Insulating a complicated roof / attic
davidl256
| Posted in Energy Efficiency and Durability on
Hi,
I need some advice on how to insulate an attic of a large home
with 5000+ square feet of living space with a non-trivial roof geometry.
The home is in climate zone 5A.
Here is an image of the home:
Closed cell spray foam is not an option. I do not believe it is safe.
I have read and studied many articles on the site, including this one:
The roof geometry is non-trivial. It has been designed to be ventilated
using soffit vents and a ridge vents; however, the geometry does not allow
each rafter bay to have intake vents at the bottom. In my opinion, the attic
is under-ventilated.
There are sloped walls/ceilings on the second level. The plan is to use baffles between the 2×12 rafters providing a 2″ ventilation channel; each baffle would properly start at each soffit and span the length of the
rafter bay until it passes the insulation or reaches the ridge, whichever happens first.
Rockwool batt insulation would fill the remaining 9 inches of space, providing an insulating value of R-38. The top plates would be properly insulated so that
R-38 would meet code requirements.
Question 1:
If I insulate the entire roof and sheath the underside of the rafters with drywall,
making it as air tight as possible, how important is it to make the baffles air tight?
This would be a conditioned attic.
Due to the size of the roof, this is a daunting task and would be costly. It would
be much easier to not worry about sealing the baffles.
However, this approach would allow placing the HVAC equipment in the “conditioned” attic.
Also, the ceiling above the second level, which would contain many recessed lights, would not need to be air tight. The air tight barrier would on the underside of the rafters.
Questions 2:
If we insulated the second floor ceiling, so that there will be a mix
of insulated sloped walls and attic floor, would this approach work better.
Is it necessary to air seal the baffles? I believe it is not since anything
above the baffle is open and unconditioned.
Is it necessary to air seal the baffles? I believe it is not since anything
above the baffle is open and unconditioned.
In this case, we would find a way to keep the HVAC equipment on the second level in conditioned space; the ceilings are 9′ tall and we may sacrifice some height
to create space for ducts and recessed lights.
Thank you.
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Replies
> the geometry does not allow each rafter bay to have intake vents at the bottom.
You can use shingle vents such as DCi SmartVent. And ValleyVent.
While air barriers on both sides is best, a good, tested interior only air barrier will work.
You do have a lot of roof there, David.
I believe that if you use air permeable insulation, it's a good idea, if not a requirement to air seal the baffles to prevent wind washing which will diminish the performance of the insulation.
I'm curious if you have considered an unvented attic which you could do with closed cell spray foam or a flash-and-batt or flash-and-fill method.
Also, this article from FHB will walk you through the math you need to do to make sure you have enough venting and shows the options that Jon suggests for venting when soffit intakes are not possible.
https://www.finehomebuilding.com/2009/07/16/venting-a-tricky-old-roof
See below for why you don't need to worry about wind washing.
https://www.greenbuildingadvisor.com/sites/default/files/Wind%20Washing%20Effects%20on%20Mineral%20Wool%20Insulated%20Sheathings.pdf
The 2015 IECC requires R49 in CZ5. I typically design homes with your type of roof, combination of vented attics and cathedral ceilings, where the ventilated attics have blown cellulose over the attic floor (ceiling). Hopefully you installed 16" heel-trusses, since you need 14" of insulation and 2" for baffle space, to meet the R49. For the cathedral ceilings, my option is to install 2" of ccSF + 10" of dense-packed cellulose or ocSF, again, to meet R49. I also like the idea of keeping the HVAC in the conditioned space, so furring down 12" for ducts, is IMO, the way to go.
FWIW, I can understand if you don't want to use spay foams, but if you read carefully about the problems with spray foams like the article you mentioned, is always about installation of the foam, the wrong foam or bad sealing job. I've done dozens of homes with the above application without a single problem.
In my opinion HVAC ducts and equipment in an attic is a design defect that should be avoided in the planning stage of your project. But it looks like you are past that now.
If you have cathedral ceiling and do not want spray foam you may put the insulation on top of the roof with foam sheets. Look for low cost recycled foam.
If you have flat ceiling maybe you can you can find a way to keep the HVAC below them.
Walta
David,
Because of all of those valleys, it's going to be impossible to get ventilation air in all of your rafter bays. This type of complicated roof cries out for an unvented solution. If you don't want closed-cell spray foam, that leaves one option: a sufficiently thick layer of rigid foam on the exterior side of the roof sheathing. This article explains what you need to know: "How to Install Rigid Foam On Top of Roof Sheathing."
For other GBA readers: Remember, you need to come up with an insulation plan before framing begins. For more information on this issue, see "Plan Ahead for Insulation."
David, I agree with Martin that the best solution for this type of roof is an unvented assembly. But if you don't want to use foam, you can "over-vent" the roof, by laying 2x material over the sheathing and adding another sheathing layer above that, open at the valleys to allow continuous airflow. With a well-installed variable permeance membrane at the interior, and with cdx being somewhat vapor permeable, it should meet code and be a safe assembly, in my opinion. But a thick layer of foam is more straightforward (and what I'm using on a current renovation with a similarly complex, existing roofline).
Thank you everybody; the situations is as I expected.
At this point I cannot install a second roof for ventilation or rigid insulation on the exterior of the sheathing. We are already installing the roof shingles and I cannot delay the project.
I am looking into the DCI SmartVent product.
What about drilling holes through the 2x12 rafters at the bottom (holes perpendicular to
the rafter). This would connect the rafter bays with ventilation to the unvented bays.
Exterior foam is the way to do this, it almost always is if the thought of conditioned attic comes up, but that ship has sailed.
With a house this big it does not necessarily make sense to use the same approach throughout per your second question. What are your expectations for energy efficiency vs aesthetic requirements for extent of cathedral ceilings or limitations on ceiling soffits? I assume this will be force hot air/AC?
You could create a conditioned mechanical room in the attic that houses the air handler and gets all the ducting going in the right direction. Run the ducts the length of the house in a second floor ceiling soffit.
Or build an insulated "soffit" on the floor of the attic for ducts out of minimal framing and Taped 2" Dow Thermax Polyiso. This should have insulated boxes that are sealed to the 2nd floor ceiling where there are penetrations for supplies and returns. Seal the duct work really well.
Any ceiling penetrations are going to need airtight boxes over them, so think about every recessed lights. Just say no to recessed lights in any cathedral ceilings as they displace insulation even if airtight.
Where you have an insulated ceiling, you definitely do not need to seal the baffles, which will be short to just clear the fluffy insulation. Fill it to R-60.
In the cathedral ceiling sections, I would consider 1-2" rigid foam insulation of the interior polyiso to manage the thermal shunting through framing if ice damn are a concern at all. Don't worry about sealing the baffles if you have an airtight rigid foam and drywall layer, rockwool is pretty good and continuous baffles would be sufficient.
If you are cathedralizing a dormered section along with the main roof, it is going to be very difficult to ventilate. The holes you suggest are going to need to be at the top edges of the rafters where your baffled space is. Large holes are generally not okay on the edges of framing members.
I would strongly consider using flash and batt in these areas with 3-3.5" of closed cell foam with low global warming blowing agent and Rockwool. I am in the middle of a smaller, but similar project using this approach. Minimizing the use of foam minimizes the risk. Installers should plastic sheet everything, use fans to negatively pressurize the space while spraying. When they are done spraying open attic windows or run some sort of duct to maximize air exchanges while maintain a slight negative in the space. I had to intervene with my installers to take the last step. The air exchange rate is more important.
Air sealing such a large complex project is difficult, get someone with blower to look for holes after the primary air barrier is defined and installed, which requires a very clear plan for what the primary air barrier is.
Have a 3rd party do an aggressive manual J and manual D to make sure your HVAC is not oversized and efficiently designed (these is really Dana's advise I am passing along). This will make locating the HVAC easier.
We have decided to take the unconditioned attic approach with insulated air tight ceilings on the second level. That roof geometry is too difficult to insulate using batts.
"Or build an insulated "soffit" on the floor of the attic for ducts out of minimal framing and Taped 2" Dow Thermax Polyiso. This should have insulated boxes that are sealed to the 2nd floor ceiling where there are penetrations for supplies and returns. Seal the duct work really well."
We are currently planning exactly this. The two air handlers will be located in conditioned space on the second floor and we will attempt to make the air sealed insulated soffits in the attic; we may build with lumber or rigid foam insulation.
"Any ceiling penetrations are going to need airtight boxes over them, so think about every recessed lights. Just say no to recessed lights in any cathedral ceilings as they displace insulation even if airtight."
Yes we'll definitely try to make that ceiling air tight; we may use two sheets of drywall
on the ceiling with overlapping seams. Maybe somebody can advise on this and whether that would be overkill.
It doesn't take 2 sheets of gypsum board to make it air tight- definitely overkill. Be downright obsessive about air sealing any & all penetrations, and the seams of your mechanical chases that house the ducts. Be sure to install at least as much insulation over the chases as the rest of the attic.
Yeah, definitely no double drywall. The leaks aren’t at the seems, they are where you put holes in the drywall. Focus on sealing around those places, or better yet, limit the number of holes.
After you are done sealing, but before you put any attic insulation in test for leaks. This could be a full blower door test with a fog machine. There are also a number of ways to diy leak search if you don’t need an actual leakage rate or aren’t ready to for one.
Thanks again.