Wednesday, November 16, 2011

House insulation & initial results

As part of our house renovation over the last two and a half years, we've added huge amounts of insulation, getting the whole house to an average of about R-30. Here's a rundown of what we did.


  • Fiberglass - about R-3.5 per inch. Not the highest R-value, and kind of nasty to work with, but cheap. We got lots for free from craigslist, and lots for nominal cost from the Rebuilding Center, a local salvage/resale place. We used this in spots where we had ample room to stuff insulation and thus could use this cheaper product.
  • Loose-fill (cellulose and rock wool) - R3 to 3.5 per inch. Our attic had a thin (~2" on average) layer of cellulose, and we got a few more bags for free from craigslist.
  • Polyisocyanurate rigid foam board - R6.5 per inch. Expensive (even used, at about $10-$15 for a 2" x 4'x8' sheet), but highest possible R-value per inch. We used this where existing framing limited our available space for insulation: original house walls and the floor joists. Most of this had reflective foil faces on both sides.
  • XPS rigid foam board (pink and blue board) - R5 per inch. Somewhat pricey. We got some used, but bought most of it new. Resistant to water uptake, and with high compressive strength, so we used this under the concrete slab in the new sunspace. Also used it in some walls where we ran out of appropriately thick polyisocyanurate.
  • Reflective "bubble wrap", similar to the commercial "Reflectix." Petsmart receives their tropical fish in 2' x3' double-layered "envelopes" of this stuff; Tulsey arranged for us to pick up big stacks every 2 or 3 weeks, saving them from the dump. We used these as air and vapor barriers, and the bubble wrap probably adds about R-1, and they may have significant value in reducing radiant heat loss. (The last claim is somewhat controversial.) We did have to buy the shiny tape to seal adjacent strips together.



Approximate R-value: 20
In the original portion of the house, we gutted two bedrooms with no wall insulation and filled them with 3" of polyisocyanurate (or in a few places, 2" of polyiso and 1" of XPS just because we ran out of polyiso materials to get us to 3".) We had to cut the insulation to size, which was not difficult, but was somewhat tedious. Next we furred out the walls with some old lathe, to make the total cavity depth a full 4". We built the east and west sunspace walls to be 8" wide, with 2 rows of 2x4 studs with 1" offset between the rows with scraps of rigid board to reduce thermal bridging. The south wall mostly has windows, so we built it with 2x6 studs. We filled all the sunspace walls with fiberglass insulation.
With all the walls, once we had the insulation in place, we ran the bubble wrap insulation over the interior face of the studs, then into the wall cavity until it hit the fiberglass or rigid insulation, then along the face of that insulation until the next stud. The goal was to create a 3/4 to 1" air gap between the shiny bubble wrap and the sheetrock attached to the studs. This air gap is critical for the reflective nature of the bubble wrap to reduce radiant heat loss.


Approximate R-value: 40. The bubble wrap acts solely as an air & vapor barrier, not as a reflective barrier.

Original House

We redid the ceiling in about half of the original house, so in those rooms I attached a layer of bubble wrap under the ceiling joists (on the room side). In the rest of the areas I nestled the bubble wrap into the joist cavities from above (similar to the application to the walls), first removing the thin layer of existing loose-fill cellulose. I'm using our loose fill to fill the 2x4 joist cavities, then laying fiberglass bats perpindicular to the joists to a depth of 6-10". (I haven't finished all the attic insulation yet--I'm about halfway done.)

In the funny little pop-out of our NE bedroom, there was no way to access the attic space after sealing it up from below. So I stuffed pink XPS between the joists to create a supportive "platform" on top of which I could put fiberglass and loose fill before applying the usual bubble wrap to the bottoms of the joists.


We built the sunspace ceiling with 2x12 joists, in part to support the ecoroof load. We also built a 2x4 drop ceiling under that, so we were able to place about 13" total of fiberglass insulation. We finished with a layer of the bubble wrap insulation attached to the bottom of the drop ceiling joists before attaching the sheetrock.


Approximate R-value: 30.

Original house

Under the original house, we had no insulation to begin with. About 60% of this area had 2x6 joists, the rest 2x8. With the help of friends, I installed 3/4" PEX tubing under the living areas, which takes up about 1" and needs another 1" airgap between it and the reflective surface of the polyisocyanurate below. That left 3.5" and 5.5" available in the 2x6 and 2x6 joist cavities; I stuffed these with 4" and 6" of polyiso (with 1/2" hanging below the joist bottoms.) I finished off with bubble wrap stapled to the bottoms of the joists.


Going with the design of our passive solar consultants (Urban Sun), we used 2" of XPS (R-10 total) under the concrete slab, and against the interior face of the stem wall. The low R-value is a little deceptive, since it's OK to use the ground itself for some heat storage in the interior of the slab. Only the outside edges will lose much heat.


I placed 2" of polyiso against the foundation wall butting into the sunspace, to reduce the heat loss from the sunspace into the crawlspace. Around the rest of the walls, we glued the bubble wrap for a little bit of R-value and hopefully some help from the radiant barrier.

Initial Results

Today marks the first time this year we've felt the need to make a fire to heat the house! We've definitely made it way later into the cold season than ever before. Some notes on our parameters:
  • We've been comfortable with the house ranging from 55F as a worst-case overnight low up to 61 or 62 or sunny days, generally in the 57-59 range during the day. Yesterday we were stuck at 55 all day long, and overnight dropped to 51, so we finally broke down and made the fire today.
  • We've been cooking and baking a fair amount on our gas stove, which adds a lot of heat to the house.
  • We've been taking hot showers every 3 days or so, which adds a lot of heat to the house.
  • Before today, we had made 6 fires, but hadn't really needed any of them for our own heating comfort. We primarily made them for open houses and house showings as we try to sell our house.

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