Attic insulation detail
I am working on the details of our victorian remodel, we’ve been insulating heavily and being very careful about air sealing. We want to use our attic space for storage so don’t want to just fill it with blown in fiberglass/ cellulose.
We had been planning to use 4″ of spf up the exterior walls and continue that right up the underside of the roof deck. We would have space to take that up to 7″ on the roof deck if needed but that could add a fair bit of weight.
The question is: can we add 6″ of fiberglass batt in the 2nd floor ceiling/under attic floor to bring our roof total R up to ~50 or could that cause problems? We are in zone 5a(?) upstate New York. We do not plan to condition the attic but don’t want it to freeze too readily. We will be installing 3 operable windows for summer ventilation.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
It sounds like what you're thinking is to have some insulation in the roof, and the rest in the floor, thinking the two will "add". It doesn't work that way -- all the insulation needs to be in one layer for it to be additive. You need to choose to either convert the attic to conditioned space, which would mean putting ALL the insulation in the ceiling/rafter area, or to leave the attic unconditioned and put all the insulation on/in the attic floor.
If you decide to convert the attic to conditioned space, you could use spray foam and go unvented, or you could leave a vent channel in every rafter cavity, and then use other types of insulation to get up to the R value you want. There are various ways to do this that will work, but you didn't mention your rafter depth so it's hard to suggest any particular detail.
If you decide to leave the attic unconditioned, you could still use it for storage. The easiest way to do this is probably to build an elevated platform in the attic to use as the "floor", then use blown cellulose on the actual floor. The elevated platform would be like a deck above the blown insulation. It's pretty easy to construct something like this, and the materials cost will probably be offset by the ability to use very cheap blown insulation for your insulating material.
Bill
Thanks for the response,
Our roof has 2 main ridgelines at a 90 degree angle in a T configuration, this creates a mess of valleys each of which has a 2x10 in it. The rafters that butt up to these 2x10s are actually only 2x4s.
My thinking was to incorporate the attic space into the building envelope but not condition it with any heating or cooling outlets in the space, just whatever passive heating occurs up through the attic floor.
From our experience so far we seem to have quite a chimney effect in the house with a stiff west wind through the existing passive vents creating a venturi and a vacuum at the attic hatch which will be replaced with an insulated model regardless of what we do but even insulated doors can leak.
Why would 2 layers of insulation with a closed air space in between not work? If outside is 5F, inside 70f, R21 in the 2nd floor ceiling is slowing heat loss to the attic some, attic would be somewhere in between because of the ~R28 in the roof deck. The attic at say 40F would be less of a gradient than if it was fully vented and also ~5F so heat flow from the 2nd floor should be reduced as compared to a vented attic.
Im trying to understand the building science so please forgive if I'm being obtuse. Would the heat loss be reduced but not as much as if the R50 was entirely in 1 layer? I know that there is a whole additional factor of dewpoints and point of condensation which is also a major factor.
I had a similar idea of splitting insulation between attic floor and roof for our attic with 2x4 rafters. I decided not to, as it seemed that convective heat transfer lops would reduce the effectiveness of this insulation method. I did not calculate this though, or even research it. The thought process I used to reject it is the fact that insulation that is not dense enough to prevent convective loops is less effective than insulation that has sufficient density ( think if you took a same mass and surface area of an insulation, and then increased the volume so it was much less dense, convection would reduce the effectiveness of the insulation). In the case of the attic, colder air falling from the roof would lead to a higher difference in temp over the surface of the floor insulation. Since if you split your insulation layers in two you have more surfaces for convective heat loss, you reduce the effectiveness of the insulation. Result is that the sum of the insulation r value for split insulation planes is less than the than the total that you would get if it was one layer. Say 2 separate layers of R10 would not equal R20 in that case. No idea what the magnitude of the difference would be, or if my reasoning is even correct (please correct me if I am wrong, as I would like to know the correct reasoning). In the end it made no difference, as I finally decided that the low cost and low global warming potential of cellulose on attic floor was a much larger benefit over storing stuff in the attic (In our case, due to the annoyance of having to use a ladder to go through a small attic hatch meant that anything stored up there was effectively gone anyway for all intents and purposes.
I suggest you just put in an air sealed attic floor if one is not there, and then insulate that plane with mineral wool board or EPS sheets, and then cover that with a protective surface/second floor layer.
David is basically correct, you get convection currents in the middle that cancel out some of the benefit of the extra insulation in the "other" place. You also have all the screwy things to think about, like differing dew points in different places. You add additional variables in the split insulation locations so that the overall performance is no longer easily predictable the way it would be if you just added R20 batts over a layer of R10 batts to arrive at a total of R30.
You are correct that insulating the roof AND the floor WILL slow heat loss, just not as much as you might expect -- and code doesn't permit this to arrive at the minimum required R value. You could put the minimum code required R value on the floor AND add the roof though, but that would be a lot of material for little benefit.
It's common to have a breeze in a vented attic, that's to be expected. What you want to do is put baffles at the eaves to protect the insulation from wind washing (forced "convection" currents within the insulation, essentially, which reduces the insulation's performance). You also want to air seal everything betwen the attic and the home.
It's not difficult to build a better sealing attic hatch. You need weather stripping around the edges, a good catch to hold it closed tightly, and some insulation on the back. I did this myself. I used a piece of 3/4" MDF (because it's heavy for it's size, and its flat), some weatherstripping around the perimeter, and several layers of XPS (I had scrap handy, otherwise I'd have used polyiso) glued to the back to get up to around R30 or so. The original hatch cover was just a piece of plywood with a batt laying loosely on top.
Bill