Why should R be greater in the attic than the wall?
Hot air rises, so ceilings would pick up more heat from interior air than walls. Also, as pointed out in the GBA article “Insulating Roofs, Walls, and Floors,” space for insulation (assuming a typical residential sloped roof) is more readily available above the ceiling than in the wall.
BUT if heat loss across a surface is proportional to the temperature gradient, the area, and the elapsed time, and inversely proportional to R, and we take the temperature gradient to be the indoor temperature minus the outdoor temperature, this formula for heat loss suggests that the optimal insulating strategy is to put the same R value in attic and wall.
Furthermore, if sunlight and imperfect attic ventilation on cold days combine to make the attic warmer than outdoors, then the temperature gradient your attic insulates against is lower than the gradient your wall insulates against, and it would seem more efficient to put extra insulation in the walls.
This is such a basic issue that I feel like it must have been answered in GBA somewhere, but I failed to find it.
Is the real reason to put extra insulation in the attic this?: it is important to prevent the attic from getting warmer than outdoors, for reasons of indoor cooling in the hot season and ice dams in winter, i. e. the reason isn’t about minimizing winter heat loss, and that applies even in my (Minnesota) climate.
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Replies
Chuck,
It's good for your house to have high R-value insulation on all six sides. The short answer to your question is that it's cheaper to put more insulation above a ceiling than it is to put it on a wall or under your floor, so that's where the code requires us to do a better job.
The other reasons aren't quite as compelling, but remain true:
1. Because of the stack effect, ceilings in cold climates are warmer than floors, and that's why high R-values in ceilings are a good idea in cold climates.
2. In hot climates where homes are equipped with air conditioning, attics can get hotter than the outdoor air temperature, and therefore high R-values in ceilings reduce air conditioning bills.
Basically, though, it's easy to put 14 inches of cellulose on an attic floor, but not so easy to put 14 inches of cellulose in your walls. The issues affecting floors are more complicated than the issues affecting walls and ceilings: basements and crawl spaces aren't as cold as the outdoor air in winter, which is why (historically) they've gotten less attention in the R-value competition.
Thanks Martin. Looks like for the most part I answered my own question. My architect is proposing R40 walls and R80 attic, which is what set me thinking about this. In the end, I'm happy to go with what he proposes.
Make sure that new house is airtight and you have adequate insulation around the foundation and under the slab. I was at a townhouse I finished in 2010 yesterday. It was built by others but I did the insulating and air sealing. The townhouse is 2 levels with the lower level about 1/2 below grade. The forced air 95% gas furnace is zoned for each level and the stair is open to both levels. I have two digital thermometers I bought at the local hardware store last year and I like to check temperature between levels of homes I work on. I put one thermometer on the dining table on the main floor and one on the basement floor in the lower level. After a time I checked and both read 69F. This townhouse has R-10 sub slab insulation and the ACH50 is under 2. The walls are R-21 and the ceiling is R-60 blown cellulose. A Venmar HRV provides fresh air.
Properly insulated and air sealed new construction will have even indoor temperatures from top to bottom.
Roof temperatures get several tens of degrees hotter than the ambient air temperature when in full sun, and dramatically cooler during clear nights, often reaching the outdoor dew point temperature, which can be more than ten degrees cooler than the air. Walls pretty much track the ambient air temperatures at night, but do experience some solar gain during the day, though not nearly as much as roofs.
But the primary reason is that the cost/complexity of insulating attics is far simpler and cheaper than insulating walls, since there are far fewer penetrations and obstacles like windows & doors to work around. From a bang/buck point of view having more R-in the attic can have a higher return on investment due to the lower cost per R.
Hi Chuck -
I think there is one more reason: before spray foams, we tended to have air permeable attic insulations. Since wall insulation is confined on all six sides, wall insulation installations are inherently more airtight while attic insulations are rarely confined on that top 6th side. Without that 6-sided confinement, we (and the building code) suggest/require greater total depth of insulation because convective losses in the top layer of insulation erode the overall R-value of air-permeable insulations.
Peter