Hot Roof

Reply

  #1  
Old 12-05-09, 09:27 AM
Member
Thread Starter
Join Date: Aug 2009
Location: USA
Posts: 4,940
Hot Roof

I really want to do a hot roof when I re-insulate my finished attic. Would using 1 inch of spray foam against the roof to seal and then backing that up with a batt insulation and VB be an effective system?
 
Sponsored Links
  #2  
Old 12-05-09, 10:39 AM
Member
Join Date: Oct 2008
Location: New England
Posts: 9,776
The concept of a hot roof is totally dependent upon sealing out the inside warm moist air. Not always as easy as it sounds. For example, you will only be foaming in between the rafters and that will leave the rafters themselves exposed, plus any leaks that are missed.

Next, you risk having a double vapor barrier. Any moisture that leaks in, needs a way to get out.

Assuming you have 6" rafters, r-15 for the remaining fiberglass, plus r-5 for the foam and you're only at r-20, when the recommended roof insulation is now in the 40 to 50 range.

IMO, taking the hot roof concept and modifying it to a flash and batt approach is taking a controversial approach to insulating and making it worse.

Bud
 
  #3  
Old 12-05-09, 11:07 AM
Member
Thread Starter
Join Date: Aug 2009
Location: USA
Posts: 4,940
I really don't know what to do. There is no way for me to provide adequate ventilation. Foaming the entire bay is going to be way more than I can afford.

I don't expect to get anywhere near the recommended levels of insulation with just 2x8 rafters. I am trying to maximize the space as best as I can. If I went the route I had mentioned, I could get around r-7 for the foam plus r-23 for mineral wool batts. Do you have any suggestions that would help me deal with this properly?

I'm trying to find a good source of info on hot roof theory as well and I'm not having much luck.
 
  #4  
Old 12-05-09, 12:29 PM
Member
Join Date: Oct 2008
Location: New England
Posts: 9,776
To illustrate what I'm trying to say, I did a quick search and here are two links and quotes from each.
http://www.uaf.edu/ces/publications-.../HCM-00559.pdf
If moisture from inside the house is prevented from getting into the roof cavity, and there is sufficient insulation to keep snow on the roof surface from melting, there is no reason for roof ventilation.

Roof Design
"As illustrated in the roof wall section, if we keep moisture from getting into the structure from the outside and prevent moisture laden air from achieving dew point in the structure from the inside, we do not need venting."

Note the similarity, both are stressing the basic need to control temperature and moisture. With r=6 foam and r=19 mineral batts, you are relying upon no air leaking through or around those batts, as somewhere on the inside of that one inch of foam, it WILL be cold enough to form condensation. Adding an inside vb, now means your foam would need to be open cell, so it can dry to the outside.

These are long term design considerations and one inch of foam is going to make it difficult to get it working right.

Keep searching and perhaps some of the others have a link that will help.

Bud
 
  #5  
Old 12-05-09, 01:04 PM
Member
Thread Starter
Join Date: Aug 2009
Location: USA
Posts: 4,940
I do have a general understanding of what conditions I need to avoid. It's the details of what I need to do to avoid them that I am trying to learn about.

I emailed the manufacturer of this particular brand of spray foam to find out what their recommendations are for my situation. I explained to them exactly what I had. They said that for flash and batt there should be a minimum of 50% of the R value in foam to avoid condensation. For my particular instance they said to do 3 inches of foam. That would give me an R-21 in foam and I can add another R-14 with a mineral wool batt for a total of R-36. Not too bad.

I asked about a vapor barrier and they said to use a kraft faced batt. Unfortunately, the mineral wool doesn't come faced and I prefer that over fiberglass. Things to think about.
 
  #6  
Old 12-05-09, 01:21 PM
Join Date: Mar 2005
Location: USA
Posts: 6,125
The "old fashioned" short term idealized R-value concept is only for insulation materials peddlers that do not concern themselves with the real world.

For insulating an attic, you have to make sure you do not have any weakly insulated areas that can create melting on the roof that can cause ice dams and major moisture problems. Heat loss through rafters or studs can be a major factor in the long run.

If you chose to go with an insulated, hot roof concept, make sure you are 100%+ insured that there is no moisture or heat loss into the attic area that could be affected by the temperature of the exterior roof temperatures/conditions.

If it does not work, you can always re-roof with aluminum/stainless panels to shed the ice and water off before there is damage to the rest of the home.
 
  #7  
Old 12-05-09, 01:56 PM
Member
Thread Starter
Join Date: Aug 2009
Location: USA
Posts: 4,940
I have very little "attic" space. I have some more research to do in regards to that space. I am considering making that a "conditioned" space to reduce those concerns.

We don't get much snow here in NJ. When we do, it doesn't stay for very long. As a result, ice dams are not much of a concern here.
 
  #8  
Old 12-06-09, 07:42 AM
Member
Thread Starter
Join Date: Aug 2009
Location: USA
Posts: 4,940
I found some more information about this technique at buildingscience.com.

BSD-149: Unvented Roof Assemblies for All Climates —

Page 6 explains "What to do". It echoes what the manufacturer told me with the exception of the VB. The paper says I won't need an additional VB if at least 2 inches of foam is sprayed.
 
Reply


Thread Tools
Search this Thread
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 On
Pingbacks are On
Refbacks are On



All times are GMT -7. The time now is 01:16 AM.