After the frame is up, you are going to have to enclose it and there are quite a few different paths that you can choose. In building a timber frame enclosure system, a couple of overriding considerations need to be taken into account, as well as the details about each system. This will affect your budget and you need to understand the options to make the best decision for your project.
Structural Insulated Panels (SIPs)
Structural insulated panels, or SIPs, have been around since the 1970s and have been growing in popularity ever since. They are basically a 1/2” layer of OSB, a layer of foam, which varies in thickness depending on your insulation needs, and another layer of ½” OSB. The layers are glued together making them excellent in structural applications such as creating roof over-hangs and shear walls. Your engineer will work with your designer and architect to apply the panels’ structural benefits to the timber frame engineering. They may possibly reduce the need for knee braces and other timbered elements, saving you some money.
These panels are produced off-site and shipped to your site by the manufacturer when you need them. They come in sizes up to 8’ wide by 24’ long with all the angles, windows, and doors openings cut out and ready for installation. They get screwed to your roof and wall timbers. They can be also used in areas that are not timber framed in place of conventionally framed wall and roof systems.
There are several different types of foam, each with different pros and cons with factors like cost and insulation values. Where you live and the local building code requirements for insulation R-value in your walls and roof will affect this decision.
SIP homes also have extremely low levels of air infiltration because there are fewer gaps to seal. This airtight characteristic makes heating and cooling your timber frame home more economical in both your monthly energy bill and the smaller size HVAC units your home requires. Homes built with SIPs are able to keep a consistent temperature and have fewer drafts and less noise infiltration than standard construction buildings. When properly installed, they maintain a higher whole-wall R-value than stud walls of similar proportions.
Types of Foams Used in SIP Panels
(EPS) 1” of foam = 3.8 R-value
This is by far the most popular type of foam used for SIPs. EPS foam is basically a high grade of Styrofoam. It is extremely stable and has been used in many other applications. It costs the least, but also gives the lowest R-value compared to the other foams, which can mean thicker roof or wall panels. These are easy to modify on site for electrical work and window and door changes.
Polyurethane (PUR) - 1” of foam = 5.0 R-value
The second most popular panel is the polyurethane-based panel. It offers a higher R-value per inch, which means that it will cost more than the EPS foam but you may be able to use a thinner panel. The extra cost with a thinner panel can be made up somewhat in window and door jamb extensions, trim details and eave details. One side note is that it is harder to modify these panels on site and your electrical system has to be figured out completely before the panels go into production.
Neopor (NEO) - 1” of foam = 4.5 R-value
This is a newer product that uses graphite to enhance the EPS foam. It gives it a grayish color and you get a better R-value than with standard EPS foam.
Concerned about wiring your electric in SIP's? Check out our article Running Wires in SIPs – Concerns for the Homeowner
Oddly, conventional framing overtook and made timber framing all but disappear from standard building practices until it was revived in the 1970s. It consists of the standardized 2x4 and 2x6 pieces of lumber for the wall framing and using 2x8, 2x10, 2x12 or pre-made roof trusses out of 2x material to create the roof. This all gets sheathed in a plywood or OSB layer that ties everything together structurally. This method is the way that at least 99% of the homes in this country are built.
In the case of incorporating them in a timber frame, the walls are constructed and stood up around the timber frame. The walls need to be spaced off the frame to the outside whatever distance the inside finish walls will be. For example, for a ½” drywall finish putting a 5/8” spacer behind the timbers will allow you to slide the drywall behind the post or plates, creating a clean look.
Some folks like to use the infill system, putting the conventional wall inside the timber frame. This is not recommended for exterior walls. As the timbers dry and move through the seasons, you will certainly get air infiltration through the gaps that are created between the walls and timbers. This will increase your energy costs as well as potentially giving you moisture problems in the future.
Electrical wires and insulation work can be completed as usual and you will have the choices of standard insulation types that you can put in the walls. To match the tightness and to get the insulation value of a SIP, you will need to use a spray-in polyurethane-based foam for your walls and your roof. After all factors are considered cost-wise, you will get a small cost saving if you choose to go with conventionally-framed walls instead of opting for the SIP wall panels.
Using conventional lumber to frame out your timber frame or create a built-up roof system with foam certainly has its place and warrants discussion. There are other ways and techniques out there that I will not mention. The two that I see the most are the following:
In a built-up roof system, one uses foam boards stacked on the roof around a 2x band board. You control the R-value of the roof by the thickness of the foam, similar to a SIP panel. You then attach 2x4s flat with long screws through the foam into the timber frame tying everything together. You will need to add lookouts to the gable and eaves to create all the overhangs; care should be taken here. All of that is then covered with your sheathing material and then underlayment. Please note that this is a vented roof system, whereas a SIP roof is unvented.
A framed system uses 2x material laid perpendicular to the timber frame roof system and then insulation is installed. Any type of insulation can be used from the high tech spray-in foam to fiberglass insulation. The denim jean factory waste insulation has always been an interesting solution. All that is finished with sheathing and the underlayment readied for the final roofing. Creating the overhangs is a bit easier with this method.
With either the built-up or the framed system you are basically creating a panel-like product, just in place and on site. To factor in the additional labor, time on the roof, delays for weather, speed and materials, I would really recommend that you consider the SIP solution for the roof over the timber frame. Integrating a SIP roof and conventionally framed roof is easy if planned out. Of course, the option to conventionally frame your walls around your timber frame and using SIPs over the roof is a great combination.