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We're building a finished shed in Austin, TX with drywall and HVAC. It will also have several huge double-paned windows. I think we'll be going with R-13 for the walls. It's generally suggested to use R-30 here for attics, but our shed has a vaulted ceiling, so no attic. Should I still use R-30 because of sun exposure? Do the windows make the rest of it kinda moot?

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  • Hmm appears to be too late now, but in hot or snowy climates a roof overhang to protect windows from sun and doorways from snow is nice. It will never snow in Texas though so there's no reason for cold weather related precautions or regulations lol. Seriously though that many windows make me think you're considering spending time in this shed, so I'd want it to be comfortable without a mint spent on A/C, so definitely at least price out white shingles and spray foam.
    – K H
    Commented Apr 13, 2021 at 22:06
  • Darn. Shed builder said there's not much difference between white and black shingles so we went with the ones that better matched the main house!
    – A_P
    Commented Apr 14, 2021 at 0:10
  • White shingles are a large aid in a hot climate, but it's not too late to use spray foam or increase insulation depth. If you don't mind a pink or blue ceiling you could use 2 inch foam sheet for example instead of wallboard in addition to the insulation between the rafters or reinforce the framing and add a layer of insulation, although you should consult a structural specialist if you're adding weight. You can put R-13 in, leave the ceiling unfinished and decide during or at the end of the hot season based on your comfort level and electricity bill.
    – K H
    Commented Apr 14, 2021 at 0:30
  • Tremendous load bearing capacity and volume can be added to your roof by giving up a portion of your vault as long as you have access on the inside. Currently it is a triange
    – K H
    Commented Apr 14, 2021 at 0:32
  • sorry I'll fix that later I'm on mobile. Currently it is a pair of triangles, one large triangle where they are attached to the walls which in the middle are not strong in the reinforcement direction. The second triangle is made with the small horizontal boards at the top and is not particularly strong because of the small triangle fighting long levers. If you add horizontal about a foot or eighteen inches down from the top you can probably bear the weight of a second layer of perpendicular framing without giving up your entire vault, adding 3.5 inches on the sides and a lot at midpoint.
    – K H
    Commented Apr 14, 2021 at 0:39

3 Answers 3

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The numbers I found for your area are actually R30-R60, so more than R30 would be ideal as air conditioning is expensive and the one-time cost of better insulation can easily pay for itself.

How deep are your rafters?

R values of common materials can be found in this chart from greenbuildermedia.com:

Insulation R values

To this chart, I'll just add that if you're wealthy there is also Aerogel to consider, with an R-value of R-20 per inch. It's still insanely expensive so the time needed for it to pay for itself might exceed the life of a normal building.

Thanks to a correction from Lee Sam, according to (See ICC R806.1.2 in the Residential Code) you need a 1" air gap between insulation and sheathing so the following numbers are adjusted accordingly:

Say you have a 6 inch deep rafter, you divide the total R-value you need by 5 inches. The result shows for 30 to 60 R-value you need a material with an R-Value per inch of 6-12, so you'd need Polyurethane spray foam or Foil-faced batts of Polyisocyanurate. You would actually need aerogel if you wanted to hit a total R-value of 60.

If you have an 8 inch rafter you need an R-Value of 4.28 to 8.57 per inch depth.

For a 10 inch rafter you need an R-Value of 3.33 to 6.66 per inch depth.

For a 12 inch rafter you need an R-Value of 2.72 to 5.45 per inch depth, so anything other than blown fiberglass or Vermiculite will do the trick. 11 inches of Aerogel batting from a reputable source will give you an R-value of 220 at a cost of $1031.25 per square foot.

If cost is an issue to you, still think about the long term cost of AC since you live in Texas, but use these numbers to check which of the materials that work are available to you. Calculate the cost per R-Value per square foot one inch depth for each material and then look at the total R value you get if you completely fill the space with each material. It might be worth spending 10-50% more per volume on a material if it lets you increase the total R value.

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    No, no, no, you cannot completely fill the joist space with insulation. You need a minimum of 1” air space above the insulation and below the roof sheathing. (See ICC R806.1.2 in the Residential Code) I’m not going to downvote your answer because I like your discussion about installing as much insulation as possible to save on future heating and cooling costs. However, most of your answer deals with completely filling up the joist space. You cannot do that and you must provide adequate roof vents too.
    – Lee Sam
    Commented Apr 13, 2021 at 5:11
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    @LeeSam I stand corrected. I'll make an edit.
    – K H
    Commented Apr 13, 2021 at 5:21
  • An easy way to solve this problem is to hold insulation down 1” from the roof sheathing AND install soffit vents and ridge vent IN EVERY JOIST SPACE.
    – Lee Sam
    Commented Apr 13, 2021 at 5:44
  • Now that’s a good answer.
    – Lee Sam
    Commented Apr 13, 2021 at 5:47
  • @LeeSam Put * around something to italicize or ** around for bold or *** for both. All caps is for DOTA lol.
    – K H
    Commented Apr 13, 2021 at 5:49
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Austin follows the 2015 International Building Code and it has local amendments that are published on the city website here. There are Austin-specific amendments to the International Energy Conservation Code (IECC).

Before you finalize your plans, it would be prudent to refer to the 2015 IECC and check on the Austin amendments. Nothing worse than following generally recommended standards and then learning that they don't meet city-specific requirements.

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  • Since it's under 200 sqft, it won't need permitting, so I'm not concerned about city code. I would like it to be a reasonably-built structure though!
    – A_P
    Commented Apr 13, 2021 at 21:26
  • @A_P You may not need a permit or an inspection, but it still helps to follow local code - both from a safety and comfort standpoint.
    – FreeMan
    Commented Apr 14, 2021 at 12:17
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Besides regulations, an easy method for a technical simulation for insulations is offered on ubakus.com

Parameters like dew points, moisture and drying time can be seen in seconds, thus disclosing any critical layer combination that could produce mold and damage to the building, f.e. the question if and where a ventilation layer is necessary

And of course the U or R values are calculated, which might be important for subsidies. F.e. some EU countries demand certain U values for public subsidies. The tool/spanner icon next to the result value offers toggling between U and R.

Best way is to open a roof or wall example and change/add layers. Each layer's width can be decreased/increased and the layer can be switched on/off by a click to instantly see any changes.

The outside and inside temperature and humidity levels (winter/summer average) should be adapted. Here is an example from another insulation question. example

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  • You've posted this exact same answer on so many questions, I'm starting to wonder if you work for the company! ;)
    – FreeMan
    Commented Apr 14, 2021 at 16:58
  • I did never and I do not work for that site. I do not have any association with that web site, I do not benefit/profit in any way from my recommendation of that site. Exception: I do benefit like any other person on this planet does if energy and effort is saved and energy consumption, oil imports, pollution, health problems etc. are reduced. The many questions (and answers) show that this site is not very known outside Europe. Unfortunately, not many other free insulation simulation sites with this accuracy can be found - if any at all
    – xeeka
    Commented Apr 15, 2021 at 13:01
  • Fair enough. Notice the ;) at the end of my first comment...
    – FreeMan
    Commented Apr 15, 2021 at 13:04

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