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Insulation R-values and cavity depths

EPA's recommended r-value for insulation in different parts of the house and U.S.

Our homemade storage building continues to be a learning experience.  When we started out, I blithely said, "Let's put in as much insulation as possible despite the cost," and Mark agreed.  What I didn't realize is that you have to plan for your insulation needs from the get-go.

The map and chart at the top of the page show EPA's insulation recommendations for new wood-framed homes when heating with gas, heat pumps, or fuel oil.  (They recommend more insulation if you heat with electricity, and don't even give you an option for heating with wood.)  We're in their zone 4, which means we should have at least R30 in our ceiling and R13 in our walls.  The latter is easy, but the former is a bit of an issue.

Putting up wall insulationAssuming you're using fiberglass insulation (which fits our wallet and our remote setting), you need thicker wall or ceiling cavities to fit more insulation.  A typical 2X4 wall will hold up to R15 --- if you try to cram R19 in, you compress the insulation and, I believe, actually get less insulative value than you would have with a lower rated batt of insulation.

Our original rafters are 5.5 inches deep, which would only allow us to put in R19 insulation up there --- makes me chilly just thinking about it (although I think the trailer ceiling has about R13.)  So we extended our rafters with some two by fours, giving us the space to increase our ceiling insulation to R30.  For future reference, here is the cavity depth you need for some common insulation r-values:

  • 3.5 inches --- R13
  • 6 inches --- R19
  • 9 inches --- R30
  • 12 inches --- R38

Most of our building project has been very forgiving of my learn-as-we-go mentality, but insulation requires some forethought.  For those who might want to try their own hand at building --- shun the fault I fell in!

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Arguably the most important part of the whole building!
Comment by Errol Sat Feb 6 07:49:03 2010

I totally agree! That's why it was the one part I was willing to spend money on. :-)

(By the way, you should have signed in, then you could have commented without being moderated!)

Comment by anna Sat Feb 6 08:46:28 2010
The weak link in your r-value chain will be your wall studs and ceiling rafters. Ever seen as snow melts of a vaulted ceiling. It always melts at the joists first. The r-value of a 2x4 stud is only about 4.5. Check out the chart at this site: Solid panel insulation has amazing r-values. Also I believe the overlooked benefit of covering the studs and creating a solid barrier with very few areas for air penetration can be well worth the cost. Install it then have Mark get out his caulking gun and seal all the corners. Sheetrock only has a r-value or about 0.5.
Comment by Erich Sat Feb 6 14:47:55 2010
We're definitely covering all of the studs (and caulking any seams) to create a solid wall on the inside and the outside. I'm sure you're right that the solid panels would increase our R-value a lot, but when I looked at them in the store, they were seriously cost prohibitive (if I'm thinking about the right thing)! I guess I should have said that we're willing to spend real money on insulation...within reason. :-)
Comment by anna Sat Feb 6 15:50:12 2010

Don't forget to add a moisture barrier, at least in the loft. The preformance of most insulating materials (with the exception of closed-cell foam) drops a lot when it gets damp. And you can get mold problems as well.

Here in the Netherlands, a moisture barrier is usually added on the inside of the insulation, because humans in this case can be seen as leaky bags of dirty water. :-) For a wooden building it might be different, I don't know. Check your local building code.

P.S. Cans of spray foam are excellent at sealing gaps and preventing draft, and the material also adds insulation and acts as a moisture barrier.

Comment by Roland_Smith Sun Feb 7 05:12:57 2010
I've been doing a lot of reading on moisture barriers, and the concept seems to be a bit controversial in certain climates. In warm areas or cold areas, everyone agrees it makes sense (although depending on which climate you're in, you either put the barrier on the inside or the outside.) But many people say that in mixed climates like ours where it gets too hot in the summer and too cold in the winter, moisture barriers might do more harm than good. As a result, I've opted to leave them off, but I'm taking my father's advice and leaving some cracks unsealed under the roof to let wet air out.
Comment by anna Sun Feb 7 10:22:46 2010
Bad idea! The only time it will be moister outside than inside is in hot weather. Please reconsider. You could put the shiny bubble pack stuff up if you've removed the vapor barrier and add an R 7 to the ceiling, which is the most important part to insulate.
Comment by Errol Tue Feb 9 08:17:43 2010
I should have been clearer --- I'm not removing the vapor barrier that's already on the insulation. I'm just not adding the house wrap some folks recommend on the outside. Better? :-)
Comment by anna Tue Feb 9 08:45:19 2010
Yep, but the house wrap (tyvek) is only a vapor barrier preventing moisture from coming in from the outside. It lets the moisture on the inside escape. (so long as you put the lettering on the outside, that is). It does prevent air infiltration, which can be important.
Comment by Errol Tue Feb 9 09:32:49 2010
If you wander around the building forums on the internet, you'll see a lot of controversy about house wrap in mixed climates. I don't have all the details, though... I figure our caulking the cracks should prevent air infiltration, I hope!
Comment by anna Tue Feb 9 11:48:40 2010

Don't seal all the cracks, or you might get more mushrooms than you've bargained for. Inside, that is. :-)

Stuff like tyvek is excellent. It keeps cold air out, but lets moisture pass from the inside out as well. I'm all for natural materials when they work, but sometimes modern synthetics are just way better.

For example, when I'm commuting to work on my bicycle, I tend to wear a polyester t-shirt and a softshell jacket. Which probably sounds really cold. But the polyester wicks the sweat away from the skin and the softshell material keeps the wind out but lets the moisture escape. So I'm dry and therefore warm. Only when it gets below freezing I have to add another layer like a fleece sweater. When I was still wearing a cotton t-shirt I always arrived at work practically soaked, and in the winter cold as well.

Comment by Roland_Smith Tue Feb 9 14:22:45 2010

I'm not knee-jerking against synthetics so much as against anything that costs extra. We've put plenty of synthetic parts into the building (like the fiberglass insulation.) But given that there is a controversy about both house wrap (as being mostly a quick fix to deal with water seeping through cracks in metal siding, which we don't have) and vapor barriers (as being possibly more harm than good in mixed climates like ours), I think we'll be fine with just putting in what's on the insulation.

I do appreciate y'all's concern, even if I'm ignoring it. :-)

Comment by anna Tue Feb 9 15:19:28 2010

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