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Old 09-14-2008, 07:50 PM   #1
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un-vented attic insulation questions....


so, i've got a 1926, 2 story colonial.
the attic is unvented, except for the new ridge vent.
no soffit vents. no gable end vents.

i would like to finish the attic, and wonder how best to insulate this attic, especially the rafter bays...

the attic floor has cellulose insulation under the plywood subfloor, but nothing in the rafter bays.

i got a few estimates for closed cell foam to be sprayed, but they want $4500 for the roughly 1000 sq. ft roof (4 inches thick foam) (24 rafter bays total, each 16' long, 24" wide.)

can i use fiberglass batts instead? useless to use rater bay vents right? or no?

any help would be greatly appreciated.
thanks.
-thomas

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Old 09-15-2008, 06:27 AM   #2
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un-vented attic insulation questions....


If you use styrofoam vents and but your fiberglass over them you would be fine. The styro vents allows the air to travel to the ridge vents.

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Old 09-15-2008, 08:06 AM   #3
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un-vented attic insulation questions....


thanks for the reply.

but here's the thing-
there aren't any soffit vents, so do i still need to use the stryofoam rafter vents?
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Old 09-15-2008, 08:46 AM   #4
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un-vented attic insulation questions....


an unvented attic? how strange. you may need to install soffit vents then? use the foam baffles because it still needs to breathe? makes sense to me, but what do i know? i once found a rainforest dripping from one guy's unvented walkway down to his tornado shelter. we vented it, of course. fine now.

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Old 09-15-2008, 09:02 AM   #5
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un-vented attic insulation questions....


un-vented. yes, strange indeed.

these old homes in my neighborhood were all made with unvented attic spaces. no soffits, just blocked of at the ends, where the rafters meet the top plate.

never been a problem around here, but now, it's code in the area to put in a ridge vent (not that it does much without air coming from the bottom to circulate...)

so, i wonder what to do- should i leave some space between the batts and the underside of the sheathing?

or right up against it?

-t
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Old 09-15-2008, 09:18 AM   #6
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un-vented attic insulation questions....


if you put it right on the underside of the roof, i think you might regret it. but what do i know? a vented attic needs to breathe, yes?

DM

it's code in the area to put in a ridge vent (not that it does much without air coming from the bottom to circulate...) <--also strange... personally, i'd add a couple of gable vents with covers for colder weather, add the rafter baffles down to floor level and vent inside that way. kinda hard to close the ridge vent in the winter though.....

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Old 09-15-2008, 09:25 AM   #7
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un-vented attic insulation questions....


Can't you drill holes and install louvered vents over them on the facia? You need to ventilate some how. I have seen people take the facia off install a 1" board as a spacer cover with screen to keep critters out then reinstall the Facia. This gives a vent all along the bottom edge.
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Old 09-16-2008, 07:27 AM   #8
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un-vented attic insulation questions....


First off, nowhere can I find your location. It's much harder to respond to insulation questions not knowing what kind of climate you are living in. It's not that unusual for an old house not to be vented. You definitely need to allow a 1-1/2" air space between the bottom of the roof sheathing and the top of the insulation. The airspace needs to be continuous from the soffit to the ridge. You can (and probably should) install some vents in the soffit. Ideally, one in each bay. Generally, old houses had enough cracks and crevices around the roof edge to allow all the venting they needed. The problem here is that you will be creating living space . You will be introducing warm moist air into a previously unheated area so you need to be very careful to keep additional moisture out of places where it doesn't belong, like the under side of the roof. Woe be unto the homeowner who ignores the rules of moisture and condensation there-of, for Mother Nature will not be fooled.
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Old 09-16-2008, 08:23 AM   #9
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un-vented attic insulation questions....


M-6,
I am located in northern New Jersey.
You can see the exposed rafter beams from the exterior of the home, and the rafter bays are blocked at the edges.
Yeah- these homes probably had PLENTY of venting since they certainly weren't that tightly built in the roof.

my concern is obviously that it will become a living space.

so, if i were to install soffit vents,how big do you think each bay vent would need to be?

in addition, if i were to go this route, can i assume that i need to place rafter bay vents from top to bottom? all 16 feet from bottom to ridge vent?

thanks again.
-t
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Old 09-16-2008, 09:35 AM   #10
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un-vented attic insulation questions....


A friend of mine had the same setup that you do, except he had no vents at all. The air was trapped and couldn't escape, and during the winter days the sun would warm the roof and decking. Since the air was trapped, when the temps dropped, frost would form under his decking. Once the sun came back out and warmed the roof again the frost melted and it looked like it was raining inside his house.

His was a vaulted ceiling and that is what you will have if you build in the attic space. Hot air rises and with the soffit vents and ridge vent, cool air will be drawn in creating an air flow which is needed to keep the frost and moisture out. Even if frost didn't form from a non-vented space for some reason, the trapped heat in the summer months would dry rot your decking.

You could install a continuous soffit vent which would be best but there are rectangular and smaller round vents on the market also. The more air that can circulate the better off you will be. The styrofoam rafter bay vents will be necessary also.

Last edited by BigJim; 09-16-2008 at 09:44 AM.
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Old 09-16-2008, 01:42 PM   #11
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un-vented attic insulation questions....


Quote:
Originally Posted by roadk View Post
M-6,
I am located in northern New Jersey.
You can see the exposed rafter beams from the exterior of the home, and the rafter bays are blocked at the edges.
Yeah- these homes probably had PLENTY of venting since they certainly weren't that tightly built in the roof.

my concern is obviously that it will become a living space.

so, if i were to install soffit vents,how big do you think each bay vent would need to be?

in addition, if i were to go this route, can i assume that i need to place rafter bay vents from top to bottom? all 16 feet from bottom to ridge vent?

thanks again.
-t
As I understand it, your home has exposed rafter tails. That means that you don't have a soffit that can be vented, however, you can vent through the wall just under the roof deck and between the rafters. Standards for venting are 1 square foot of intake and 1 square foot of exhaust for every 300 square feet of attic floor space. By rafter bay vents, I assume you mean baffles to prevent insulation from reaching the underside of the deck. You need to maintain 1-1/2 inches of clearance the whole way from the soffit to the ridge. Either by installing baffles such as Durovent or by keeping your insulation thickness small enough to maintain the clearance. My personal preference is to use Durovent for the first 3 feet, so that i can get insulation over the top plate of the house and then use thinner insulation the rest of the way.

Last edited by Maintenance 6; 09-16-2008 at 01:49 PM.
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Old 09-17-2008, 01:19 PM   #12
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un-vented attic insulation questions....


exactly.
exposed rafter tails. NO soffit.
it's amazing how many people dont understand this type of construction.
they continue to suggest cutting the soffit, the fascia, etc.

i've got none of these!

so i guess i'll try to ventilate via the wall boards. i'll install durovents and then insulattion on top.

6 mil vapor barrier between insulation and wall board?
what's your thought on that?

thanks again for the help
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Old 09-18-2008, 06:54 AM   #13
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un-vented attic insulation questions....


Cut your vents through the wall up close to the underside of the roof deck. That way you can get the most insulation in that space. The area between the top plate and the roof deck is the hardest place on a building to insulate. 6 mil poly is fine for a vapor retarder. Just be sure to use unfaced insulation. You don't want two vapor retarders in the same space. Keep the poly to the warm in winter side of the insulation. For venting, I've used small round vents that weren't very noticable. You might need several in each bay to get the area you need.

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