Adding Venting to Unvented Roof?
atlanta86
| Posted in Energy, Heating & Insulation on
I’m renovating a 1975 Midcentury Modern house. It needs both a new roof and new roof insulation, but the current roof is not vented.
Based on advice I’ve gotten on this forum, there are two ways to go about this: spray foam or “cut and cobble” with rigid foam. The spray has the advantage of not requiring venting, but the rigid foam does.
For a variety of reasons, my preference in this case is for rigid foam. Can I simply add venting at the ridge and soffit or am I creating more problems than I’m solving?
I appreciate your assistance.
Replies
In a building the structural elements that touch the outside and the inside becomes a thermal bridge. Outside heat can enter the interior through the thermal bridge. The warm or cool interior air can also move through the thermal bridge to the outside. That is why insulation on the exterior surfaces is more effective than any insulation between the structural elements. The exterior insulation layer prevents heat transfer through the thermal bridges. It works so well that you do not have to insulate the space inside the wall. Even if you filled the hollow spaces in the wall with closed cell foam you will not reach R-30 because all of the vertical structural elements are thermal bridges. The thermal bridges reduce the r-value of the insulation in the wall spaces.
Attic spaces do not need to be vented if the ceiling is sealed of from the attic. That also includes wires and piping coming through the top plates and any chimneys going through the attic space. All of these allow warm moist air in the living space to get into the attic. Once all leaks are sealed then the attic will not have moisture in it.
If you are going to put on a new roof where the roof is a wall that faces the sky, the most effective insulation will be on top of the roof sheathing with the rigid insulation seams taped. Using more than one layer with the seams offset between the layers will eliminate any heat transfer lines visible in the cold weather on the roof exterior. With exterior insulation there is no venting need. in order to apply the shingles, you will need an additional layer of sheathing on top of the insulation. Add a water resistant layer on top of the sheathing. If you want you could use Huber Zip sheathing or plywood and a peel & stick material. Then if there was a leak, it won't come through the roof. The top sheathing will need to be screwed into the framing below if you want a wind resistant roof. Add ties between the rafter tails and the outside wall framing while you are at it.
If you want to add rigid insulation to the inside or under the roof and you put insulation on the outside simply add it to the faces of the roof joists then the enclosed space can become a channel to vent any moisture between the interior insulation and the roof sheathing. There is an article in Building Science that addresses the removal of moisture accumulation at the peak of a roof by cutting a vent and covering the cuts with a vapor permeable barrier material that is then roofed over but with a vent strip at the peak. But it isn't open; the barrier material prevents any wind driven rain from getting in while allowing vapor out. The barrier material keep out the critters too.
These are the recommended practices now. Lots of appropriate guidance at Building Science.