Hello again everyone.
After making a VERY hasty decision to head out west to work with my father-in-law doing flood/fire remediation work for a month, I am back and at it with the blog.
Today, my fiancé needed a break after hitting the books pretty hard this week studying for her final real estate exam. We have been talking a lot about the kind of house that we want when we settle into life in Ottawa, get our careers in order and figure out where exactly we want to live here. We do not want to repeat the mistake we made with our old home, rushing into home ownership when we don't know a town very well. Better to pay rent for a while.
One thing that ranks at the top of my list (after south-facing windows and yard, more on that in upcoming posts) is an unfinished attic. I have spent the last few months (minus my brief trip out west) working with a local insulation installer evaluating people's insulation and making recommendations when needed, and I have noticed a few things as my knowledge or roof and attic systems has grown.
An unfinished attic, I may have mentioned before, is a space that can act as an appliance in your home, that protects and preserves your roof. When people expand their homes to include a conditioned attic space, they can create a whole host of issues, unless they are very careful about it.
A properly ventilated attic will draw air out the ridge, gable or chimney vent, which will in turn draw air into the soffit vents at the bottom of the roof. This keeps the attic cold enough to prevent snow melt on the roof.
Without proper ventilation, the attic cavity will become warmer than the outside air, and can lead to snow melt off the roof. Adequate insulation (we recommend R50 to R60) will slow heat loss into the attic, making the work of the ventilation system easier. And even better, less heat lost means less money lost!
If the ventilation system is not adequate or is blocked by poorly installed insulation, heat escaping the home will build up in the attic, warming the roof and melting snow. This can lead to ice damming, peeling shingles and even black mould.
Taking care of your ventilation will add life to your roof, and increasing your insulation will slow down damage as well, in addition to saving you money.
Here's a classic roof showing symptoms of being underventilated. The roof is warm, and the snow had melted. At the bottom of the roof, forming a ring above the soffit is a cold zone not heated by the home underneath. Water will run down to this point every time the temperature gets close to zero, and then it will freeze and form an ice dam.
And how much money is pouring out through that roof all winter? It takes a lot of energy to drive that much snow melt!
When an attic is finished, it involves squaring off the triangular roof so that the walls are reasonably flat. Here is an image that I found doing a quick search:
They even stole my punch line! Knee walls are rarely insulated properly, and this means that heat loss and moisture problems can develop.
There are two common approaches, I'll start with the one that I like less, for purely practical reasons.
The first is to insulate the roof the whole way up. This creates a kneewall space that is basically a closet. It is INSIDE the house, which means that it will (in theory) be heated.
The difficulty here is that now the ventilation system of the attic, if it is going to act properly, now needs to be entirely contained within the rafter depth in order to keep the roof cool and prevent snow melt. Having no cooling can lead to a great deal of melt, and ice-damming and roof damage.
In this case a barrier, such as a soffit baffle, can be used to keep the insulation from blocking fresh outside air from moving from the soffit to the upper attic and top vent.
This will keep the roof cool, preventing snow melt and all associated problems.
A bonus to this design is that the kneewall spaces will be kept warm and allow for conditioned storage.
If this ventilation is blocked, then heat will migrate through the insulation (more slowly as R-value increases) and reach the roof sheathing and contribute to snow melt. The attic space above will also be warmed by lost heat and not cooled by ventilation, leading to further melt unless additional vents are added.
This can lead to otherwise preventable snowmelt, and potentially shingle damage, ice damming and even mould growth.
Here we see a home where the kneewall is part of the inside conditioned space. However, it looks like there is not adequate ventilation along the roof sheathing to keep the roof cool. It is very early in the season, and there is already a significant ice dam building up and quite an icicle!
More worrisome might be the amount of money pouring out through the melted strip of roof. Yikes!
The second method is to insulate the kneewall separating the room being built from the kneewall space, and the floor of the kneewall, such that the new space is essentially OUTSIDE the house. The big benefit here is that you have cut off a bunch of air volume that you do not have to pay to heat! You have created two triangles of unconditioned storage space on each side and a small attic on the top. The benefit of this is that you have more freedom to increase the insulation value of your walls, and you can control the ventilation more easily.
If the walls of the new room are insulated, ventilation must be provided to connect the soffit vents with the top attic space.
This will keep the roof cool and reduce the air volume that will have to be heated.
If ventilation is not adequate, insulation will actually provide a contact path for heat to move to the roof sheathing.
In the case drawn here, the kneewall is shown ventilated, as in the photo below.
In many cases, however, the kneewall may require further ventilation, and without it the kneewall may get warm enough due to heat lost through the ceiling of the floor below and the wall of the new room that snowmelt may be generated through the full height of the room.
Now in this case, we actually see a home where the kneewall space itself is well ventilated, but the sloping ceiling in the finished attic show signs of not being ventilated at all.
Just going by the line of snow melt, I could make an educated guess about where the kneewall ends and the ceiling follows the roof.
The kneewall is ventilated by the soffit and vents on either end, providing cool air and keeping the roof above cool. Unfortunately, the roof above the room and the small attic that remains do not seem to be staying cool, and snow melting from over these spots will run down and form ice dams over the cool kneewall. So close!
Not shown is the case where the room is not square, but instead has a sloped ceiling that follows the roof line. In this case, ventilation must be run the entire length of the slope above the room, to connect the cool kneewall with whatever attic remain above the room. Otherwise melt will occur.
That is all for now. I will put up some more pictures from my walkabout soon. There are so many roofs and so little time!
