials of some type on the front and back of each sheet, and @ 1” thickness has an R-value of about R-6.5 per inch and a perm rating of .03, taking it into the vapor impermeable (Class I vapor retarder, considered a vapor barrier) category regardless of thickness. A single 3” wall sheet of Thermax (trade name) will yield an R-value of approx. R-19, while the perm rating remains unchanged due to the facings. A roof assembly incorporating double layers of 3” Thermax will yield a cumulative R-value of approx. R-38, also with an unaltered perm rating.
It would seem that using a single 3” layer of Thermax in a side-wall application offers the most time-efficient, one-step method, as most code R-values are met with one-sheet installation, the facing materials act as both an interior vapor control layer and an exterior drainage plane (assuming all seams are well taped), while also eliminating the additional costs and associated labor that these steps would normally require.
*It is my understanding that when incorporating multiple layers of rigid foam sheets in order to achieve a given R-value, and these sheets are faced both front and back, and regardless of whether it is EPS, XPS, or polyiso, that there should not be INTERIOR (sheet to sheet contact) facing material present within the mix.*
For more definitive information, please consult with the insulation manufacturer for specifics about their respective products’ performance and applications and also your local building code for further guidance.
There seems to be some disagreement among building professionals regarding the suitability of dedicated vapor barriers within certain wall/roof systems. The debate centers on the use of plastic sheeting (6-mill polyethylene) as a commonly used vapor barrier. The claim against their use is that they are climate dependant, and should be limited to only extremely cold climates (Canada). Opponents of polyethylene vapor barriers also say that while they do prevent walls from getting wet during the winter, they also prevent them from drying out during the summer, when vapor drive is reversed. Keeping in mind that water vapor seeks to travel from warm to cold, a vapor barrier is meant to prevent the movement of water vapor from the warm side of a wall/roof to the cold side, where it can condense to a liquid. Therefore building codes tell us to place plastic sheeting on the warm side of any wall/roof assembly. In my climate zone of N.E. Ohio, the warm side of the wall/roof is not the same in July (assuming an air conditioned interior) as it is in January. So theoretically, for half the year, this vapor barrier would be on the wrong side of the wall.
A sound, general strategy that seems to make sense is that we should be looking at ways to prevent wall/roof assemblies from getting wet from the interior and the exterior, and in the event that they do get wet, allow them to dry to the interior and the exterior. In a layered system of rigid foam sheets, with the assembly functioning in a impermeable to semi-impermeable nature, both the interior and exterior sheet faces would function as vapor retarders in this type of insulation system. In the summer months the exterior face of the rigid foam would prevent warm, humid air from entering the cooler air conditioned space, while during winter months the interior face of the foam board would prevent vapor drive to the exterior.
For further assistance and/or additional information in regards to rigid foam insulation, vapor barriers, moisture control strategies, etc, go to www.buildingscience.com. They are largely recognized as the pre-eminent authority on the issues mentioned. They can also provide architectural services and design an insulation system and moisture-control strategy that are tailored to your specific project. In addition, visit the U.S. Dept. of Energy’s web site www.energysavers.gov and search “foam board insulation”, and also www.dowbuildingsolutions.com and search “rigid foam”. Along with www.buildingscience.com, these three web sites have provided the bulk of the technical information contained in this write-up.
interior it still may be a good idea to use the 2x6 T&G as the wall sheathing (as opposed to plywood or OSB). Even though it will not be visible, the 1-1/2” thickness of the T&G allows confidence when hanging kitchen cabinets and various wall fixtures. Also, the interlocking T&G boards contribute to overall stiffness and lateral stability of the wall unit.
The concept of whether roof assemblies should be vented, resulting in a cold roof, or un-vented, giving you a hot roof, is dependent on a variety of factors including final roof covering and the type of insulation to be used. The most important reasons for venting a roof are removal of moisture and keeping the underside of the roof sheathing as close as possible to the exterior air temperature, an effective strategy in controlling ice dams, a topic which will be addressed a bit later. The assumption can be made that it is almost impossible to control moisture entry into a roof assembly, so by creating vent paths you then have an effective means of moisture removal. Most building professionals recommend that you vent your roof when using rigid foam insulation, as do almost all SIP manufacturers. In a cathedral ceiling design such as is common with exposed timbered roof systems, certain roof coverings, notably wood shingles or shakes, standing-seam metal, tile, and asphalt shingles perform best when the underside of the roof deck is adequately ventilated. Cedar shakes/shingles and standing-seam metal roofs may benefit from a less labor
One of my primary objectives, along with building a finely handcrafted timber frame or log cabin, is to ensure that all clients are fully informed and knowledgeable from start to finish in every aspect of their intended building project, so that every decision you make will be rooted in solid, fact-based information. In that regard, I’ve gone to great lengths in doing much research on the enclosure & insulation issues that will impact the completion of your project once it leaves my hands. I certainly do not claim to have all the answers as they pertain to these peripheral building matters, as they are outside my level of expertise, so in that regard I have not included a definitive recommendation concerning what is the best enclosure system or insulation technique.Since the skeletal timber-framework is a non-standard form of construction, the process and materials required to insulate and enclose your timber frame can differ a bit from conventional stud-framing. So as you look over the following information, you’ll notice some very non- traditional insulation components and methods that have been adopted by the timber frame community and pared up with this very traditional building technique. All in the name of increased energy efficiency, while at the same time maintaining the timbers and their joinery as the focal point of your structure.
The various concepts, material characteristics and techniques are presented here so that you can determine what methods and components are most suitable for your particular situation. This is in no way meant to be a comprehensive, authoritative guide, but simply my intent to provide accurate information to guide those that will be acting as their own general contractor or to offer some helpful insight to those that will be hiring a GC to complete their project, so that when finished, your timber frame structure will function at an optimal level.
Not to be overlooked in the entire construction process is the issue of quality workmanship. Finely built structures are not created from a list of materials and drawings. The most effective methods and quality products can be rendered ineffective unless attention to detail, proper installation, and careful building practices are followed.
When covering your roof, the materials and assembly process are similar to how the walls were constructed. First, 2x6 T&G is fastened to the rafters, then sheets of rigid foam of your choice and in the thickness required are laid down. Fill any gaps at the ridge or elsewhere with canned expanding foam. Screw 2x4 strapping (24”oc, laid flat vertically) through the insulation to the 2x6 T&G, followed by a layer of 7/16” OSB as your nailbase, then a layer of #30 felt. Stop the felt about 36” from the eaves, and install Grace Ice & Water Shield www.grace.com on the lower 36” of the roof. When used in conjunction with adequate amounts of insulation and proper ventilation techniques, this self-sealing waterproofing membrane offers excellent back-up protection in the event that ice dams may form. This roofing system with strapping and the resultant air space is an example of a ventilated or cold roof, so airflow in the ventilation channels will need to be maintained by soffit vents and ridge cap vents that are equal to the cross-sectional area of the channel. This is just one requirement that must be met for a vented roof assembly to function at an optimum level. Other conditions to be looked at include properly installed and adequate amounts of insulation, sealing or eliminating the sources of heated indoor air leakage that will promote roof warming, and the ability of the assembly to remove moisture faster than it is accumulated.