Insulation installation

[BillGalbraith]

I hope to install my attic insulation in my new garage soon. This might be worth 1,000 words:

I've seen faced insulation installed with the kraft paper stapled to the bottom of the truss, so that when the drywall is installed, it would be in contact with the kraft paper. From what I've heard lately, this ISN'T the way to go. It should be stapled to the side of the truss.

This got me thinking. Insulation works because it provides a dead space of air. So, wouldn't it be best if I could staple the kraft paper to the TOP of the lower truss, thereby increasing the dead air space to 3.5 inches over stapling it to the bottom of the truss (which isn't the right way to go anyway)? As I work my way across the attic, it wouldn't be hard to reach 2 feet over to staple it down, or I could staple it to the side of the truss at the very top.

What do the experts think?

Thanks

 

[IP2GSR]

I hope to install my attic insulation in my new garage soon. This might be worth 1,000 words:


This got me thinking. Insulation works because it provides a dead space of air.

Yup The air space within the insallation is what provides the ability to not conduct the termal energy.

So, wouldn't it be best if I could staple the kraft paper to the TOP of the lower truss, thereby increasing the dead air space to 3.5 inches over stapling it to the bottom of the truss (which isn't the right way to go anyway)?
What do the experts think?

Well I don’t know about the expert part but here is my opinion and worth what you are paying for it…. LOL

I think your dead air space beside the truss is the worst case scenario....
The problem is that dear air space will contain moisture and if there is a large temperature difference between the surface of the drywall and the surface of the truss or insulation then condensation will form. This in turns will rot the truss over sufficient time. . If the temperature outside and the temperature inside was always constant you would have nothing to worry about. However sun heating the attic or a furnace/air-con heating and cooling the living space would create this “sudden “ swing in temperature.

In addition the insulation is cut to just fit between the truss on 24"or 16" OC. Stapled to the top of the truss will leave a gap between the adjacent sections and provide a "bridge" between the attic air and your dead air space. The only thing separating it would be the kraft paper. not good IMHO

You should consult contractors in your area as well with regards to what side to place the kraft paper or vapor barrier. My understanding is the barrier is normally placed on the warm in winter side and is normal for most of the northern states and Canada. Since you are in Florida I have seen it reversed but you have to check the local building codes or contractors as I really have no experience in your region.

Dave

 

[timgr]

When you insulate a roof, your main concern will be condensation on the underside of the roof sheathing.

The usual approach is to create an air space above the insulation and below the sheathing, that is ventilated - typically the attic in a stick-built building. This is a COLD roof, because the sheathing is at the same (cold) temperature as the outside. The air flow and temperature in the attic (cold or hot, depending on the outside temperature) prevents humidity from the occupied space from condensing on and rotting the sheathing.

You can also have WARM or HOT roofs. A hot roof is popular in arctic climates and has the insulation sitting on top of the sheathing, with the inside of the sheathing exposed to the living space. The insulation on top of the sheathing (typically rigid foam) forms a vapor barrier on top of the sheathing.

A WARM roof has the insulation applied directly to the underside of the sheathing. This works fine as long as there is an impermeable moisture barrier between the humid living space and the sheathing. Any leaks will allow humid air to contact the underside of the sheathing, and the condensation rot the sheathing.

So, you can take any of these approaches, but you have to do one of them. Paper won't stop moisture, so if you use paper faced batts in any configuration, you'll need to have effective ventilation in the roof, typically soffit vents, ridge vents and end wall vents, location and size determined by code.

 

[BillGalbraith]

OKay, thanks for that information. I have vented soffit and off-ridge vents.