Non-standard low slope roof — best way to insulate and ventilate?
I am trying to find a solution to properly insulate and ventilate an attic in my house. We purchased the house two years ago. The house had a history of ice dams. Before the first winter in the house, based on recommendation from the energy auditor, we had the house air sealed, added cellulose to the attic and additional vents in the roof (turtle vents) installed. Unfortunately, it did not help and with all the recent snow we are dealing with now (Boston area) there is another ice dam damage. The temp. in the attic is 40F when the outside temp is 20F.
The most problematic area (with lots of icicles and ice dam damage) is is the low slope roof (approx. 4 feet high and 15 long). The attic is unconditioned with a number of AC ducts. There is probably 2 feet on insulation there. The roof has an eve, but no soffit vents or any type of opening at the edge (everything is filled up to the roof deck with cellulose) . The only vents are: 1 gable vent and 2 roof vents (2 roof vents recently added). This attic is connected with an adjacent attic (also unconditioned) where the air handler is located. The left hand side of the roof meets high wall, the top of the roof meets another wall. There does not seem to be an easy way to install any type of ridge vent.
I am thinking about the following fix:
1) Remove all old insulation and air seal all air penetrations
2) Build a “box” for the AC ducts from polyiso boards and make them a part of “conditioned” space of the house
3) Install soffit vents at the eves (each rafter) and add baffles
4) Close the connection to the adjacent attic, close gable vent
5) Replace existing turle roof vents with tall roof vents (www.ventilation-maximum.com), so they do not get covered with snow
I am worried if two roof vents will be enough to remove the warm air from underneath the entire roof deck (due to rafters blocking the flow). Does this solution make sense?
Any help or input is greatly appreciated. I am attaching two pictures of the house (the problematic roof is colored in orange).
Thank you!
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Replies
Marcin,
Here are the most important steps you need to take:
1. You need to make sure that all ducts and air handlers are inside the conditioned space of your home.
2. You need a very tight air barrier adjacent to your insulation.
3. You need thick insulation that at least meets minimum code requirements.
In the areas of your attic with ducts and the air handler, you really need to move the insulation layer to the rafters if you want to solve the ice dam problem. Halfway measures won't work.
In areas of your attic without ducts or an air handler, you might be able to leave the insulation on the attic floor -- but only if you can build a tight air barrier and insulation layer that walls off the part of your attic that is conditioned from the part of your attic that is unconditioned.
Don't worry about venting. Venting never solves an ice dam problem.
The best solution, by far, would be to install a thick layer of rigid foam insulation above the roof sheathing, followed by a second layer of roof sheathing and new roofing. Not cheap, but effective.
For more information, see:
Prevent Ice Dams With Air Sealing and Insulation
All About Attic Venting
Creating a Conditioned Attic
Insulating Low-Slope Residential Roofs
Thank you so much for the reply Martin! I am determined to fix the issue - whatever it takes. I wish the solution was recommended to be me when I was doing energy upgrades two years ago. It looks that everything needs to be redone....
I my climate zone, it's recommended to use R-60 insulation in attic. This would mean 4 (2 inches thick) polyiso board on the top of the roof deck (8 inches total). As an alternative would it be possible to use only 2 polyiso boards (4 inches total thicknesses) on the top of the existing roof deck, install, roxul insulation between rafters (R-15) and add one more layer of polyiso underneath the rafters? I would get approximately R60 total and the roof would be raised by 4 inches only. I am not sure about a moisture that may get trapped between "sandwiched" rafters.
Also, any recommendation on how to find a builder knowledgeable in the area of energy efficiency.
Thank you again for help!
Best,
Marcin
Marcin,
Building codes require at least R-49 of roof or ceiling insulation in your climate zone (Zone 5). You might find it easier to aim for R-49 rather than R-60, especially if the insulation is being installed along the roof slope.
If you install 4 inches of polyiso (about R-26) above the roof sheathing, you could install the remaining R-23 under the roof sheathing -- for example, by installing about 6 or 7 inches of mineral wool or fiberglass insulation. That's probably a better approach than making a foam sandwich with rigid foam on both sides.
If you are looking for a contractor in the Boston area, I can certainly recommend Paul Eldrenkamp at Byggmeister, Inc., or Gary Bergeron and David Joyce at Synergy Construction. You can Google the company names for contact information.
This is great info Martin! Thank you so much for your help on this issue.
Best,
Marcin
You need an architect/engineer to design plans that are submitted and approved by the proper authorities and then a proper qualified builder with great insurance builds a great project.
Plan first, hire pros.
Martin,
I have one more question (after reading more on this topic of unvented roofs). If I understand it correctly, if my roof is not ventilated the temperature of the roof will be controlled by the amount of insulation, depth of snow coverage and temp. difference between indoor and outdoor ( I found this formula on the internet)
For example:
R50 insulation on the unvented roof deck
20 inches of snow (R20)
Total R value = R70
Temp inside 70F
Temp outside 20F (temp difference 50F).
Temp. at roof sheeting: 20/70 * 50 + 20 = 35F. Won't the melting happen at the roof line anyway?
Thank you,
Marcin
Marcin,
Yes, a roof that has a deep layer of fluffy snow is susceptible to melting at the roofing/snow interface. There are three ways to deal with this problem:
1. In climates where 20 inches of rooftop snow is rare, ignore the problem. Occasional melting is unlikely to lead to a roof leak.
2. In climates where 20 inches of rooftop snow is frequent, install a type of roofing (for example, EPDM or standing-seam metal) that resists ice dam leaks.
3. In climates where 20 inches of rooftop snow is frequent, install a generous ventilation cavity (at least 2 inches deep) directly under the roof sheathing. This method only works for roofs without valleys, hips, dormers, or skylights.
Hi Martin,
Thank you again for all the input on this project. Per your recommendation, I have decided to go with two layers of polyiso on top of exiting roof (no venting) and use EPDM instead of shingles. This was also recommend by the builder. I also want to add mineral wool between afters to get to R50 of the total assembly.
Since the roof with be "air-sealed" from the top with hard-board insulation, do I need to cover mineral wool with some type of air barrier from the inside of the attic? If yes, could/should I use something like ClimPro Intelli Plus barrier? The access to certain section of the attic is not good (very little space between floor and roof)
Thank you!
Marcin
Marcin,
An air barrier under the mineral wool is a good idea, but is not strictly necessary, especially in areas with difficult access. Just make sure that some method is used to ensure that the mineral wool batts remain in physical contact with the roof sheathing, and that the batts don't slump or slip.
Hello Martin,
Thank you again for all the help on this project! The work has finally started and I am moving forward with the recommended approach. There is going to be 6 inches of polyiso at the top (R26) and and approx. R24 of fiberglass between the rafters.
There is still a lot of insulation on the attic floor (6 inches of faced fiberglass) and piles of cellulose. Total average depth is around 12 inches. Shall I be worried about this extra insulation at the ceiling level?
Thank you,
Marcin
Marcin,
Q. "Shall I be worried about this extra insulation at the ceiling level?"
A. No, you shall not. Leave it where it is.
Thank you Martin!
I am thinking about pushing most the cellulose towards the soffit area with (corner with very limited access). Cellulose could entirely fill the floor to rafter cavity in that tight corner, but if would result in a small section of that roof (1 foot by 30 feet) where I have between 12 to 15 inches of insulation directly under the roof deck (R-50).
Considering I only have R-26 (4 inches of polyiso) on the roof deck, will it still work, or 40/60% above/below roof deck insulation rule needs to be enforced?
Thank you and have a nice weekend!
Regards,
Marcin
Marcin,
I think that your plan to shove the cellulose in the corner of your attic will work. It doesn't sound risky to me.
Thank you Martin.
On the outside, the plan is to put ice and water shield on the entire roof, so the layers will be:
- Roof deck 1 (zip systems OSB, taped on seams)
- 4 inches polyiso boards
- Roof deck 2 (plywood)
- Ice & water shield (entire roof)
- Asphalt shingles
Questions:
1) Is there any risk, the water (leak) moisture can get trapped between the foam and ice and water shield causing problems?
2) My second roof will have similar layers but plywood instead of ZIP systems as roof deck 1. Would ice and water shield work on the entire roof too?
Thank you for any feedback!
Marcin
Marcin,
Q. "Is there any risk, the water (leak) moisture can get trapped between the foam and Ice & Water Shield causing problems?"
A. Not really. You have three waterproof layers -- the asphalt shingles, the Ice & Water Shield, and the taped Zip System sheathing -- so the likelihood of water leaking through these three layers is very low.
Q. "My second roof will have similar layers but plywood instead of Zip System sheathing as roof deck 1. Would Ice and Water Shield work on the entire roof too?"
A. I'm not sure what you mean by "work." It may be overkill -- but it will certainly "work."