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best way to insulate vaulted ceiling?

 
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Hello,
We are building a straw bale house with standing seam roof.  I recently posted about ventilation with a spray foam house and now I am even wondering if that is really a good option.  I am now looking into the rockwool insulation which seems pretty ideal. However, I am clueless on how to properly install either of these methods with a vaulted ceiling. I have been researching the ventilation baffles...is this a must for both scenarios or just the wool? and if so, does there need to be one in EVERY rafter bay?  Our house we are building is a clerestory type roof system so there is a knee wall (can this act as an attic? how?) connecting both roofs as well as a typical gable style garage roof.
If anyone can lead me in the right direction as I feel there is not enough solid information out there and others just do it however they think works when in all reality there is more than likely a better option to keep moisture out of the sheathing, etc.
Thank you,
Theresa
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Hi Theresa, just to check your situation...   You have a building going up (pictured) and you want to insulate the left and center roof sections with Rockwool.  You're wondering how to do that and if you need airflow baffles in every rafter bay.

Assuming I understand, the answers are:  Pretty easily and I dunno.  

The Rockwool should be slightly wider than your roof trusses/rafters, assuming they are 16 or 24 inches on center.  It will friction fit up in between the rafters and stay there.  If needed you can put strapping or other temporary supports in until you're ready to put a finished ceiling on.

I'm not sure if you need baffles.  I think they're normally needed for standard trusses where it's easy to blow insulation into an attic and block off the airflow from the soffits up into the attic.  If your rockwool is thinner than the rafter bay is deep, you'd naturally have a good air channel between the insulation and the roof.  I'm assuming you have some sort of ridge vent and soffit vent so that air can travel under the roof until it goes out the ridge vent?
 
Theresa Lantz-Huffman
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im starting to really understand how to correctly do this with wool in general but not with my roof system!  heres more pictures to shiw why i am question (only the lower roof is the question as far as getting the air to flow out anywhere)
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Mike Haasl
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Hmm, you're past my level of experience now.  Hopefully someone who knows a bit more about it happens along soon...
 
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If necessary small vents can be installed in the roofing to allow air to move from the soffits up the incline and out of the roof at the top. But in your case each section is not connected to the other so you would need many vents.
If a hot roof is your concern, look at fitting a 'safari ' roof over he finished roof to provide shade. I have lowered roof temps from 76C to 38 C which vastly reduces the internal heat build up.
 
Theresa Lantz-Huffman
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ok thank you, ill look into both options.
 
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Insulation and air movement are critical building functions, getting them wrong can have tremendous (and expensive) consequences.

Rockwool insulation is a great product - just be sure to have a cheap bread knife (the serrations are critical) to cut it.

It looks like your roof framing is 2x8 or 2x10 - creating a much deeper cavity than the r23 Rockwool batt.  thus as Mike says if you push the batt in betwern the framing and keep the batt flush (or nearly so) with the interior plane you will have a good 2+ inches of air space under the roof. BUT here's the rub - EACH pair of framing members creates a little "attic" - there is no way for all that air to mix and mingle and flow.  Further, I see some structural blocking between the roof framing, further segmenting your "attic" into a bazillion pieces.  

To create air flow the attic needs to have low-point air entry and high-point air-exits so that as the air in the attic warms it just pumps itself out.  I would have used shorter blocking (flush on interior, gap on roof side). Fortunately you don't need the full height of those blocks, so get a hole saw and drill away the top (I'd think 2 x 3" holes wold do).  The roof on the left of the picture is complicated because it abuts another living area; the roof on the right looks like you could have both air entry and air exits in the roof fascia (so long as you don't have blocking in the way).

There are fancy products for making a continuous vent along the peak of a roof. Turns out the same idea can be used here ...  the roof sheathing would need to stop 2-3" from the wall intersection, creating a window for all the air to escape.  The vent layer (a corrugated plastic sandwich thing, about 1/2" thick) would be placed at the edge of the sheathing.  You would need your flashing to come down the wall to extend down the roof far enough to cover the vent.  Yep, that'd do it.

And yes, others have thought of this.  Excellent. For instance  https://www.gaf.com/en-us/roofing-products/residential-roofing-products/ventilation-and-attic-vents/exhaust/hip-vents/cobra-hip-vent



 
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It's not very natural but we used 6 inches of foam board insulation panels on the roof deck of our straw light clay house. That took  care of the thermal bridging issues of insulating between the rafters. We put down 1x4 sleepers on top of the insulation and installed the standing seam panels on top of that. There's also a continuous soffit vent around the entire perimeter of the roof.

Here's an article I wrote about it:

Exposed Rafters and Roof Deck Insulation

And here's a timelapse of the build:

Roof Build Timelapse

I struggled with natural roof options but eventually folded. I rationalized that any off gassing from the foam would outside of the building envelope and roof should outlast me.
 
Theresa Lantz-Huffman
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Eliot Mason wrote:Insulation and air movement are critical building functions, getting them wrong can have tremendous (and expensive) consequences.

Rockwool insulation is a great product - just be sure to have a cheap bread knife (the serrations are critical) to cut it.

It looks like your roof framing is 2x8 or 2x10 - creating a much deeper cavity than the r23 Rockwool batt.  thus as Mike says if you push the batt in betwern the framing and keep the batt flush (or nearly so) with the interior plane you will have a good 2+ inches of air space under the roof. BUT here's the rub - EACH pair of framing members creates a little "attic" - there is no way for all that air to mix and mingle and flow.  Further, I see some structural blocking between the roof framing, further segmenting your "attic" into a bazillion pieces.  

To create air flow the attic needs to have low-point air entry and high-point air-exits so that as the air in the attic warms it just pumps itself out.  I would have used shorter blocking (flush on interior, gap on roof side). Fortunately you don't need the full height of those blocks, so get a hole saw and drill away the top (I'd think 2 x 3" holes wold do).  The roof on the left of the picture is complicated because it abuts another living area; the roof on the right looks like you could have both air entry and air exits in the roof fascia (so long as you don't have blocking in the way).

There are fancy products for making a continuous vent along the peak of a roof. Turns out the same idea can be used here ...  the roof sheathing would need to stop 2-3" from the wall intersection, creating a window for all the air to escape.  The vent layer (a corrugated plastic sandwich thing, about 1/2" thick) would be placed at the edge of the sheathing.  You would need your flashing to come down the wall to extend down the roof far enough to cover the vent.  Yep, that'd do it.

And yes, others have thought of this.  Excellent. For instance  https://www.gaf.com/en-us/roofing-products/residential-roofing-products/ventilation-and-attic-vents/exhaust/hip-vents/cobra-hip-vent

thank you for such thorough reply, i feel like there is most definitely hope out there for the wool! what type of insulation would you do if this was your house?

 
Eliot Mason
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Theresa:

You're welcome - glad my reply was helpful to you.

I regularly use Rockwool in my projects.  Its the only material I've used in cavities since 2014.  I also use it in interior walls as a sound insulator.

In regards to "what would I use" well, that depends.  As Aaron noted, sometimes there is a call for placing the insulation on the outside of the structure and that completely changes the discussion.  For sheet insulation I've used a lot of PolyIso - it has a higher R value per inch than just about everything else but I'm still not sure where it falls in terms of toxic-ick.  I know that the omnipresent "blue board" or XPS (eXtruded Polystyrene) has an awful environmental footprint (b/c of the gas they use to expand the foam), although there is now some sort of alternative XPS using a different expansion agent that is supposed to be significantly better.  EPS (Expanded Polystyrene - and yes, really, two products that start with the same letters...) looks like a classic bead-based white insulation, er "coffee cup".  EPS is a lot harder to cut and break, but has a much lower footprint and similar R values.

There are many other types of insulation - blown cellulose, embedded straw, actual wool, cork boards, chopped blue jeans - some of them are pretty esoteric but they can all have a place depending on a design and goals.
 
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Theresa Lantz-Huffman
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If I go the rockwool route, it looks like I can still do the R30 at 7.25" thick correct(our rafters are 2x10)? i am unsure how much soffit ventilation I need???

We have some extra work ahead of ourselves now beings we have all of the OSB up already on all roof systems and making holes in our blocking (thankfully it is just one row), but I am happy to have found an alternative to the more toxic spray foam AND it is half the price! 5k for rockwool and was estimated 10k for spray foam
 
Mike Haasl
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If you're going to cut holes in the OSB for ridge vents, do it before you put the roofing on

To cut holes in the blocking, I'd be tempted to use an oscillating saw to cut a vertical slice from the OSB down about 2".  Two slices maybe 6" apart at the center of the rafter bay.  Then pray that you can knock out the bit between the notches and/or chisel them out.  Upon further reflection, making multiple slices 2" apart with the saw would make it easier to knock them out.   Otherwise you may need a lot of big holes to equal the area you can get from a 2" by 6" rectangle
 
Theresa Lantz-Huffman
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thank you for that extra advice, im sure hubby will make something work between the two! whats your thoughts on how much soffit ventilation would be needed?

and yes we will for sure have that part finished before we put the metal roof on!
 
Eliot Mason
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I'll second two of Mike's points:

1) don't you dare roof until you've got that OSB trimmed at the top - and for that you'll need to settle on a product and follow the instructions.
2) a saw may be faster/easier than a giant hole saw.

On cutting the blocking... hubby may find it easier to just whack the blocking out with a hammer, pulls nails, cut down, replace.  Otherwise its a lot of ladder work.

Second, in terms of sizing holes/gaps ... the more the merrier. You really want that air shooting through the system with as few impediments as possible.  Your bottle neck is mostly likely your air entry system on the low side.  Make a decision on what you'll install there and then keep the same size all the way through.

In terms of gap between the insulation and roof osb ... 1.5" or better I'd say.  I've had roofing systems spec'd for me with 1/2" air spaces, so you don't need a giant space, but the greater cross section translates into less friction/resistance, and since the rockwool won't be perfectly installed (we are human... and its a soft, squishy thing....) having an error absorbing margin on top is a good idea.
 
Mike Haasl
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Our big box home improvements stores sell ventilated metal soffit in 10 or 12 foot lengths (can't remember which).  You cut it into sizes to match the width of your soffit and just staple it up.  So if your overhang is 3 feet, cut a bunch of 3 foot pieces of soffit and tack them up.  The fascia will hold them up and enclose them on the outer edge, your siding or a F channel can hold them up on the wall.  That way the entire soffit is venting.
 
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We're in the process of adding rigid insulation on the exterior of our roof deck right now with standing seam over it for a vaulted ceiling. If you can get enough R-value from just the mineral wool then that would be easier, but we're trying to work with an older house without as much depth as you have for the rafters.

If you do go with exterior then keep in mind that your roof deck will now be drying to the inside, so don't use poly under the roof deck (which it doesn't sound like you would be doing). Also, take into account how exterior rigid will change your dew point. The colder it is, the more rigid you need in order to keep the dew point out of your sheathing. We're close to Canada, so by spec rigid should supply 61% of our total R-value to ensure the dew point is kept far enough away. Also, polyiso was mentioned here, but a mix of EPS with ISO under it will perform better in extreme cold temperatures (if that applies to you) than just ISO since ISO can lose up to 25% of its Rvalue when cold enough. Some EPS above it will help it maintain a higher average temp and thus more R-value.

We'll also have a 1" channel between the rigid and the roof, so air can flow through that to give us a cold roof instead of this terrible hot roof setup (read ice dam and leak maker) that came with this 1930 house. The proof will be in the pudding, but I'm hoping for no ice dams this winter. Good luck over there!
 
Theresa Lantz-Huffman
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me again,
soo I have researched more into the rockwool and am now reconsidering due to all the added binders/chemicals. Has anyone ever used sheeps wool or know of how efficent it is?
Rockwool      ~$4000(R30 - 7.25")
Sheep Wool  ~ $5600(R20 - 5.5") or $7000(R26 - 7'')

We were originally quoted $10,000 for spray foam so these prices aren't completely scaring me away ...We have cut our OSB and our blocking so we are ready for tin this weekend!!  Thank you all for the information on venting, it has been such a great help.
If anyone has any advice in regards to sheep wool that would be wonderful as well, also if you think R20 (5.5") could be sufficient in a 2x10 rafter?!

 
Mike Haasl
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Is R20 enough for your part of the world?  Maybe I missed it but where is your house located?  R20 may be fine for Tennessee but might not be for North Dakota...
 
Theresa Lantz-Huffman
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Haha I wasn't sure if maybe R20 was a possibility but sounds like its not, thats what I needed to hear!  your right, we are in zone 3/4 north dakota
 
Mike Haasl
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Yeah, I'm down south here in northern WI and I think they want R40+ in our ceilings.  I could be wrong but I think R20 is a bit low for your area.  Paying particular attention to sealing the attic space from the interior air is very important.  Every penetration (light boxes, wires passing through top plates of studs, etc) should be sealed with caulk to keep the stack (chimney) effect from pumping warm air into the attic and cold air in through the walls down low.

Your pictures looked like somewhere cold but I wasn't actually thinking ND.  Lucky guess!
 
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