Before I get started on the insulation talk , I thought I’d share a picture of the outside of the house with all new hardie siding. We still have the front and back of the house to install, but Daniel’s hard work at leveling out the barge board sheathing is evident in the this picture of the North elevation. That’s about as straight as we could have hoped for given that the house has a pronounced lean and 90 degree angles are hard to come by!
We have also made progress in the last couple weeks on restoration of the windows and doors. More info will follow in posts by Daniel, but here is a picture of windows and doors sanded and primed.
And now for the fun part – insulation. I will break this into two parts – walls and attic space.
For the walls, we decided to go with BIBS or blown in blanket insulation. This is an affordable, easy to install alternative to spray foam that provides a more consistent R-value than the common pink fiberglass batts because it fills the entire cavity. It is a two step process – install a fabric liner on the outside face of the studs and then pump in the “powder puff” through a hole cut in the fabric at each cavity. Powder puff is a technical term for BIBS that was taught to me by the installer. The result is an R-15 insulation value (2×4 walls) or an R-23 (2×6 walls). An added bonus is that your walls feel like mattresses. The fabric liner even closely resembles what you’d find on the bottom of a box spring.
The attic presented another obstacle. Because we made our attic space accessible, we went from having a vented attic (non-airconditioned space with open air flow to the exterior) to a non-vented attic due to conditioning the space. In order to have a truly unvented attic assembly, you have to provide a full air and moisture barrier.
In your walls, you want the option of vapor permeability so that if and when when vapor occurs within the walls, it has a way to escape prior to doing any damage. Here is a great article on why condensation occurs in walls and how to avoid it.
Your roof line on the other hand should be vapor and air impermeable when conditioned. The most widely recommended insulation to achieve this is close celled spray foam. As mentioned in a previous post, the State Historic Preservation Office who is administering our tax credit program does not allow the use of spray foam in historic buildings. They feel that it is detrimental to the old wood and irreversible should a problem arise.
In its place, we elected to use vapor impermeable polyisocyanurate foam board insulation. The right product for this is Dow Thermax sheathing. It is foil faced on both sides and comes in various thicknesses. Other (more affordable) rigid board insulation products can be used in the walls, but the polyiso boards are the only ones that meet the requirements of a true air and vapor seal. I was warned endlessly by the insulation subcontractor at the problems that can arise with moisture if this is not done correctly.
Our application is 2″ thick sheets between the rafters – sealing all major gaps – with a second 2″ layer attached to the underside of the rafters. On the second layer, we alternated the joints from the first layer and sealed (I should say… are sealing as we speak) every joint with a fiber mesh tape covered with a fibrous mastic. The mastic is the same stuff used by HVAC contractors to seal duct connections.
Since the rafters are 6″ deep, we installed blocking to keep the first layer in line with the base of the rafters. An added bonus is that the foil face of the polyiso provides a radiant barrier in the air space between the polyiso and the roof, though I’ve heard that a radiant barrier facing up can collect dust over time and not function as well due to loss in the reflectivity.
In order to offset the costs of the insulation (the polyiso is more expensive than the spray foam, mainly due to added labor), we are installing the attic insulation ourselves. I have to give Daniel most of the credit here – I have the very good excuse of finalizing wedding plans keeping me from crawling around in hard to reach places in an unconditioned summertime attic space. And when I say hard to reach, I mean extremely difficult and claustrophobia inducing spaces that require shimmying between the roof rafters and large piece of HVAC equipment with pieces of foam board, drill and sealant. I did get recruited for long enough to experience that part!
To end on a fun note, here is a view out of our attic bedroom window.