There are good things about capes....and then there are bad things.
The good: when done right, they can be just about the most efficient homes out there. Capes offer efficient living space in a tightly packaged shell as they basically utilize the exterior walls (including the roof) as the thermal envelope (air, vapor & thermal barrier).
The bad: in order for them to be efficient, they have to be insulated and air sealed a very specific way...which most of them have not been, making the classic cape notorious for high energy bills despite their small footprint, inconsistent seasonal temperatures and comforts, and bad ice damming.
A sign of heat loss and energy inefficiency: inconsistent snow melt on a roof (see photo above). This is the exact type of snow melt is what leads to ice damming.
The primary issue with most Cape homes is a combination of having no established thermal envelope and excessive and mismanaged airflow. Capes suffer from decades of backwards thinking, the thought has always been "we have snow melt and ice damming on our roof, we must need more venting". It just doesn't work that way with a cape. Since the thermal envelope IS so tight and close to the livable space, the only way to address these issues is through strategic air sealing and proper insulation selection and installation.
The photos below shows excessive air flow entering through the soffit vents (left photo), supposedly going up the roof deck into the attic crawlspace above, but instead it is surfing the ceiling cavity between the finished 1st and 2nd floors (bottom right photo) = not good.
How is this happening? Well, once we gain access into the knee wall area of the 2nd floor, we can see one of the ways cape are classically insulated. Fiberglass under the knee wall floor deck (under the green carpet) and fiberglass on the the actual knee wall (not visible, right edge of left photo). This scenario lets endless amounts of air enter into the building and wander freely....and yes, that is a mouse on the carpet. Since when does air travel in straight lines?....light a match, does the smoke go straight up when air is moving about?
So, what happens with all that air entering through the soffit vents? It's meant to enter the knee wall and run up the roof deck towards the attic crawlspace above, and maybe some of it does, but certainly not all of it. Some of it can enter the house through built-in dressers (left photo) and some maybe through improperly sealed knee wall hatches (middle photo). Side Note: any area where cold air can enter, as shown in these photos, is also where heated air can exit the building shell - so it works both ways depending on a variety of different factors (temperature, wind pressure, exhaust fans, etc).
What happens to the soffit ventilated air that does make up the roof deck? The upper right photo shows devalued insulation due to airflow up the enclosed ceiling slope. This is the classic cape scenario. Ventilation is installed to stop snow melt and ice damming, and all it does is further devalue existing insulation making it much easier for heat to conduct through the enclosed slopes to the roof deck, still producing snow melt and ice damming.
Many people have a hard time understanding this approach as it goes against classic building standards, so just trust us. First thing you need to do is block ALL soffit venting on the sloped sides of the house (if you have a big shed dormer, then maybe you can leave that soffit venting in place, call a pro). To properly ventilate the attic crawl space, depending on its size, you can simple install gable end vents and potentially a ridge vent as well.
- Block all soffit venting & remove the existing ceiling slope insulation (if existing) from the knee wall slopes & enclosed ceiling slopes. The rafter cavity should be empty from the soffit to the attic flat.
- Proceed with all air sealing of the attic flat (chimney chase, lights, fans, electrical wiring chases, plumbing stacks, etc)
- Install air tight and insulated attic hatch (4-6" rigid board insulation with weather stripping) and install a barricade for new insulation to abut
- Install either 5"+ of closed cell spray foam insulation to the knee wall rafter slopes OR if enough space is available install 2" thick rigid board strapped in place to the rafters - creating an enclosed rafter cavity within the knee wall. NOTE: if the rafters are greater than 2x8" then you can simply install Intello Plus smart barrier to the rafters to create the enclosed cavity; however we like rigid board as it decreases thermal bridging through the rafters.
- Insulate the rafter cavity from eave to attic flat with dense pack cellulose (if you foamed the knee wall slopes, then only dense pack cellulose from the top of the knee wall slope to the attic flat.
- Add R49 of loose pack cellulose to the attic flat.
Additional Notes: There can be many other steps/details that go along with these solutions to ensure proper air sealing and insulating, because of this an Energy Audit is always recommended and only choose insulation contractors who have insulated other homes in this manner.