Roof

The first purpose of the roof is to keep water out of and away from the building. The second is as an environmental separator more generally for the top of the house.

So the first priority is its water management features. Because this renovation will keep heat in the house in winter, it will put the bricks at greater risk of freeze-thaw cycle damage (by keeping heat in the building, we will end up with bricks which are substantially colder in winter, and thus less inclined to dry when wet). We thus need to do everything possible to keep them from getting wet.

The single most effective tool for keeping walls dry is generous roof overhangs with effective gutters. So step one is to add roof overhangs and effective gutters.

But what happens if the roof fails and allows some water to get under the shingles and through the ice and water shield material via nail punctures, or what if wind-driven rain enters the ventilation openings? The roof boards and anything else that gets wet, such as roof insulation, will need to dry. Thus the underside of the roof-deck will need to be ventilated to the exterior.

(Additionally, our roof envelope assembly will be open to vapor diffusion, allowing vapor to be driven down and into the house interior if necessary. This vapor openness is likely not necessary here, but it is necessary in other part of the building envelope, near masonry walls, and we are aiming for a simple, continuous envelope design.)

For reasons discussed elsewhere, the house's attic will be conditioned, interior space, on the inside of the building envelope. Thus roof ventilation will not be able to be simple attic ventilation through a few openings, as is the case in most Chicago bungalows.

Rather, individual ventilation channels will have to be included in the space between each and every roof rafter. The air intake will be at the eaves, through a long, continuous opening supplying air to each rafter bay. The air exhaust will be through continuous ridge vents, which must be designed in such a way as to stop the entrance of wind-driven rain through small wind baffles.

So much for the asphalt shingle roof, roof sheathing, and ventilation system.

The next layer of control will be thermal insulation, reducing the entrance/exit of heat through conduction. Rockwool batts, friction-fit between each roof rafter, but not thick enough to block the ventilation channels, will provide the first layer of thermal insulation. Rockwool is highly water-resistant through their lack of adsorption ability. Furthermore, if they are forced to adsorb water through a catastrophic water even (e.g. tree damages roof during deluge), they will remain undamaged, and return to their thermal insulating properties after eventual drying. They will not support mold growth while wet.

A second layer of thermal insulation, in the form of rigid Rockwool boards, will be hung from the underside of the roof rafters by metal "skewer" fasteners, this time continuously, without thermal bridging occurring through the roof rafter wood.

Next we turn to heat's second way of entering/exiting the building -- radiation. Reflecting radiant heat back to where it came from requires a reflective material with an air space to one side of it. Without the air space, the radiant thermal energy will simply transition to another mode of movement (conduction) and be conducted by the interface of the radiant barrier material (aluminum, and very good conductor).

Long screws will be used to fasten 3/4" wood boards to the underside of the continuous Rockwool insulation layer, these boards will (1) provide the air gap for the radiant barrier, (2) provide a stapling surface for the radiant barrier, and (3) provide a stapling and tape-rolling surface for the below application of the air, water, and vapor control layer. The boards will thus be at least wide enough to facilitate rolling out of this seam tape, and spaced perfectly for alignment with these seams.

The radiant barrier will then be staple to these boards.

Next we turn to the air, water, and vapor control layer. Dorken Delta Vent S was chosen for its high quality, ready availability, and relative vapor-openness, while remaining totally air and water tight.

In addition to preventing air infiltration/escape for energy efficiency, we need to prevent warm, humid air from the interior coming into contact with the cold surface of the roof sheathing and condensing, leading to roof damage. Likewise, in the summer, we need to prevent this condensation occurring on the inside, colder layer of this assembly.

The Vent S material will be stapled to these boards on top of the radiant barrier, with care taken that all staple punctures are covered with fully adhered seam tape. The material is shingled, and the seams are taped meticulously.

This assembly only achieves approximately an R-27 level of thermal conductance insulation. Expert recommendations for roof assemblies recommend R-60. We will thus need to add additional R-30 or so of thermal insulation. Insulation fasteners can be attached through the Vent S material and into the boards with careful taping and caulking to air-seal the punctures.

Because this additional insulation is on the interior side of the now fully sealed up building envelope, it does not need to be water resistant, and so cost-effective fiberglass can be used. The outside of the Vent S material will be sufficiently insulated that it will not reach the dew point, and thus warm house air will not condense on this surface in the winter.