# Insulating a sloped ceiling



## Guap0_ (Dec 2, 2017)

R21 in walls with 2x4 studs? That will compress the insulation way too much AFAIK. The same concern for R38 in the ceiling. Maybe something has changed over the years but I didn't hear about it.


----------



## Swizzle1! (Oct 5, 2017)

The walls are 2x 6 sorry..


----------



## Guap0_ (Dec 2, 2017)

I think that would be R19 but check with the manufacturer &/or the architect who drew the plans.


----------



## jlhaslip (Dec 31, 2009)

The roof rafters are too skinny. Not possible to get R 38 in a 2 x 10 space without using foam. Batts won't work to achieve the desired values. You will need an inch and a half of ventilation space above the insulation as well.


----------



## Swizzle1! (Oct 5, 2017)

So, if the proposed R38 isn't possible , what is the appropriate R value/ product for this ?? any idea??
Thank you


----------



## jlhaslip (Dec 31, 2009)

9 1/4 inches less 1 1/2 leaves leaves 7 3/4 inches.
Two thicknesses of R 12 max. for total of R 24.

Whatever you do, do not let them stuff the rafter bays full of insulation without the venting space above. Major condensation issues result.


----------



## Bud9051 (Nov 11, 2015)

They do make r-21 high density insulation for 2x6 walls but it must state so on the packaging.

As for the cathedral ceiling with 2x10 rafters your climate zone and local codes will dictate what is required. The link below is for the most common guidelines used the 09 codes but may have increased to 2012 or 2015 (soon 2018). Climate zone 5 would be r-38 and zone 6 and above would be r-49 but neither will fit into a 2x10 cavity and allow for air flow above.

There is an exception allowed if the vaulted area is within some limited area of the total roof area. However, taking that exception doesn't necessarily help the energy efficiency of your home, just the bottom line of the builder. Remember code requirements are considered minimum standards. Those rafter channels need chutes all the way up and not exposed insulation and no insulation can be placed against the bottom of the roof deck. And, IMO, no recessed lights in the slopes.

Bud
https://energycode.pnl.gov/EnergyCodeReqs/


----------



## Swizzle1! (Oct 5, 2017)

Ok so that sorta confirms that we NEED to have at minimum R38 (zone 5) in our roof..... BUT it sounds like that's not possible with 2x10 rafters??? was this roof built incorrectly ??? or is his approach wrong, I want to be sure this is done right so how can this be achieved ?? clearly vaulted / sloped ceilings are not uncommon , there's got to be a way to insulate this ...isn't there????


----------



## Bud9051 (Nov 11, 2015)

This link will help to bring you up to speed.
http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling
There are different ways to approach those ceilings and we really don't know what your builder has in mind.
1. Is he taking the allowance and using less than R-38?
2. Are those bundles of R-30 even intended for the roof?
3. Is he planning on a layer of rigid insulation on the inside to bring the total up to required?
4. Will he be strapping the ceiling to increase the depth?

You mention soffit to ridge venting so that path needs to be created and protected from expanding insulation, in other words it needs baffles or a built in place baffle arrangement.

You need to ask him what R-value he intends to install and how he intends to accomplish that?

The link above will help.

Bud


----------



## Swizzle1! (Oct 5, 2017)

You are spot on Bud... I am going by the drawings and they say R38... so this is my assumption, but I WILL address this with him, THIS is why I come here, it helps ME to have an understanding of what is supposed to be done vs what someone thinks they will do.

Much appreciation to you all


----------



## Bud9051 (Nov 11, 2015)

Fiberglass insulation is usually around R-3.5 per inch. A 6" wall is 5.5" times 3.5 equals about r-19. 2x10's are about 9.25" real dimension. Subtract the 1.5" and multiply by 3.5 and you get R-27 way short of the needed R-38.

If your plans call for r-38 and that is what the approval was based upon, that is the minimum you should get. I did a search for the language involved with the limited area allowance and this was reasonably close. "The IECC allows, for example, that in a cathedral ceiling, you can use R-30 and still comply with the Code provided that the area of the cathedral ceiling with less than R-38 insulation is less than 500 square feet. (404.2.2 2006 IECC)" But that is for a specific vintage energy code and we don't know what vintage code you are under or what other prescription paths are available. Sometimes a trade off is allowed but I would have expected that to have been reflected on the plans, not switched in the field.

Good luck and let us know what they are planning.

Bud


----------



## Swizzle1! (Oct 5, 2017)

Thank you 

I am really hoping this builder surprises me and has a good plan, ill be sure to let you know .


----------



## jlhaslip (Dec 31, 2009)

Code up here is R40 in a ceiling.
Typically, we would use 14 inch or taller I Joist material for the rafters. As well, we spec the rafters with holes in the top of the web material for cross bay ventilation. Also, the top flange is 1 1/2 inch thick, so we also cut thin material (osb) and nail or staple it up under the top flange to create the ventilation trough.

You might be able to cross strap the top of the 2 x 10's to act as insulation stops and then strap it again above them to create the vent space. Seen that done a few times.


----------



## Swizzle1! (Oct 5, 2017)

Ok so I have a question,
before I speak with the builder, like I said he subcontracts this work to a local insulation company, I want to watch out for our investment, so my concern is making sure this addition is comfortable and not at risk of any issues, I am open to paying above the cost of the contracted R38 to ensure this, by any chance would a spray foam application be of any benefit in the slope or cantilever ?? we have a cantilevered floor in this addition, its 2ft along a 21ft wall, the wall above is our window wall with a 4ft picture paired with flankers and a half round over , this addition has a lot of glass, this paired with sloped ceilings has me VERY conscientious about ensuring the best way to insulate, not only meeting "code" or the agreed contract, I'm certain its easier and less costly to do this now than when were sheet rocked, so please and thank you let me know your thoughts on other methods such as spray.


----------



## Bud9051 (Nov 11, 2015)

Spray foam is usually very good as it has a higher r-value per inch (closed cell) and air seals those areas. BUT, I don't like installing it when it is cold out and I think the foam mfgs have guidance on temperature. 

For the overhang and all of that glass that space will be a challenge to keep comfortable. Between the low r-value of even very good windows, the radiant heat loss, the difficulty of getting the heat down to the floor level, IMO, cold room. Throw in that a vaulted ceiling allows the warm air to be pushed up by the cold air falling off of the windows and it becomes drafty from internal convection.

Better to know now than later.
Bud


----------



## Swizzle1! (Oct 5, 2017)

Yeah, I already had that inclination, ... this is why I'm trying to do what's best now, 2 bright sides to this, 1.we'll have a large ceiling fan & 2 the addition faces south so we'll heating from the sun in the winter that will help during the day.


----------



## Bud9051 (Nov 11, 2015)

Most overhangs suffer from the natural direction of air flow, cold air pushing in the lower areas and forcing the warm air up and away from where you want it. Making sure that area is perfectly air sealed from the outside, along with super insulation, will help. If the basement area is heated that can help, but different approaches depending upon who you read.

Bud


----------



## Swizzle1! (Oct 5, 2017)

Ok , I stand corrected... apparently our rafters are 2x12 ... still too small for standard R38 but will the r38c for cathedral ceilings work ??


----------



## Bud9051 (Nov 11, 2015)

The nominal depth of a 2x12 is 11.25". Allowing 1.5" for ventilation, I prefer 2", leaves you with 9.75" of insulation. Even with a high density fiberglass insulation that doesn't make the r-38. If they reduce the vent gap to 1" and find an r-38 high density material they get very close. But you said the insulation there was r-30. not sure what they intend to do with that.

One option might be a layer of 5.5" r-23 Roxul plus a second layer of 3.5" r-15 Roxul. That total would be 9" allowing room for the baffles and good air space.

Bud


----------



## Swizzle1! (Oct 5, 2017)

http://www2.owenscorning.com/litera... (for 2x12 Cathedral Ceilings) Data Sheet.pdf

what about this???


----------



## Bud9051 (Nov 11, 2015)

The R-38C they offer seems to be intended specifically for 2x12 rafters. It would definitely be better than the R-30 you said they have on site, maybe that is intended for elsewhere. The only aspect I don't care for would be omitting the baffles or chutes and I like them larger than 1" and so do the experts. Citations available. Their 10 1/4" batt is 1" less than the 11 1/4" depth you have and requires the ventilation to flow directly above the fiberglass insulation. In reality the colder air will filter down through the insulation reducing its performance, thus some sort of chute is desired along with protection at the bottom to keep incoming air off of the end of the insulation. Here is one specifically listed for cathedral use:
http://www.bestmaterials.com/detail...MIj_ODuoCD2QIVFLjACh3h1QWiEAQYASABEgK3YvD_BwE

Should be available at other locations.

Bud


----------



## Swizzle1! (Oct 5, 2017)

Well, I spoke with said builder, and learned his plan ( as advised by Insulating company)
they are suggesting to use R30 for ceiling as the R38 wont fit in rafters with baffle. 
I inquired about the R38C which is intended for cathedral with 2 x 12 and he still thinks there will be a vent spacing issue, but is going to check with the insulation company, he did say foam will cost more ... but that doesn't tell me much of anything , 
This is leaving the brunt of this on me, I will NEED to figure this out myself it seems and request they do what I want as opposed to what they *think*, I told him I'm not interested in getting "by" any code ..I live here and want it done the BEST way possible, I am convince of a couple things here,
1) builder is inexperienced AND has issues in personal life so not as dialed in as needed.
2) Insulation company are idiots... if they suggested the R30 ..when code is R38
So I' m now re reading and scouring the internet trying to find the solution to present to them to be sure our insulation is done well.
It sure does suck having ZERO faith in someone in charge of your dream.
so that's all for now
any insulating company want to give me the recipe???


----------



## Bud9051 (Nov 11, 2015)

First, it shouldn't be just you. Being a new addition there should be a permit that was pulled and typically associated with permits are inspections. Assuming yes, then locate that inspector or department and ask them what they need to see to pass inspection.

Second, you have a set of plans calling for r-38 and I assume that was what the contractor bid on and agreed to and I assume the insulation contractor looked at the same plans. Both of them are in this business as professionals and should have been planning on r-38.

The Owens Corning insulation you linked: http://www2.owenscorning.com/litera... (for 2x12 Cathedral Ceilings) Data Sheet.pdf

Looks like exactly what you need. It is a high density product designed to provide more r-value in less space. 10¼", R-38C EcoTouch ®Insulation Batts. It should have baffles all the way to the upper vent area to be sure it does not infringe on the vent space or suffer from "wind washing" at the lower end as air comes in. All standard practices they just need to get onboard.

Print out or send them the link to that product. A quick search indicated it is available from Home Depot, confirm that for your area.

Bud


----------



## Swizzle1! (Oct 5, 2017)

Yes Bud, a permit has been pulled, I've yet to see the inspector, I'm calling tomorrow to ask what the inspection schedule is, not sure if you know from previous posts but I also have a foundation issue with this build, (it has a leak on both corners where it joins our existing house) I don't even think the inspector came out to see the foundation. Maybe the builder hasn't called him...(wouldn't surprise me at this point),, allow me to clarify something.. I refer to him as "the builder" when in actuality hes ore the gc... we .have a very skilled carpenter here who has been doing everything... (mostly alone) the the "builder" is seldom here, the thing is this "builder" contractor is the one hiring subs for this ...and I'm shocked an insulation company would suggest R30... oh the R30 here is for the cantilever, I learned the plan was to put that along with plywood sheathing ,Tyvek , & vinyl to insulate the cantilever... something else to request differently I imagine.. does it matter which vents they use ?? I heard properVent mentioned, but looked at the liknk for the one you mentioned, I printed the spec sheet to show him, is there a "better" one ???
Thank you for your help, I appreciate it.


----------



## Bud9051 (Nov 11, 2015)

There are several varieties of vents but I personally like to create my own with 1" rigid insulation held off of the roof by 1.5" or 2" strips if rigid. It depends sometimes how the depth works out with the insulation I want. Been awhile but if I were building today I would be using Roxul. For your 2x12's (11.25") I would probably use 5.5" plus 3.5" plus 1" rigid. That would be r-23 plus r-15 plus r-5. I would rip 1" rigid to give me 1.25" strips for the edges to space the air gap. Insulation and then a vapor barrier depending upon what is required/desired.

The important parts of a cantilever are insulation and air sealing, both are emphasized in this link. https://basc.pnnl.gov/resource-guides/cantilevered-floor
I like a solid covering on the bottom like plywood and no ventilation.

Before I forget, are your 2x12's regular wood or engineered "I-Joists"? I-Joists would change your approach to insulating a lot.

Bud


----------



## Swizzle1! (Oct 5, 2017)

The 2x 12's are reg wood


----------



## Swizzle1! (Oct 5, 2017)

So it sounds almost like the cantilever plan is ok ?? I briefly looked at the link and it sounds similar to their plan, they already have blocking from the basement room side, so all work will be done from outside.


----------

