Vapor Barrier with Metal Roof on Old House???

Reply

  #1  
Old 06-16-15, 10:36 AM
Member
Thread Starter
Join Date: Mar 2004
Location: Southern NJ
Posts: 137
Received 0 Votes on 0 Posts
Question Vapor Barrier with Metal Roof on Old House???

Hi Everyone,

I am rehabbing a very small home in Southern NJ that was built in 1885. I guess for lack of a better description, the house is a "Cape" style in which the upstairs (2 bedrooms) follow most of the roofline/attic area. The plaster does level off at about 7' in the middle of the house so there is about 3' of additional roof pitch that is above the plaster/unfinished. I'm hoping that makes sense so far

I am going to be installing a metal roof on the house. Currently there is no insulation or vapor barriers. There currently is an asphalt roof on top of a cedar shake roof installed on top of purlins that are in good shape. The inspector said (I do have proper permits pulled) since the metal is so light I can put the metal on top of the old roof if I install new purlins on top of the asphalt shingles. Or I can also tear off the old roof. I'm leaning towards tearing off the old roof because then I can just put the metal on top of the existing purlins AND I can insulate the space. I don't really have access to insulate most of the roof unless I tear off the old roof.

My major concern is condensation between the metal roof and the conditioned space. I really only have room for 4" of insulation since most of the plaster is against the 2x4 (actual size) rafters. Towards the peak I could add a bit more insulation but that's only 1/3 of the interior surface area.

Should I leave the old roof in place just to add the extra layer of "vapor barrier?" Should I tear the old roof off and put a plastic vapor barrier down (against the back of the plaster) and insulation on top? Or should I just use paper-faced insulation for 2x4 walls? I definitely cannot make the vapor barrier seamless since the plaster and purlins are already there. I would essentially be doing the best possible with trying to tuck it into existing cavities.

Any advice would be appreciated!
 
Sponsored Links
  #2  
Old 06-16-15, 10:52 AM
Temporarily Suspended
Join Date: Jul 2008
Location: NY
Posts: 10,986
Received 0 Votes on 0 Posts
My major concern is condensation between the metal roof and the conditioned space.
Does the house have soffits?
 
  #3  
Old 06-16-15, 11:52 AM
Member
Thread Starter
Join Date: Mar 2004
Location: Southern NJ
Posts: 137
Received 0 Votes on 0 Posts
Yes, but small and unvented currently. They probably extend maybe 4-5" and were retrofitted at some point with aluminum. I guess theoretically I could add those small circle vents in there if that's what you're getting at. But I'm not sure what type of lumber is buried at the ends. I assume just some kind of 1x trim board.
 
  #4  
Old 06-16-15, 12:57 PM
Member
Join Date: Oct 2008
Location: New England
Posts: 10,483
Received 33 Votes on 30 Posts
Definitely more planning is going to be needed before you get too far.

You have permits, so how much insulation is the inspector requiring. 4" will not meet the codes as I see them.

Air permeable insulation cannot be installed against the bottom of a roof, there needs to be an air gap for ventilation. A gap of 1.5" to 2" is usually recommended, but an absolute minimum of 1" may do.

I don't have a complete picture of what you have, but in all cases dealing with vapor barrier issues, air sealing is very important. Moisture vapor can move through materials, but it does so slowly. An air leak however will deliver moisture laden air directly to a cooler condensing surface.

What is the asphalt roof attached to, those shakes? Is there a layer of plywood between the shakes and the asphalt?

Venting at the eaves will require more than just a few of those pop-in vents. If you dig out their effective vent area, called net free area (NFA), they are at about 20%. A 3" diameter hole with a pop-in gives you about 1.5 in of vent area. You will need 1 ft of vent area for every 150 ft of attic floor or slope. Then the same NFA is required for high venting, ridge of gable.

Installing a metal roof over exposed fiberglass insulation is an invitation for bugs and bees to take up living space. If you cover your roof with plywood and then add the metal, they may still under the metal, but that will be outside of the insulation.

More as needed.

Bud
 
  #5  
Old 06-16-15, 02:50 PM
Member
Thread Starter
Join Date: Mar 2004
Location: Southern NJ
Posts: 137
Received 0 Votes on 0 Posts
Thanks for the very thorough information Bud! Some answers:

You have permits, so how much insulation is the inspector requiring. 4" will not meet the codes as I see them.
I never did ask that question. To my knowledge, none. I gave them a whole plan/drawing and didn't say any insulation. My local inspectors are very reasonable when it comes to renovating older homes and grandfathering them to a reasonable extent. E.g., I can't really make the rafters larger without significant effort and I didn't remove the plaster from the walls. So I have limited abilities in this situation anyway without extensive work like sistering the boards.

Air permeable insulation cannot be installed against the bottom of a roof, there needs to be an air gap for ventilation. A gap of 1.5" to 2" is usually recommended, but an absolute minimum of 1" may do.
The rafters are 4" deep. Then the purlins are another 7/8" deep, of course running the opposite direction. So standard 2x4 wall cavity insullation at 3.5" deep would still leave a bit over an inch from touching the roof. Certainly not up to modern codes but better than it has been for the last century!

I don't have a complete picture of what you have, but in all cases dealing with vapor barrier issues, air sealing is very important. Moisture vapor can move through materials, but it does so slowly. An air leak however will deliver moisture laden air directly to a cooler condensing surface.
Exactly why I am asking the question! I'd rather have no insulation and no leaks due to condensation than to try to insulate and have issues. Though I realize theoretically the insulation could help but the way I see it is I don't have any moisture issues now so if I leave it the same, it shouldn't change (though I think metal roofs are a bit more susceptible).

What is the asphalt roof attached to, those shakes? Is there a layer of plywood between the shakes and the asphalt?
Yes and no respectively

Venting at the eaves will require more than just a few of those pop-in vents. If you dig out their effective vent area, called net free area (NFA), they are at about 20%. A 3" diameter hole with a pop-in gives you about 1.5 in of vent area. You will need 1 ft of vent area for every 150 ft of attic floor or slope. Then the same NFA is required for high venting, ridge of gable.
It happens to be a really small house. The second floor area is 210 sq ft. It has 3 ft kneewalls on the outside walls, then follows the roofline on the angle (about 11/12 slope), then the plaster and lath is attached to the underside of the collar ties. So technically on top of the collar ties there's about 3' maximum "attic/unfinished" area. Hope that makes a better picture

Installing a metal roof over exposed fiberglass insulation is an invitation for bugs and bees to take up living space. If you cover your roof with plywood and then add the metal, they may still under the metal, but that will be outside of the insulation.
That's a great point. Maybe that's a good reason to put the metal on top of the asphalt roof. It would limit my ability to insulate, but at 3.5" of insulation, maybe the old cedar shakes and asphalt roof would do just as good of a job as fiberglass anyway! The problem with plywood is I won't be able to walk on it with the angle. So I would need to add plywood and then purlins on top just to walk. Or do a chicken ladder which is a PIA too. I was trying to plan it so I could walk on the purlins as I install the roof from the side.
 
  #6  
Old 06-16-15, 04:43 PM
Member
Join Date: Jan 2011
Location: USA
Posts: 4,561
Received 42 Votes on 37 Posts
No way would I do it like that.
I'd strip all that old roofing off first.
That attic would need to be air sealed and insulated to R-50, proper soffit vents.
Only would have standing seam metal roof with a ridge vent.
 
  #7  
Old 06-16-15, 04:52 PM
Member
Join Date: Oct 2008
Location: New England
Posts: 10,483
Received 33 Votes on 30 Posts
Thank you Joe, I should have said it, but was avoiding the conflict.

Bud
 
  #8  
Old 06-17-15, 09:29 AM
Member
Thread Starter
Join Date: Mar 2004
Location: Southern NJ
Posts: 137
Received 0 Votes on 0 Posts
That's all great but how do you air seal an attic that's already finished without demolishing the finish work? Soffit vents and ridge vents are doable but the R-50 and air sealing seams next to impossible without many $thousands. When retrofitting sometimes you do have to compromise between bad and good when "best" isn't an option. 100+ year old houses don't practically allow for some of the modern best practices. Is there a middle ground or is this all or nothing in your books?

There's no conflict. I come here for great advice. Ultimately if we disagree it's the thread-starter's (in this case me) ultimate decision which direction to take - whether at our own peril or not
 
  #9  
Old 06-17-15, 09:54 AM
Member
Join Date: Oct 2008
Location: New England
Posts: 10,483
Received 33 Votes on 30 Posts
There is of course some middle ground and air sealing isn't rocket science. That's as long as your code people are happy. It may not seem fair, but they have to answer to all future owners who may ask why it was allowed to be completed if it didn't meet the codes. But anyway, I'll add a link on air sealing which may help.

The condensation issue you are concerned about generally occurs when warm inside air that is loaded with moisture finds a way through the walls, ceiling, and/or insulation, and reaches a cold metal roof. Simple physics, you get water or ice. Plumbing and electrical penetrations are some of the biggest sources, also chimneys, exhaust fans, vent stacks are others. when you get down to the fine details, that's when you start gluing the drywall to the studs. not going to happen here.

Bud
http://www.efficiencyvermont.com/ste...ide_062507.pdf
 
  #10  
Old 06-18-15, 11:39 AM
Member
Thread Starter
Join Date: Mar 2004
Location: Southern NJ
Posts: 137
Received 0 Votes on 0 Posts
Thanks Bud. That article is really helpful. I understood some of it already but it definitely teaches a few things. I think after all of your information, knowing the limitations, and trying to do the right thing within reason I will:

- Tear off the old roof.
- Install as much insulation as possible - above the collar ties I can get to R50. But below them where the interior finish follows the rafter I will have to keep to 3.5" insulation.
- Install metal roof with a ridge vent
- Add soffit vents
- Make sure interior is sealed well - shouldn't be too hard with the plaster already in place according to the article.
- Cross fingers and hope that going from 0 insulation to some insulation and 0 vents to some vents will keep my moisture problems at 0 like they are now!

One last question - do you think I'm better off adding a plastic vapor barrier against the back of the plaster and around rafters and then putting unfaced insulation on top of that? Or is it better to just used faced insulation? I'm thinking at least the plastic would seal a bit better than I can get with retrofitting faced insulation.
 
  #11  
Old 06-18-15, 12:02 PM
Member
Join Date: Oct 2008
Location: New England
Posts: 10,483
Received 33 Votes on 30 Posts
The construction industry is swinging away from vapor barriers, recommending them only in the far north and deep south. Instead, they emphasize air sealing. The other factor is that your painted plaster is considered a class III vapor retarder so you should be all set.

For your 3.5" insulation, they make a high density fiberglass in 3.5" at r-15 and Roxul is also r-15 for the same space. I think the Roxul is better, denser, and easy to work with and it comes unfaced which is fine.

Bud
 
  #12  
Old 06-18-15, 12:23 PM
Member
Thread Starter
Join Date: Mar 2004
Location: Southern NJ
Posts: 137
Received 0 Votes on 0 Posts
Thanks Bud. That sounds perfect! I will look for Roxul or at least an R15 competitor. That's good to know it's out there.
The other factor is that your painted plaster is considered a class III vapor retarder so you should be all set.
And add the many layers of lead paint underneath and I should be set against moisture and x-rays

Thanks a lot for your recommendations!
 
  #13  
Old 06-18-15, 07:44 PM
Member
Join Date: Apr 2014
Location: Nova Scotia
Posts: 750
Received 0 Votes on 0 Posts
This sounds like a perfect job for some spray insulation. Tear the roof off, have the guys come and fill your cavities, giving you an air/vapour barrier all in one with the maximum amount r value you can get for the space.

Failing doing that, I would use 3" of rigid foam cut to fit between the rafters tight to the plaster, and seal all the seams with caulk.
 
  #14  
Old 06-19-15, 02:23 PM
Member
Thread Starter
Join Date: Mar 2004
Location: Southern NJ
Posts: 137
Received 0 Votes on 0 Posts
Spray foam would be great except logistically it's almost impossible. I'd have to tear the roof off entirely, have someone be there at a perfect time to spray foam, and then get the entire roof back on. I was planning to do the roof myself and spanning it over at least 2 weekends. Doing a bit of tear-off, putting new panels up, more tear-off, etc. If only I had my own spray foam rig!
 
  #15  
Old 06-19-15, 02:46 PM
Member
Join Date: Apr 2014
Location: Nova Scotia
Posts: 750
Received 0 Votes on 0 Posts
I guess I misunderstood, as it sounded like you were planning to rip the old roofing and sub roofing off anyway so you could insulate the cavities.
 
  #16  
Old 06-22-15, 11:42 AM
Member
Thread Starter
Join Date: Mar 2004
Location: Southern NJ
Posts: 137
Received 0 Votes on 0 Posts
You understood correctly because I think I am going to end up ripping off the roof. But do to how much I can get done in a day it probably has to be in sections so I have time to tear off, insulate, and put new roofing up in the same day so as to not leave it exposed for very long. So it will be at best 1 side in one day and the other the next day. But it could take even longer, e.g 1/2 one side in one day, etc. So just the logistics of coordinating a spray foamer at the exact time that I'm ready and on more than one day would be impossible, at least with considering budget as well.
 
  #17  
Old 06-22-15, 03:03 PM
Member
Join Date: Apr 2014
Location: Nova Scotia
Posts: 750
Received 0 Votes on 0 Posts
In that case I would still highly recommend using rigid foam. It will give you more r value and a better air/vapor barrier.
 
  #18  
Old 06-23-15, 10:06 AM
Member
Thread Starter
Join Date: Mar 2004
Location: Southern NJ
Posts: 137
Received 0 Votes on 0 Posts
Just stuff the foam between the rafters so it sits tight? Is there any wood-to-foam tape I can use where they meetup? Even if I do a very good job cutting, there will obviously be voids between the foam and the rafters. I saw something where someone used can spray foam for that but I feel like I'd have to buy 100 cans!
 
  #19  
Old 06-23-15, 01:16 PM
Member
Join Date: Apr 2014
Location: Nova Scotia
Posts: 750
Received 0 Votes on 0 Posts
You could use spray foam from a can, and it would be preferred, but as long as your cuts are fairly tight, a simple latex caulking will do the job at the joints. It would be best to run a bead around the edges of the foam before you set it in place.

A can of spray foam will go a long ways. You would not need much for each peice and could probably do the entire roof with less than 10 cans. The good thing about the spray foam is it will be 100% airtight, and it is also an excillent adhesive.
 
Reply

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


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: