Best choice / strategy to re-insulate existing unvented attic?
Hey follks, I’m currently working on a remodel of a 1920s stud-framed house, about 1200 sq ft, in climate zone 6 (Montreal, Quebec). The house will be rewired and new air conditioning unit installed in the attic. The GC decided to pull the insulation to facilitate re-wiring and hvac installation which presents a great opportunity for a ‘green’ retrofit. I just really want to give the client good advice!
The attic appears to be unvented and was originally fitted with wall to wall floor boards. Since then a few boards were pulled and cellulose was blown in around the perimeter and several different layers of insulation laid on top of the floor boards. No particular attention was given to air sealing. There is also a forced ventilation system of some kind consisting of one inlet in the attic and an outlet going through the roof (but no fresh air intake that i can see. does that mean it’s just sucking air through all the penetrations in the ceiling).
As I see it, since we have an unvented attic the only options are closed cell spray foam on the back of the roof deck or start cutting soffit and ridge vents. The cost of both of these options will probably startle the client but I can’t in all good conscience just put the old insulation back in as it was.
Any thoughts would be greatly appreciated, especially if they make me look clever.
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I am in the same predicament. There is no easy solution, just some that are not as hard. The BIG deal is that the inside of your roof will bleed water in the winter if the roof is lined with insulation. If it isn't, it's no big deal. If it is, and finished, it is called a cathedral roof in the venting websites.
I live in Oregon, temperate climate, and mine still bleeds. I had batt r-21 installed on the inside of my roof when I bought my house. It bleeds water every year.
So this is my proposed solution this summer/fall: Build knee-walls all around (I have 4 gables... makes it a much bigger job) and pull down the batts from the lower section and let flop down and it can be the knee-wall insulation. So now I have a triangle-profile crawlspace in the eaves of my attic all around.
Then fur out the rafters with 2x2. I'll use 1x2 on edge across the rafters, and cut the 2x2 into 14 1/2" chunks, to bracket the 1x2 cross-members. The 1x2 is to hold the insulation up, and the 2x2 is to give more rafter depth to give some breathing room between the batts and the roof. That has to have an air gap.
OK... so far all I've done is create a crawlspace along the eaves, and get the batt off of the roof. Now to vent. You have to have a balanced system. So Air Vent II Shingle-over ridge vents all along. Then I am going to ventilate those crawl spaces with attic louvers to the same combined net free area as the ridge vent (9 inches per foot each side). They will not be in the peak, obviously, but in the lower corners of the gables near the eaves. As I back out of the crawlspaces, I will lay batt on the floor. Then close up a rigid foam-fixed door, and see what happens.
Oh, and to cover the insulation, I do plan to sheetrock the kneewalls. I have to cut the sheetrock into 2' strips to get it in the attic... steeeeeep winding stairs, small windows, etc. But for the inside of the roof I plan to cover it with canvas. Staple it on, then put shellacked 1x2 strips along the rafters for a neat effect as well as help the staples. Then spray it with a water bottle to let it shrink, then prime and paint. I might skip the spray if I am using latex primer. I did talk to the air vent rep, and he was not sure about the intake. He said, "You have the intake requirements, but not sure about the flow." That puzzles me... if I have a well-ventilated knee space, and free air flow. Oh well. I'm still pondering that.
They do offer EdgeVent, but that is normally installed with a new roof. And for an older home without soffits, you have to put it in way upstream to actually get it over the attic. Challenges, and much more expense. Home Depot has some galvanized metal attic vents for $25 or so apiece.
http://www.homedepot.com/webapp/wcs/stores/servlet/Search?storeId=10051&langId=-1&catalogId=10053&keyword=construction+metals%20gable%20vent&Ns=None&Ntpr=1&Ntpc=1&selectedCatgry=SearchAll
Blake,
From a building science standpoint, there is only one answer to your question. You are describing an unvented attic that contains HVAC equipment (an air handler and ducts). That means that you need to insulate the roof slopes, to create a conditioned attic.
There are basically two ways to do this: you can either install spray polyurethane foam on the underside of the roof sheathing, or you can install rigid foam insulation (and new roofing) above the existing roof sheathing.
For more information, see Creating a Conditioned Attic.
Thank you Meryl and Martin. I figured spray foaming the roof deck was the thing to do. Assuming the client won't want to spend the money (we must be talking a good $8000 for that much spray foam). Are there other options I can propose?
Also can y'all direct me to some numbers or case studies that would justify that kind of expense?
Blake,
I think it's nuts to put an air handler in a hot attic, and that it's also nuts to put cooling ducts up there. Researchers estimate that ducts in unconditioned attics waste about 20% of the output of a furnace or air conditioner. If there is an air handler up there in addition to ductwork, it's reasonable that more than 20% of the output of the equipment will be wasted.
For more information on this topic, see Keeping Ducts Indoors.
If you are one of the contractors working on the house, omitting the insulation that would bring the air handler inside the home's thermal envelope would amount to (in my opinion) a type of malpractice. It's your duty to do quality work, not to accept slip-shod approaches and halfway measures designed to save the customer money.
You asked whether there are any options other than using spray foam. As I explained in my last answer, there is another option: installing a thick layer of rigid foam above the existing roof sheathing, followed by a layer of plywood or OSB and new roofing.