Re-insulating Attic: Radiant Barrier? Vapor Barrier?

[djmcmath]

I'm refinishing the attic in my old house. Someone else, probably about 50 years ago, had done this job, but had done it pretty poorly. The electrical wiring was all aluminum, the insulation was all asbestos (and poorly distributed, so it was pretty hot up there), and the walls were covered in some kind of chintzy particle board. The windows were starting to rot in the sills, and needed to be replaced.

So I'm most of the way through tearing everything out, and trying to figure out the right way to do insulation in a modern world. There's no ridgeline vent, no soffet vents, and currently, no insulation between the roof and anything else. The construction is pretty simple -- it's just a big triangle-shaped roof, no truss-work or anything.

So my thought is:
1 - Install soffet vents all the way around. Is there a thumbrule for sizing and number of these?
2 - Ridgeline vent.
3 - Lay in 1x2's next to each of the trusses, then attach vapor barrier onto the 1x2's. This gives me a small air space between the incredibly hot roof and the inside, where (hopefully) air will flow through instead of sitting and heating the house.
4 - Radiant barrier goes on next, presumably the kind that isn't a vapor barrier since I'm doing a layer of plastic vapor barrier first. I think I could combine these two, but I'm not sure.
5 - F/G insulation batts, also with no vapor barrier.
6 - Drywall the whole thing.

What am I missing? What's going to give me grief 10 years from now? What should I do differently? What am I going to learn while doing this that I really would have wished I had learned before I started?

Thanks,
Dan

[BlueSkyJaunte]

I will be keeping a close watch on this thread. Our attic needs an overhaul as well.

[Heel n Toe]

Originally Posted by djmcmath

1 - Install soffet vents all the way around. Is there a thumbrule for sizing and number of these?

I heard about this product on the Gary Sullivan radio show (website is http://www.garysullivanonline.com/main.html ), and the text here says you should split the square inches of openings between the ridge vent and the soffit vent (or this product) 50-50:

http://www.dciproducts.com/html/smartvent.htm

[Heel n Toe]

Loads of "attic stuff" on the GS site: http://www.garysullivanonline.com/pages/gary_attics.html

Here he says the incoming-outgoing split should be 60-40:

This past weekend we had a lot of calls regarding a very hot attic that was making the AC run continuously. There are two things you can do to address that problem. First make sure the attic area is well insulated. The Department of energy has standards in regards to how much you need in your home. If you fall under their recommendation add either fiberglass or blown in insulation. It is also very important to have proper ventilation in the attic. Hot area rises and by having vents at the base of the roof that will push the hot air out of the attic will also help. For every 300 sq ft of home footprint you need 1 square foot of ventilation. 60% of that ventilation should provide incoming air and 40% should be outgoing air.

http://www.garysullivanonline.com/pages/gary_attics.html?feed=297384&article=4174044

[Burnin' oil]

Originally Posted by BlueSkyJaunte

I will be keeping a close watch on this thread. Our attic needs an overhaul as well.

+ a gazillion. Our attic is insulated with spider webs, cow manure dust, regular dust, dead bugs, live bugs, feathers, straw, redwood shavings, bare wires, and various odds and ends. But I have plans . . .

[Neilk]

What can you do about the older non-insulated CAN lights? I am sure those aren't helping either.

I was thinking about putting 5 gallon buckets around to insulate them somewhat. Otherwise, you aren't supposed to put insulation near them.

Neil

[djmcmath]

Ok, so for a 24x28 house, that's a 672 sqft footprint, which makes for 1.34 sqft of ventilation. That's 193 sqin of input and 129 sqin of output.

So, if I use 3" soffet vents, that's 21 of them. If I use the smaller 2" size, I'll need 30. I have about 20 trusses, so putting one in each truss bay seems logical, though a little overkill.

On the output side, the ridgeline is 28 feet long. If I open a 1" vent, that's 336 square inches of output -- more than double what I actually need.

Ok -- those are good numbers -- thanks for passing that link along.

Dan

[masraum]

My house was built in '67 and is pretty inefficient. We've got a 2 story house with one AC unit in the attic (except the outdoor unit, of course). The attic has a crappy mix of (90-95%) shredded (blown in) insulation and a little bit (5-10%) of the pink rolls.

We have a new roof with ridgevents (but the roof is angled front to back and side to side, so the vent is only 5'-6' long if that. We have soffit vents all of the way around the house.

I put a temp sensor in my attic to see what the temps up there did. The attic gets up to at least 120*+ during the day with full sun. At night, the attic will actually get a few degrees cooler than ambient, I assume for two reasons, heat going into the house, and the AC ducting in the attic being unsufficiently insulated. (all of it is wrapped).

I'd like to put in new pink rolls (assuming that's good stuff), reinsulate the AC ducts, and do something to the bottom of the roof to help keep heat out.

When we had the roof put on we were poor and didn't have them put in the insulation (wish we had).

So, is there anything that I can do to the bottom of the roof to help keep heat out, relatively easily?

Reinsulating the ducting seems like it shouldn't be that hard (other than working in the roof being horrible).

Rather than taking out the blown in insulation, could I just put more insulation on top of it?

[Heel n Toe]

Originally Posted by masraum

So, is there anything that I can do to the bottom of the roof to help keep heat out, relatively easily?

Yup... radiant barrier.
http://www.garysullivanonline.com/pages/gary_attics.html?feed=297384&article=4153939
http://www.ornl.gov/sci/roofs+walls/radiant/index.html

Reinsulating the ducting seems like it shouldn't be that hard (other than working in the roof being horrible).

Wait and do it in the fall or early winter.

Rather than taking out the blown in insulation, could I just put more insulation on top of it?

Yup... use blown-in cellulose.
http://rvalue.net/page5.html
http://www.diynetwork.com/how-to/how-to-install-blown-in-cellulose-insulation/index.html

::::::::

[svandamme]

is the attic used for living or not?
if not, why bother with a vent?
if it is, why not simply install a decent double glazed skylight?


if not used for living, not heated
you simply insulate the snot out of it, and that's it.
Then you insulate between the used/lived part of the hose, and the unused part.
That's where you put the vapour barrier.

you do not want moisture from the heated area condensing in the unheated part.
Do that, and you don't need a vent, hence you have even better insulation...

there's no use whatsoever building a vapor barrier in an unused, unheated attic, since that will simply cause more problems then do good.

[dad911]

I don't quite understand what you are trying to do with the 1x2, but vapor barrier should be on the conditioned (living) side of the insulation.

Per national building code:

Enclosed attics (and rafter space, like a cathedral ceiling) require cross ventilation. 1 square foot of ventilation for every 150 sq. feet of attic.

This can be reduced to 1 foot per 300, with 50-80% of the ventilation up high near the ridge, and the balance at the eaves/soffit.

Insulation shall not block the free flow of air. Minimum 1" space between insulation and roof sheathing.

If you are trying to get airspace, there is a product for that http://www.owenscorning.com/worldwid...f.30011372.pdf

[dad911]

Originally Posted by Neilk

What can you do about the older non-insulated CAN lights? I am sure those aren't helping either.

I was thinking about putting 5 gallon buckets around to insulate them somewhat. Otherwise, you aren't supposed to put insulation near them.

Neil

Frame around them, leaving an airspace.

[Racerbvd]

I have blown insalation in mine(about 13 years ago), just installed an solar attic fan & adding a few more vents to ensure a god flow.
A friend just had his attic strayed with foam http://www.advancedinsulationinc.com/newconstruction/sprayfoam/default.htm
and is very happy with it, says it did make a noticable difference, and here inn FL. that does ean something...

[djmcmath]

Originally Posted by svandamme

is the attic used for living or not?
if not, why bother with a vent?
if it is, why not simply install a decent double glazed skylight?

It is used for living. Or at least, it was designed that way, sort of -- we're pretty sure that the dormer was part of the original bungalow design, though we're less certain of the original access to the upstairs/attic. In any event, it's final purpose will be finished space, albeit somewhat shorter than optimal finished space.

Not sure how a skylight fills the same design purpose as vents -- seems like the vent concept is to get air flowing past, while a skylight is designed to let light in. ???

you do not want moisture from the heated area condensing in the unheated part.
Do that, and you don't need a vent, hence you have even better insulation...

there's no use whatsoever building a vapor barrier in an unused, unheated attic, since that will simply cause more problems then do good.

So in the final design on this thing, I'm basically finishing it (and conditioning the space) all the way out to the rafters. So the vapor barrier lives on the far side of the insulation because I don't want condensation from the vent path settling into the insulation. Right? Or do I put the vapor barrier on the occupied side, to prevent moisture from ... ?? Now I'm confused.

Dan

[djmcmath]

Originally Posted by dad911

I don't quite understand what you are trying to do with the 1x2, but vapor barrier should be on the conditioned (living) side of the insulation.

Per national building code:

Enclosed attics (and rafter space, like a cathedral ceiling) require cross ventilation. 1 square foot of ventilation for every 150 sq. feet of attic.

This can be reduced to 1 foot per 300, with 50-80% of the ventilation up high near the ridge, and the balance at the eaves/soffit.

Insulation shall not block the free flow of air. Minimum 1" space between insulation and roof sheathing.

If you are trying to get airspace, there is a product for that http://www.owenscorning.com/worldwid...f.30011372.pdf

The 1x2 is something for me to staple the vapor barrier or insulation to. It provides, if nothing else, a uniform guideline for the air gap between the roof and the first layer.

This is a little different than an enclosed attic -- it's finished space. That changes the requirement some, doesn't it? Or is the "1 foot per 300" still a valid thumbrule for a finished attic?

Dan

[svandamme]

Originally Posted by djmcmath

It is used for living. Or at least, it was designed that way, sort of -- we're pretty sure that the dormer was part of the original bungalow design, though we're less certain of the original access to the upstairs/attic. In any event, it's final purpose will be finished space, albeit somewhat shorter than optimal finished space.

Not sure how a skylight fills the same design purpose as vents -- seems like the vent concept is to get air flowing past, while a skylight is designed to let light in. ???

well, i installed a velux, i called it a roof window, then somebody told me you guys call em a sky light...

i guess velux is more then a skylight then ?


you can open them, but they also have a vent function where they aren't open , yet still vent

[jyl]

My attic has five skylights. Two open for light and air, three are just for light. If you go with skylights, install blackout blinds under them as well. Not expensive.

I can't quite tell from the pic, but the Velux appear to have the blinds integrated, and seem to open at both top and bottom. Very clever, those Old Worlders.