Thursday, April 23, 2009

Chimney Effect

Following up from my last post, what exactly did my Dad and I do for three days if we didn't rip up the attic floor? We worked sealing the eaves from the attic underfloor.

The image below shows how the exterior walls of the house are constructed.

On the outside of the house there is a wythe of brick. I believe that this brick is held in place with metal ties. There appears to be a very thin layer of air between this brick and the main structural terracotta block walls of the house. Attached to the interior of the terracotta walls are 2 x 2's. Nailed to those are the lathe and then the plaster is applied over this.

Any cracks in the exterior mortar allow the wind to penetrate into the first air gap. Any additional cracks in the terracotta mortar allow further penetration into the second air gap. These air gaps terminate in the attic, between the attic floor joists.

During the winter, the warm interior air seeps into the second air gap from cracks in the plaster and along the base board. Warm air from the basement also seeps into the second air gap. What ends up happening is a chimney effect. The warm air from inside rises up to the attic, pulling cold air from outside to replace it. This makes our interior walls cold and allows a lot of warm air to escape the house.

Because of our cantilevered roof, cold air is also vented into the attic from vents in the overhanging soffits. Which means a whole lot of cold air circulating in the attic and along the walls. The R-19 pink stuff is better than nothing, but one problem with pink stuff is that it is terrible at stopping air infiltration.

My dad and I cut 2 x 6's and installed them between the joists to try and stop the cold air from the soffit vents from mixing with any air that might be leaking into the attic joists. We also did this in preparation for possibly putting in polyisocyanurate spray foam. I didn't want the spray foam to ooze out into the soffit or down into the first gap between the brick and the terracotta.

Here's a picture of what it looks like at present that will hopefully make it clearer. The clean, shiny boards are the ones we put in. The are positioned on top of the terracotta wall.

And this is what one of the four corners in the attic looks like. It also shows how it would be impossible to remove the attic floor without causing my roof to collapse.

And that's what took three days. Taking up the perimeter floor boards, cutting around sixty five 2 x 6's and then nailing them in place. This will hopefully block wind from coming up the soffits and the first gap and mixing with heated air in the house. We still need to plug the second air gap though.


Ms. Huis Herself said...

Nice diagram, hon! Colors & everything!

I'm a little disappointed you didn't use one of the pix I took of your dad up there for scale so folks could see the teeny tiny space he was trying to work in. :)

Christopher Busta-Peck said...

Very helpful. This makes me think that fixing some of the cracks in the mortar will help address (some of) our issues with drafts. One more project for the list!