Insulating the Attic

The Bathroom Project which led into removing old insulation and rewiring, now is almost back on track to focus on the bathroom. Over the last month L and I installed new denim insulation into the attic crawlspace next to the bathroom. What a difference it makes!

I spent quite a while researching insulation options, including a really helpful article in Fine Homebuilding (November, 2009) about how insulation works. What I got from that was that spray-foam would be the most efficient, cellulose-type second most efficient and fiberglass least efficient. I couldn’t do spray-foam because I don’t have full walls. The back of the wall and the top floor of the crawl space are unfinished, of course. Also, spray-foam would make it hard (I’m guessing) to add wiring or pipes down the road. And the more I thought about it the less I wanted to work with fiberglass. My choice was to pay a pro to install fiberglass or we could install cellulose ourselves.

I saw the denim batting at a friend’s house and it looked soft enough to sleep on. Comparing the costs and thinking about what I wanted in there, I really prefer something less harmful and less toxic. There’s a local resource (EcoHaus) for Bonded Logic’s UltraTouch Natural Cotton Batt insulation. L called them up and got a quote. The price was reasonable and they deliver. Easy to get. They even got  it here in two days.

For the walls to meet code they needed to be R19. EcoHaus was out of that so we got R21 instead. As you can see above, the insulation is 5-1/2″ deep. This isn’t going to fit in a normal 3-1/2″ studwall so I can see where this may not be catching on as fast as it should. But our walls are open so it was easy enough to rig up some supports where needed. Note that it’s important not to compress the insulation. So we built supports that stood out from the wall rather than just attaching pushing a 2×4 against the insulation and the studs. I think they also sell metal rods that you can bend in between the studs but as we had to fur out anyway, we didn’t get those.

You may not be able to see from the photo but none of the studs in this old house are equally spaced. We had to cut each piece of batting to the right length and width. EcoHaus sold us a cutting tool which is basically a hack saw with a knife blade (i.e. smooth, not serrated) and a sharpening tool. L said she had to sharpen the blade about every dozen feet of cutting. We also found that with the R14 (on the floor) we could rip it by hand which was a hell of a lot faster than cutting with the blade. I suppose an perfectionist would say that leads to poorer insulation as it’s not flush against the joists. But the joists aren’t exactly smooth and even either.

The floor of the crawl space / attic was originally framed in 2×4 joists for the old bungalow part of the house. When they extended the first addition, they put in 2×6 joists in the new construction and possibly rotated the roof. I say this because the 2×6 joists run perpendicular to the room peak which the 2×4 “joists” run parallel to it. In order to make the old ceiling stable enough for a floor, and to make it level, they laid 2×4’s down sideways across the 2×4 “joists” leaving a nice grid of wood.

This made for some interesting challenges in insulation. We opted to get R14, because it’s 3-1/2″ thick, and lay it into the 2×4 channels the long way. You can see this in the first of the floor pictures above. This makes for a pretty dense and thorough insulation and wasn’t all that hard to do really. Then we took R30 (which is 8-1/2″ thick) and lay it between the grid lines towards the roof. We cut it straight such that the far corner was about 1/2″ from the roof sheathing. I figured this gave the requisite space for air circulation. I didn’t bother cutting an angle on the and of the insulation (to match the roof angle) because that part of the roof overhangs the outside wall of the house anyway.

Here are some things we found about our new insulation:

  • While installing the wall insulation the room rose five degrees in the course of the day we installed it
  • Once complete the entire upstairs is regularly about ten degrees warmer than it was before
  • The sound from the traffic on the street outside is almost gone; in fact I hear the nightstand rattle more than the car drive by outside
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