Attic insulation – R-21 and radiant barrier
Hi I know this topic has been asked a lot but I was hoping to get some info I could not really find in other posts.
I live in Queens NY – climate zone 4, I have a tall pitched non-vented attic. The attic is really a 4th floor with a smaller crawl space attic on top and closed un-vented spaces on the side these spaces get really hot and heating up the room. BTW I have no duct work in the un-vented attic spaces but we do use it for storage. The house is from 1945 – used to have a stone roof but the previous owner replaces it with a shingle roof maybe 8 years ago (only the shingles and roof paper not the sheathing were replaced) and I am in the process of redoing the insulation, the roof joists are 2×6 so I am using r-21 faced batts, not wanting to go with spray foam for various other reasons
My issue is the house is in direct sunlight all day and gets really hot just from the radiant heat. I wanted to use a radiant barrier since the sun is so strong but since its an un-vented roof I am trying to figure the best route. I tested the following scenarios today and these were my results
Outdoor temps today got up to 82 degrees, these were the inside temps:
1 – bare underside of roof – roof got up to 107 degrees !!!
2 – old foil faced R7 insulation – 88 degrees
3 – new R-21 kraft insulation – 83/84 degrees
4 – New R-21 with 1/2″ air gap and Radiant barrier – 88 degrees ??
5 – Radiant barrier alone – with 6″ air gap – 91 degrees
As you can see the Radiant barrier worked – by itself, it didnt help when put after the R-21 (even though I left a gap.
What i am proposing and think would give the best effect is to install the radiant barrier first – with 3/4″ gap on the underside of the roof, then install the r-21 faced insulation this way I am insulation 91 degrees instead of 107 degrees. My problem/concern is that the air will be trapped in that 3/4″ air gap between the radiant barrier and the roof sheathing (actual wood slats not osb board) I am concerned that the super hot air will cause damage to the roof as it has no where to go.
Interesting to note – the old R7 foil faced insulation was stapled so there was a 2″ air gap behind it but I really dont understand why since there was no place for that air to go.
Any thoughts and info would be greatly appreciated.
Thanks
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Replies
M. Simm,
Several aspects of your post are difficult to understand.
What do you mean by a "stone roof"? Do you mean a slate roof?
Are you talking about triangular attics behind 4-foot kneewalls, along with a cramped attic under a gable roof above a flat ceiling, as is typically found in a Cape Cod building? Or are you talking about one big attic space?
If you are trying to insulate a sloped roof assembly that is unvented, and you are working from the inside, there is only one safe way to proceed: You need to install an adequately thick layer of closed-cell spray foam on the underside of the roof sheathing. If the closed-cell spray foam is thick enough -- most codes call for R-38 or R-49 minimum -- it can satisfy minimum code requirements.
Otherwise, you can use the flash-and-batt approach.
For more information, see these two articles:
How to Build an Insulated Cathedral Ceiling
Flash-and-Batt Insulation
Hi Martin,
Thanks for the quick response. To answer your questions
- Yes I did mean slate
- Apparently I am talking about a gable roof, didnt know thats what they are called. Difference here is the kneewall is 6' tall so there is quite a bit of space. Its basically a 3,4,5 triangle. Knee wall is 6', floor above ceiling is 8' long and pitch is about 10'
- I know closed cell would be the correct approach here but not looking to deal spray foam for the time being.
I am not really looking to achieve the rated r value as I know I wont be able to without the spray foam.
What I am most interested in knowing is if I can install the radiant barrier maybe with the solid panel foam board backing with an air gap underneath the sheathing from inside and then put the r-21 under the radiant barrier. Basically what it would look like from outside int is the shingles/roofing then sheathing, then air gap, foam board, then r-21 batts.
Thanks
M. Simm,
A radiant barrier will in fact provide limited R-value. If installed with an adjacent air space, the air space will provide an R-value of about R-2 or at most R-3. It's better than nothing.
You should realize, however, that attempts to insulate a sloped roof assembly without following the rules can put your sheathing at risk. If interior moisture has access to cold sheathing, condensation and rot can result. Because of this possibility, I'm leery of home-brew solutions, and urge readers to follow the rules.
Martin, I attached 2 drawings to help you understand the situation. I read those articles but could not confirm whether I can put the radiant barrier in the location I am suggesting.
Thanks
I totally hear what you are saying, I am not providing the radiant barrier for the R-Value it is to block the radiant heat which as my experiment from this morning showed - worked very well, but not with the fiberglass in the picture which I why I was proposing the fiberglass after the radiant barrier to deal with the conduction and convection. The barrier brought the temps down from 107 to 91.
Regarding the moisture I am weary of it as mentioned in the article but I thought that was the point of a vapor barrier ? I was thinking about lining the whole assembly with 1" rigid insulation after the fiberglass and taping up the seams. Is moisture really going to seep through this ?? and even if so - so much that it will cause a problem ??
Thanks for your input, greatly appreciated
Sorry I was a little unclear there - the r-21 is faced and has a vapor barrier, the rigid insulation would do on top of that.
M. Simm,
Is the air gap under the sheathing connected to a soffit vent at the bottom and a ridge vent at the top? Or is this unvented?
If it is unvented, you have to stop trying to break the rules. You can't insulate unvented rafter bays with fiberglass batts. As the articles stated, you need closed-cell spray foam on the underside of the roof sheathing.
If any of this is unclear, re-read the articles.
If I take moisture out of the problem, is there an issue with heating the underside of the sheating too much due to the radiant barrier reflection the heat.
Ok. thanks for the answers.
Hi Msimms, How did you measure the temperatures you reported? Did you use an IR gun? If so be aware the radiant barrier has vastly different emissivity values from what the gun was likely calibrated against.
msimm15,
I'm in the process of doing pretty much what you're doing. I have an old house with a minimally insulated attic and 2x6 rafters. I'm doing a radiant barrier spaced 1/2" away from the roof, but I vented the ridge and will vent the eaves in the spring. The remaining 5" of each rafter bay will get R21 fiberglass. Should be a pretty nice setup given that all the hot air will be vented out the ridge vent.
They say radiant barriers increase your shingle temps considerably... something like 10-15 degrees. I'm hoping since I vented my roof that won't be the case. My only concern with venting is that it could allow moisture in that could make the fiberglass insulation less effective. But I have a layer of perforated radiant barrier and a layer of RamBoard between the vented space and the fiberglass, so not too much of a concern.
Let me know if you have any updates and I'll do the same when mine is done and I've seen the results.
- Jake
Jake,
I don' t think I'd be following msimms15's example. Ignoring moisture concerns when designing a roof is risky.
A 1/2" vent channel is too shallow to work effectively and doesn't meet code.
Except in very hot, humid climates the moisture that affects roofs doesn't enter through the venting, it comes from the house below. You want the baffles to be vapour-open to allow the moisture to move into the cavity and be removed by the venting, and you need a good warm side air-barrier.
Malcolm,
In my scenario, I'm using breathable radiant barrier and Ramboard, which is also breathable. On the room side of that I'll do faced fiberglass insulation which will hopefully function well as a vapor barrier to keep the insulation dry-ish. I'll cover all of that with painted tongue and groove wall board which should further help seal off the insulation from the moisture of the finished space. If I'm doing this wrong definitely let me know.
You might have seen the pictures I posted that show I'm using 1/2-inch rigid insulation as spacers to keep the radiant barrier from making contact with the hot roof, and also to create an air channel that I can use to evacuate hot air and moisture.
As far as code, I didn't know that a vented roof was even required by code in Illinois. To my knowledge it isn't. I just think letting the roof breathe and cool itself is a smart idea. Plus the necessity to leave a space between the roof and my radiant barrier makes it a no-brainer to vent even that small of a space to let heat and moisture escape.
I'm new to this so any feedback is appreciated. I'm not the touchy type so brutal honesty is always appreciated. Thanks for reading.
Jake,
With the exception of the depth of your vent space above the radiant barrier, and the lack of a good air-barrier, I think your proposed assembly will work fine, although from your explanation I'm not sure you understand the mechanisms at play.
"My only concern with venting is that it could allow moisture in that could make the fiberglass insulation less effective. But I have a layer of perforated radiant barrier and a layer of RamBoard between the vented space and the fiberglass, so not too much of a concern." You have mixed up two things. The venting doesn't introduce moisture, and the baffles (in your case the radiant barrier and Ram Board) are there to stop wind-washing and keep the vent space open, not to stop moisture from above.
You are using "breath" to refer to both air-movement and vapour-permeability. They are two separate things, and each requires a different response.
From what I can see different parts of Illinois have adopted different versions of building codes. I don't see any that allow un-vented roofs with permeable insulation. US Codes set minimum depths for vent channels at 1" because below that it is ineffective at removing moisture. Increasing the depth would mean less insulation in your rafters, but your proposed 5" of batts isn't really adequate anyway. I would furring down the rafters to try and get close to code minimum insulation levels for your climate.
The roof needs a good warm side air-barrier. Faced insulation and wall board are hard to detail effectively as that. I would include a variable-perm membrane or drywall above the finished ceiling.
The best thing you could do to cut down on roof deck temperatures is to convert to a proper vented roof assembly with soffit vents at the eaves and a ridge vent up at the top. You need, per code, 1" minimum vent channels with such an assembly, and more is better. I like to use 1.5" vent channels since cheap 1x2 furring strips make great 1.5" spacers for this purpose.
A 1/2" air gap won't actually allow for much, if any, circulating air currents. Keep in mind that the reason that windows often used 1/2-3/4" air gaps between panes is that convective currents don't circulate in such small air gaps. Such a small air gap is an INSULATING LAYER more than it is a VENT CHANNEL.
The issue I see with your "breathable" assembly is twofold: 1- the roof deck is exposed (i.e. no fully adhered material like spray foam), and it's going to get down below the dew point in the winter for certain. 2- moist air is less dense than dry air, which means moist air rises. Moisture is going to rise up from inside the home (the usual source, and older homes tend to have poor air sealing making this problem worse), migrate through your breathable insulating assembly, and then condense on the cold underside of the roof. You are very likely to get mold on the underside of the roof deck with this assembly.
My first choice (assuming there is no way to insulate the attic floor and vent the entire attic) would be to convert to a vented assembly. The vent channels then provide a pathway for moisture to escape, which solves all kinds of problems. These vent channels would need to go from eave to ridge, and you'd need to add soffit and ridge vents for this to work. My second choice would be closed cell spray foam applied directly to the underside of the roof deck, which is the usual (and safe) way to do a hot roof.
I would air seal the attic floor. The primary source of moisture in the attic is from the living spaces within the home, and you want to minimize how much of the air from the conditioned part of the home makes its way up into the attic. This will cut down on moisture accumulation in the attic regardless of how you vent (or not) and insulate the roof.
I think your proposed assembly is very risky, especially in climate zones with a longer heating season.
Bill
I have not been impressed with the independent test results for radiant barrier and only then in the deep south. Yes, they have a good sales pitch but I don’t hear people talking about great results.
R 21 seems pretty pathetic goal and unlikely to be code compliant.
Will the attic become living space? If not put the insulation on the floor.
If you must insulate the roofline, consider one if the 5 plans in this article with at least R38 as a goal.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Walta
Walter,
The floor is already insulated. I'm trying to do the best I can with 2x6 ceiling joists. The attic will be an office/storage area, maybe a bedroom occasionally.
Radiant barriers do seem to be pretty optimistic in their claims. We'll see what the results are.
Jake,
As soon as you add insulation to the roof and condition the attic as living space, any insulation in the floor effectively ceases to matter.
Having some insulation on the floor and more at the roofline is a risky game in that the air in the attic is semi conditioned. The risk is that the relative humidity of the attic air can get very high and some surfaces in the attic may fall below the dew point and condense water.
Malcome, If Jake said he planned on conditioning the attic I missed that.
Walta
Walta,
I assume he is incorporating it into the living space of the house, as in his reply to you he said it would be used as an office, for storage, and as a bedroom.