Insulating A Semi-vaulted Ceiling - Insulation - DIY Chatroom Home Improvement Forum
Advertisement


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Insulation

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Like Tree3Likes
Reply
 
Thread Tools Search this Thread
Display Modes
Old 05-23-2018, 03:59 PM   #1
Member
 
Join Date: Apr 2009
Location: KS
Posts: 995
Rewards Points: 986
Default

Insulating a semi-vaulted ceiling


1915 1-1/2 story bungalow. I'm redoing a bedroom on the 2nd floor that had no insulation in the walls and very light insulation in ceiling. Walls are a no brainer IMO. The ceiling is what I'm struggling with a bit. I believe they call for about R30 in ceilings in my area....south central KS.

The issue is the ceiling joists are 2x4 lumber, so not much room for insulation and parts of the ceiling, as mentioned in the title, are vaulted. The original decking on the roof was 1x4 lumber with 1-2" gaps. Back 12 years ago a hail storm came through that took many roofs out and the city said everything needed to go to OSB/Ply decking, so the companies just left the 1x material and overlayed it with OSB.

I know a lot of people say leave an air space between insulation and the decking, but I've also read several articles, by what seems to be creditable sources that says IF you can create an air tight insulation that is better yet. I believe they basically talked cellulose or spray. Spray is very costly and not sure in my budget. I've even priced the DIY kits and they seem high.

I have been doing some research on different R-Values of insulation and it seems that rigid foam provides about as good, if not better than most anything. I have about a dozen sheets of 2x4x8 DOW insulation I came across for an excellent price of $1 per sheet several years ago. It has an R Value of 10. This leads me to the debate I'm having with myself....

If I fir out the studs to a solid 4" I could double that up to get R-20. Not yet R-30, but close. If i fir it out another 2", so a 2-1/2" firring strip I'd be at R30. My issue is that would leave the air gap between the 1x material next to the OSB decking. So now I'm considering maybe a layer of 2" Rigid over the joist and just spraying cellulose and trying my best to dense pack it. We aren't talking a huge difference if it's not to dense pack standards. BUT I see that Polyisocyanurate Board foil faced is R value of 7.0 per inch. Can that be stacked and retain the R rating or is that stand alone only?

Am I thinking in the right direction?
Master Brian is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 05-23-2018, 04:09 PM   #2
Guapo
 
Join Date: Dec 2017
Posts: 5,940
Rewards Points: 11,950
Default

Re: Insulating a semi-vaulted ceiling


Is there a ridge vent? Are there gable vents or soffits?
Guap0_ is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 05-23-2018, 04:14 PM   #3
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 37,899
Rewards Points: 408
Default

Re: Insulating a semi-vaulted ceiling


The air gap should go from vents in the soffit to the vents near or on the peak. Adding to the rafters is a good idea for what ever insulation and covering last with foam board will work, if you tape the joints you will keep house air out of the system too.
The make 3" drywall screws to hang drywall over foam board.
Nealtw is online now   Reply With Quote
Sponsored Links
Advertisement
 
Old 05-23-2018, 06:00 PM   #4
Member
 
Join Date: Nov 2015
Location: Maine
Posts: 10,667
Rewards Points: 136
Default

Re: Insulating a semi-vaulted ceiling


Check with your local inspector where you pulled your permits and ask if there is any work-around for those 2x4 frame members.

Failing to follow codes is adding a skeleton to your closet that may show up when inspected or when you sell. Then it gets really expensive.

Bud
Bud9051 is online now   Reply With Quote
Old 05-24-2018, 03:53 PM   #5
Member
 
Join Date: Apr 2009
Location: KS
Posts: 995
Rewards Points: 986
Default

Re: Insulating a semi-vaulted ceiling


Quote:
Originally Posted by Guap0_ View Post
Is there a ridge vent? Are there gable vents or soffits?
There are no ridge vents nor any soffit vents. I do have two turbines on the roof and a couple of standard roof vents.
Master Brian is offline   Reply With Quote
Old 05-24-2018, 03:58 PM   #6
Member
 
Join Date: Apr 2009
Location: KS
Posts: 995
Rewards Points: 986
Default

Re: Insulating a semi-vaulted ceiling


Quote:
Originally Posted by Nealtw View Post
The air gap should go from vents in the soffit to the vents near or on the peak. Adding to the rafters is a good idea for what ever insulation and covering last with foam board will work, if you tape the joints you will keep house air out of the system too.
The make 3" drywall screws to hang drywall over foam board.
The air gap is the thing. This is something I've been looking into for a few years as it's been a plan for a while. I've seen articles for both the air gap and not. The latest article said no air gap is better, but you have to insure no air in insulation, hence no fiberglass batting. I'll see if I can find and link it.
Master Brian is offline   Reply With Quote
Old 05-24-2018, 04:01 PM   #7
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 37,899
Rewards Points: 408
Default

Re: Insulating a semi-vaulted ceiling


Quote:
Originally Posted by Master Brian View Post
The air gap is the thing. This is something I've been looking into for a few years as it's been a plan for a while. I've seen articles for both the air gap and not. The latest article said no air gap is better, but you have to insure no air in insulation, hence no fiberglass batting. I'll see if I can find and link it.
I have read some on it, it never applies where I am because we worry about ice dams so best you get advise from people who have more to do with that system.
Nealtw is online now   Reply With Quote
Old 05-24-2018, 04:27 PM   #8
Member
 
Join Date: Nov 2015
Location: Maine
Posts: 10,667
Rewards Points: 136
Default

Re: Insulating a semi-vaulted ceiling


@MB "but you have to insure no air in insulation, hence no fiberglass batting."
I don't believe that was the intention of that statement. Yes fiberglass insulation is very air permeable but so are almost all others. Closed cell spray foam or closed cell rigid foam are both good air barriers and vapor retarders. Mineral wool, cellulose, and others like these permit far too much air infiltration. Those buildings that choose to eliminate the air gap go to extremes to eliminate air leakage from the conditioned space below. Many example of a small mistake taking out an entire roof assembly.

http://www.greenbuildingadvisor.com/...hedral-ceiling

A properly ventilated roof assembly has years of success to justify advising that as best approach.

Bud
Nealtw likes this.
Bud9051 is online now   Reply With Quote
Old 05-24-2018, 05:14 PM   #9
Guapo
 
Join Date: Dec 2017
Posts: 5,940
Rewards Points: 11,950
Default

Re: Insulating a semi-vaulted ceiling


Quote:
There are no ridge vents nor any soffit vents.
In that case, it may not be a good idea to insulate.
Guap0_ is offline   Reply With Quote
Old 05-29-2018, 02:50 PM   #10
Member
 
Join Date: Apr 2009
Location: KS
Posts: 995
Rewards Points: 986
Default

Re: Insulating a semi-vaulted ceiling


Quote:
Originally Posted by Guap0_ View Post
In that case, it may not be a good idea to insulate.
Like I said, one article, wish I could find the one i read a few weeks ago, saying better to NOT have the air gap. I'll look in a few and find it. The one that I believe was quoted above says,

"Can I build an unvented roof assembly?
It is quite possible to design an unvented insulated roof assembly that performs well, as long as you get the details right. In recent years, most building codes have begun to allow the construction of unvented insulated sloped roof assemblies. Many such roofs have failed over the years, however, so don't get creative. Follow the rules."


I realize they say DON'T get creative, which maybe I'm trying to do, but I'm just trying to learn and ask questions so don't take it as me thinking I know it all on this because I realize I don't, which is why I'm here asking.

All of that said, the portions of my roofline that are in contact with the roofline currently have fiberglass batting shoved into the bays and this was done about 40yrs ago in the 80's from what I gather. I see no signs of any damage to the roof, though it was overlaid with 7/16 OSB back in 2006 after a hail storm that was right after code in the area changed and all the roofs that were previously without sheathing had to be updated. Prior to this, my house was likely just 1x4 with a layer of cedar shingle. The 1x4's are still there, they just pulled the 1st layer off and went over the 1x's, which is what was common. With no signs of any failure on this, some might say then leave well enough alone, but there is way too much heat transfer through the roof. In the summer that plaster ceiling gets very warm to the touch!
Master Brian is offline   Reply With Quote
Old 05-29-2018, 03:03 PM   #11
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 37,899
Rewards Points: 408
Default

Re: Insulating a semi-vaulted ceiling


I prefer the tried and true of venting and venting can be added in a bunch of ways but here is some talk on the subject.
http://www.greenbuildingadvisor.com/...hedral-ceiling
Nealtw is online now   Reply With Quote
Old 05-29-2018, 03:04 PM   #12
Member
 
Join Date: Apr 2009
Location: KS
Posts: 995
Rewards Points: 986
Default

Re: Insulating a semi-vaulted ceiling


Tomorrow morning I'm meeting with a local insulation company that does what they call a comfort blanket, which they say is basically a net they place on the wall and spray fiberglass into. They say they have done many of these in the area for the cathedral ceilings and had great luck. He said he can do a small spray foam job, but thinks their system performs better than foam and cellulose. He said on a 2x6 ceiling joist they can get an R23 value. If that is true then I'd think going with a 7" joist would net me R30. I'll discuss this with them and if I have to fir onto my existing joists or even build a structure inside the structure to achieve the desired R value, I'm game.

The one thing that seems to be different here is that our heat and cool seasons are about equal in length, so things seem to be a bit different than places like the Northern states where they have more cool weather than warm. On hold with the permit desk to see if I can get an inspector and get his take as well....
Nealtw likes this.
Master Brian is offline   Reply With Quote
Old 05-29-2018, 03:07 PM   #13
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 37,899
Rewards Points: 408
Default

Re: Insulating a semi-vaulted ceiling


Be care full of a guy that says he has been lucky, It is you that is gambling on him.
Master Brian likes this.
Nealtw is online now   Reply With Quote
Old 05-29-2018, 04:11 PM   #14
Guapo
 
Join Date: Dec 2017
Posts: 5,940
Rewards Points: 11,950
Default

Re: Insulating a semi-vaulted ceiling


An insulation guy is going to sell insulation. Keep that in mind. The way that I understand attic insulation is that venting is needed otherwise moisture can be trapped. If the ceiling is hot, it sounds like an exhaust fan might do a better job than insulation. That way, there is no chance of moisture being trapped.
Guap0_ is offline   Reply With Quote
Old 05-29-2018, 04:22 PM   #15
Member
 
Join Date: Apr 2009
Location: KS
Posts: 995
Rewards Points: 986
Default

Re: Insulating a semi-vaulted ceiling


I just called and spoke with an inspector and his only advice was to be certain they follow the 2012 IRC section R806.5. He said our area didn't adopt the Energy Code. He wouldn't say if he has seen better results one way or another, just to go with the insulation company.

What I'm reading in that is Unvented attic and unvented encolosed rafter assemblies are doable. It says...

1. The space needs to be completely contained within the building thermal envelope. I planned on that.

2. No interior class 1 vapor retarders on ceiling side or attic floor side. might answer questions about certain rigid foam?

3) deals with wood shingles, which I don't have.

4) deals with climate zones 5, 6, 7, 8. We are in zone 3/4

5) talks about air permeability of the insulation directly under the roof sheathing.
5.1 Air-Impermeable insulation only. Insulation shall be applied in direct contact with the under side of the structural roof sheathing. (I'm taking this as spray foam and/or rigid foam)
5.2 Air-permeable insulation only. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.5 for condensation control. (I'm not re-roofing my house, unless a hail storm pops up and requires it, so I'll ignore this, BUT this sounds like the spray in fiberglass won't work unless maybe with the next step!)
5.3 Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation. (sounds like the spray foam and/or rigid board needs to go up first per the table.)
Table R806.5 says Zones 1, 2A, 2B, 3A, 3B, 3C can use R-5 minimum rigid board on air-impermeable insulation (not sure if zone 3 or 4 because different maps show it differently, so I'll go with Zone 4 as that's what I've always believed.) Zone 4C is R-10 rigid board. Zone 4A, 4B says R-15 rigid board.
5.4 Where performed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer. (so if I go with the foam board I have, I'm assuming spraying the edges with spray foam small gaps should suffice. What about a caulk of some sort? The spray foam cans to seal might be as cheap and easy.)

So, if reading that correctly, it sounds like I was basically in line with my thoughts. I might have to cut a LOT of little strips to fit between the 1x material, but cost won't be too bad and honestly only a few hours of work. The 2" dow Foam i have is R-10, then I'd just need a 2nd layer of 1" to finish it off, then maybe batts or spray in cellulose or the fiberglass to get the additional R-Value.

Any comments on this?
Master Brian is offline   Reply With Quote
Sponsored Links
Advertisement
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Miami Vaulted Ceiling Insulation (need to seal?) ShenAmateur Insulation 4 08-06-2014 01:21 PM
framing a vaulted ceiling dwoolver Building & Construction 5 05-26-2012 04:40 PM
Crown moulding with a vaulted ceiling? cstrat Carpentry 34 06-17-2011 06:51 PM
HELP! lighting question for vaulted ceiling pauls General DIY Discussions 5 07-03-2010 04:53 PM
Insulating Vaulted Ceiling cibula11 Building & Construction 11 01-31-2010 08:48 AM




Top of Page | View New Posts