Finished Attics, Part 3: Construction
Updated: Apr 9, 2020
Having discussed the finished attic topics of zoning and planning, and
stairs & windows from my earlier posts, we now move on to the last topic: Construction, specifically framing, insulation & heating/cooling. I will not be discussing electric and plumbing, since, for these trades, the work is generally not attic-specific.
If the attic remodel project includes dormers or if the roof itself is getting raised, a framing plan will need to be created which includes the proper sizing of all new roof framing elements. Our Maplewood attic remodel/addition (below) required an entire roof reframing, but the larger space afforded two new bedrooms and a full bath.
If the project has no plans for dormers or any other expansion, there still may be some required framing work. It is important to take a close look at the framing in the existing space. If the attic is framed with trusses, it cannot be finished without reframing the roof. Even if there are no trusses, there may be posts, support walls or horizontal cross members (called ridge ties) within the unfinished attic space. These are there to support the roof, so they cannot simply be removed without reframing portions of the roof. Are there any sags in the roof rafters, ridge board/beam or attic floor? These sags will need to be addressed prior to any work, as they will only get worse with the additional weight of drywall or flooring.
The new finished attic floor itself needs to be able to support new living space. By code, unfinished attic space without storage is only required to support 10 pounds per square foot (psf) of "live load" (people and furniture); attics with limited storage require 20 psf. Given this, most unfinished attic spaces have relatively shallow floor joists - 2x6s or 2x8s. For the future bedrooms/closets, 30 psf is required and for other rooms 40 psf is required. Depending on the structure of the floor below it, the existing attic floor joists may or may not be deep enough to support the new habitable space requirements. If the existing attic floor is determined be insufficient for the new habitable space, it must be reframed. There are a few ways to accomplish this. Once common common way install new, deeper, joists next to each existing joist. This will raise the height of the ceiling floor by the difference of the joist depths. This may require significant work to an existing attic stairway so that the top step of the stairs does not have extra vertical space.
Insulation. Insulation for attic remodels can be very tricky. In the first place, the insulation of the roof is the most important place for insulation during winter months given the fact that heat rises. Generally, unfinished attics are uninsulated and the insulation should be in the attic floor in order to keep the space below it insulated and reduce the size of the insulated "envelope" of the entire home. An uninsulated attic generally has either windows or vents in the gable walls, eaves, roof surface, and/or along the ridge to create a large vented space. This vented space provides colder air beneath the roof in winter to prevent condensation on the interior of the attic as well as to prevent ice dams. In summer, the venting enables the space to release any hot air build-up, helping to keep the lower floors cooler and helping to preserve the longevity of asphalt shingle material. When heat builds-up underneath asphalt shingles they deteriorate at a much faster rate; without ventilation, an expensive 30-year asphalt shingle roof may not make it past 20 years.
When an attic space is to be finished, the insulation must be reconfigured entirely. The home's insulated "envelope" now expands all the way up to the roof rafters and out to the gable walls. The gable walls must now be insulated and the rafters must be insulated in a way to enable as much insulation as possible, but to still keep the air flow underneath the roofing material. Roof rafters can be as shallow as 2x8s or even 2x6s; this depth is too shallow to house the typical batt insulation (the pink pads) needed to provide code-required energy efficiency or R-Value. In New Jersey, our Rehabilitation code allows us to only have to provide the batt-insulation that will fit in the existing roof-rafters, but this rarely gets to the code-required (and recommended) R-Value, resulting in high January energy bills.
The two common options we use to get that desired R-value are either to increase the depth of the roof-rafters, or use spray-foam insulation. If you have a bit of headroom to spare and the existing rafters are almost the depth needed to get the batt-insulation you want, you can simply fur down the bottom edge of the rafters with 2x lumber along the edges of the rafters to create a deeper cavity for insulation. Alternatively, the more expensive spray foam insulation (above) can get the required R-value in a fraction of the depth. With either insulation choice, baffles are required between the insulation and the roof decking above it.
Baffles are thin pieces of styrofoam or other material that rest up against the roof decking in between the roof joists and keep the insulation from choking the air flow; the baffles allow channels of air to flow from the bottom of the roof rafter to the top. Shown here (left) are Owens Corning Raft-R-Mate baffles. At the top/peak of the roof rafters a continuous horizontal path for air flow must be created and vented out the sides or at the top via mechanical vents or a continuous ridge vent.
Heating & Cooling: I want to mention a few things about heating and cooling specific to attics. Given this is a separate floor, it is generally zoned separately. If not on a separate zone from the floor below it, it may be considerably warmer than the floor below it if not balanced properly. The attic is often done with a completely different system altogether than the rest of the existing home. Heat is often accomplished with electric baseboard or wall units. Cooling can be a separate split-unit system, wall units or a feed off of an attic forced-air system.
A forced-air heat/cooling system is often located in an attic to be able to heat and cool floor(s) below. When this is the case, the system itself as well as the ducts have to be carefully planned out in the attic space. Ducts running parallel to joists can generally fit between the joists, but when ducts cross joists they have to be carefully located and planned for.
In this Verona remodel (above) , a window bench was created under an existing gable end to enclose a cross joist duct, and built-in toy chests were built on either side.
While an unfinished attic can provide a large amount of new living area for family rooms, offices and bedrooms, it is important to understand all the elements of planning and construction that make these great new spaces possible.