Attic Insulation: benefit of only adding to part?

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runner9
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Attic Insulation: benefit of only adding to part?

Post by runner9 »

Every year I think about adding insulation to our roof. I can only add and plug air gaps in part however, so I’m wondering what benefit that is.

The house is a bungalow with an addition. So, the original part is basically 1.5 stories, the second floor has kneewalls and then where there’s actual ceiling sloping upwards there’s only room for R-13 and a very small air gap, which is what’s there. All the areas are connected, if only by this little section.

Right now the flat section has about 7 inches of insulation. I could seal around wires, etc and then add more insulation, but want to know what overall benefit this will have as I can’t do the whole “envelope.”

However, what benefit will it be to really beef up the insulation in only part? For instance, if I get 60% of the attic to R-50 and the only 40% stays at R-13 is that 60% as effective, or 0% as warm air can still more easily move through the R-13 section?

Thanks!
adamthesmythe
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Re: Attic Insulation: benefit of only adding to part?

Post by adamthesmythe »

Apart from complications (convection comes to mind) heat will be lost at surfaces by conduction. You should imagine summing up the heat loss on all surfaces.* That means that if you perfectly insulate half the surface area the heat loss drops by roughly one half.

My thinking would be (1) insulate as much of the surface as you can and (2) if there are highly localized leaks (switch boxes, etc.) then be sure to address those.

Keep in mind that there is an issue of decreasing returns- if you insulate one area well enough that it accounts for a small part of the total heat loss, then ADDING additional insulation there is pointless.

* If you are technical, think of it as an electrical parallel circuit.
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runner9
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Re: Attic Insulation: benefit of only adding to part?

Post by runner9 »

Thanks for the response! Any other with the same or other advice?
hicabob
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Re: Attic Insulation: benefit of only adding to part?

Post by hicabob »

I agree w/ Adam that if you insulate x% (x<=100) it will be about x% as effective as if you insulate all of it. In my house I have 1 bathroom w/ minimal ceiling insulation and it gets significantly colder than the rest of the house which is well insulated..
ourbrooks
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Re: Attic Insulation: benefit of only adding to part?

Post by ourbrooks »

You might want to consider foam insulation for the sloping part. Per inch, the R value is double that of fiber insulation, but, as important, it seals off air movement.
It's also two or three times as expensive as fiber insulation but it might be worth it for the draft reduction if not for the actual energy cost savings.
ralph124cf
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Re: Attic Insulation: benefit of only adding to part?

Post by ralph124cf »

A couple of weeks ago, I had an insulation contractor inspect my attic insulation. Of course, he said that it was woefully inadequate, and I needed a couple of thousand in additional insulation.

I have a very precise remote reading thermometer, so that night, -10F, I checked the temperatures on the second floor. The furniture in the rooms read mostly 68, which is where I had the thermostat set, the outside walls read 67, the middle of a window read 64, (triple pane argon filled), and the ceiling read 68. I declined the salesman's recommendation.

Ralph
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just frank
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Re: Attic Insulation: benefit of only adding to part?

Post by just frank »

Generally speaking, uneven insulation is not the most cost effective....literally like wearing a warm jacket and bermuda shorts.

In practice, most older homes lose more heat to the attic by air leakage than through insulation. A pro airseal is usually cheaper than deep insulation, and will save much more energy. If you are handy, it is also DIY-able if you google up some online resources.

IOW, airseal the whole thing before you think about upping the insulation anywhere.
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runner9
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Re: Attic Insulation: benefit of only adding to part?

Post by runner9 »

By airseal you mean plugging air leaks, correct?

I've considered getting a temp sensor if it's not to expensive, if you have a recommended one.
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just frank
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Re: Attic Insulation: benefit of only adding to part?

Post by just frank »

Yes. Most houses built pre-2000 have 5-10 square feet (!) of opening between the conditioned space and the attic space.

Closing this up can save 20-30% of the energy bill in such houses, and resolves a lot of problems with attics like condensation, mold and ice dams. Does not require specialized tools, but is a bit tedious to do completely. One feature is the 'top plates' of the walls on the upper story...there is usually a gap left between the wood and drywall that is 0.1" or so times the length of your walls.

http://www.greenbuildingadvisor.com/blo ... ling-attic

If you have knee walls, often the floor joists under the heated room are wide open to the unheated knee space.

http://www.energysmartohio.com/blog/how ... all-attics

If you decide to DIY (I did) buy good tools (power caulking or foam gun) and a good valved respirator (not the paper masks). Might be less than $100 total extra, but will be well worth it.
gd
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Re: Attic Insulation: benefit of only adding to part?

Post by gd »

ralph124cf wrote: The furniture in the rooms read mostly 68, which is where I had the thermostat set, the outside walls read 67, the middle of a window read 64, (triple pane argon filled), and the ceiling read 68.
I wouldn't speculate as to the cost-effectiveness of more insulation, but wouldn't you expect a well-insulated ceiling to be warmer due to rising heat?
Ninegrams
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Re: Attic Insulation: benefit of only adding to part?

Post by Ninegrams »

I can speak to this directly to your question if only anecdotally. At the end of last fall, I brought my attic from a very rough R30 or so to R-49 with blown-in( after previously air sealing it as best I could ). There was(is) a 15" wide crawlway ( plywood resting on the rafters ) doglegging down the center of the length of the house which had R-13 under it. At the time, I figured it would be pointless to cover the crawlway with the blown-in, since it would hide the crawlway and using it would pack the blown-in over time anyway, plus it seemed like such a small percentage of the total insulated area, so I left it uncovered. The upgraded insulation did in fact lower my heating bill and raise comfort level a bit but nothing dramatic. Like you, it bothered me though that I wasn't getting the full benefit of my upgrade even though the area that was R-13 was little more than 5% of the ceiling area. So a couple of weeks ago, I came up with the idea( :idea: ) to put R-30 unfaced roll batts( which will not pack down like blow-in, and will indicate visually the line of the crawlway) onto the crawlway bringing total insulation to R-45 or so on the crawlway, and last week I went ahead and did just that . We almost instantly noticed a 2 degree rise in measured house temps( over previous to my latest insulation work) for a given heat setting. I am looking forward with some anticipation to my coming utility bills. So the short answer is I can predict that you will benefit a lot more by getting as close to %100 coverage of your R-value target as you possibly can.
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