Tuesday, April 01, 2008

Winning the war against cold.

OK, technical update for those interested in remodeling an old house.

As you can see from the photographs posted by my lovely wife, we gutted our dining room. I tore the plaster off, then the lath boards, then removed the (spotty and inadequate) cellulose insulation. So, right down to the bare studs. There is nothing quite as dirty or quite as satisfying as tearing plaster off a wall with a sledgehammer and a crowbar.

I debated for a time about leaving all the old stuff in place, and just building the additional wall to give us 12” thick walls when finished. The old walls would contribute some insulating power, and the new insulation/wall would do most of the work.

The problem is two fold. First, I really want to control air infiltration. On a windy day, the effectiveness of most insulation can drop dramatically if air infiltration is not controlled. This is especially true for fiberglass insulation. The reduction can be as much as 50% and it happens when you need the r-value the most.

With new house construction, this is easy. Just wrap the house in Typar or Tyvek on top of the plywood sheathing (before the siding or bricks), and you’re done. Of course, I don’t have that luxury since I am not tearing the siding off my house. As an alternative, I put the “house wrap” on from the inside. I cut a piece of Tyvek for each stud bay and staple it up. If you examine the photos, you see that the Tyvek makes a “C” shape in cross section, since it’s installed on the left stud, the wall sheathing and the right stud.

Once the cellulose is blown in, this will push the Tyvek tight against the studs and sheathing, and will help seal the house wrap to the framing, reducing air infiltration. Without tearing out all the old plaster, insulation, etc, I could not install the Tyvek in the proper place.

The second half of the problem is the fact that this is an old house. I really want and need to know what’s inside those walls. So far, we haven’t found any bad concealed termite damage except in the basement. That’s the nasty “swiss cheese” wood you see in the other photo, now gone/replaced. But the previous insulation job was absolutely terrible. Either they did it themselves, and badly, or a contractor did it and figured the owner wouldn’t know the difference because you can’t see it. I would estimate that 30-40% of the wall was uninsulated.

So the general idea is to tear the old stuff out right down to the studs. Line the wall with Tyvek, build a second wall to make the total thickness ~12”, install all new wiring and plumbing as needed, then a new vapor barrier, then dry wall. Then we blow a foot of cellulose into the wall, finish the drywall then paint and trim. This technique works well for big older homes with nice big rooms, since you will lose at least eight inches on every outside wall. For a house with smaller rooms, you need other techniques like adding the insulation on the outside, or using super high r-value insulation where you can get away with a 6-8” total depth.

That's it for now, tune in soon for further exciting developments.


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