Insulation question

ChrisJ

Ok I'm kind of confused and hoping someone can help me out.



In our house we have an unvented and uninsulated attic, I know, not the best situation.  I realize the reasons an attic should be vented but at the same time we are currently using ours for storage as we have nice stairs going up and a floor to walk on.



What confuses me is, if my attic is unvented and sealed fairly well doesn't the air gap between the ceiling and roof have insulating properties?  My understanding is every inch of air gap gives you 1 R value.  So anywhere I have a foot of air I should have an insulating rating of R-12 as long as there is no infiltration, correct?



This would mean most of my attic actually has an R value of between R-24 and R-60 if I am understanding this correctly. 



Again,  maybe I'm looking at this all wrong?

STEVEusaPA

in theory you're right

Don't forget you need to consider that yes, it's an air gap, but also, heat moves to cold.  So when the attic is colder then the rooms below it, the heat will migrate into the attic, and vice versa.

More important reason for attic venting is moisture control.  Condensation will accumulate on the undersides of the roof sheathing and other surfaces, with mold right behind it.  So be on the lookout for mushrooms and mold.

One solution, albeit pricey, would be to have an insulation contractor come in and spray foam all the ceiling joist bays, and gable walls.

Less pricey would be to insulate the bays, making sure you provide an air space between insulation and the underside of the roof sheathing, put vents in the soffits, and use either a gable vent or full length ridge vent--and also insulate the gable walls.

ChrisJ

condensation

Mushrooms eh?



I kept an eye on it this past winter and never noticed any condensation or dampness.  Though at the same time my wife hated me as I refused to run a humidifier as I was aware of the problem and didn't want any extra moisture in the house.

icesailor

In Theory you're right/wrong: (take your pick)

Geez, I can't count high enough to tell you how many houses (up to and over 200 year sold) that have not a shred of insulation on the underside of roofs or any insulation at all. I can't then begin to tell you about all the houses I see that are less than 30 years old that are loaded with rot from being too tight.

Some one (maybe you) posted this discussion recently and showed a 50 to 100 YO house, uninsulated attic that had not a sign of rot or mushrooms growing anywhere on the roof structure.

If the floor between the ceiling below and the attic is insulated, the heat loss into the attic is nothing. If you had the old IBR H-22 Heat Loss Guide, it is explained and has tables showing it. The "new

 H-22 guide is made for low information technicians. Whenever you have an enclosed and unconditioned space, between the inside and the outside, the space is figured at one half the loss to the outside because if the room is 70 degrees inside and the outside is zero degrees, the average temperature in the enclosed space is 35 degrees. If you leave a pot of water on the floor, and it is zero outside, the water will not freeze. And I have never seen it freeze.

In spite of what all the experts are now saying, moisture from inside the house travels through the insulation and wall board, and ends up migrating through the sheathing to the outside. This is (in my opinion) a good thing. Stopping the natural migration is not a good thing. Spill water on your floor in the morning and leave it there. At the end of the day, it is gone. Where did it go? It was absorbed by the atmosphere.

SWEI

too tight

It's not all that difficult to get waterproof without being completely vapor proof, but it doesn't lend itself to the simple plastics we've been using.  There are both natural and engineered materials which can give us the best of both worlds (think Gore-Tex.)

icesailor

Insulation:

Fiberglass insulation is like Gore-Tex.

"Icy" insulation is an expensive $15.00 vinyl rainsuit from WalMart.

MikeG

Air Currents

As other have said in theory an air space provides some R value. The bigger the space creats the opportunity for convective air currents.  The underside of the roof will generally be colder than the floor so you will get air movement.  The same thing can happen in a wall cavity, and in some windows that have a large air gap between panes.  

ChrisJ

Interesting

I appreciate all of the responses and think I have an understanding now.



Thank you.

Roland_18

Insulation

Additional reading;

http://www.buildingscience.com/doctypes