what do I get if I press R19 into small space

walenta

I am wondering if I should just get R13 batts (which I think are 3.5 inch deep) or R19 - but if I press R19 into 4 or 5 inch space - how much R value is actually left - there does not seem to be a big price difference between R13 and R19 - is there enough gain to use "squashed" R19 over R13?
if there a formula?

chris

Clutchcargo

You get about R3.0-3.5/inch. So if you take an R23 and jam it into a 3.5" space, you'll only end up with about R13. In your case you'll get a bit extra because you have a little more room to fill.
There is a point of deminishing return, however, I'm not sure how much you need to compress it before that starts happening.

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RemodelMan

Considering that R value is based on how much heat is retained, you need to realize that the ratings are based upon ambient air flow from one area to the next. The purpose of insulation is to slow this heat(warm air) transfer from point A to point B. The more the fiberglass insulation is compressed the less air pockets are available. It's the multitude/volume of air pockets that slow this transfer of Radiant heat. I suggest that you install foam board/panels to compensate for the difference in the walls. You could also staple in a layer of "Reflectix" aluminum foil covered air bubbles or consider spray foaming first for a tighter seal.

AtlanticWBConst.

FWIW: If anyone were to use the Reflectix, it must be installed with another solid nailing surface attached over it (like strapping) - when sheetrock is intended to be applied.
That is per Reflectix's manufacturer direction.
(It doesn't say it on the packaging, we had to call the plant directly and speak to their technical engineer about that. Our concerns were regarding the drywall screws "popping", if sheetrock were to be applied directly over Reflectix, which is over studs/rafters/joists. Reflectix confirmed that they did not recommend sheetrock be installed directly overe that product for the same reasons... use strapping)

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- Build Well -

NateHanson

Is that also the case with rigid foam under drywall? Or is that stiff enough that an inch of that would be ok, without causing popped screws?

scorrpio

The real insulator is air. However, air all by itself tends to move, creating convection, drafts, and taking heat away from warmer surfaces and giving it off to colder ones.

The purpose of fiberglass, foam, etc, is to capture that air and keep it from moving around. You want to break up air into as small pockets as possible, while minimizing the amount of space consumed by non-air.

What does this mean? That a non-compressed R-13 sitting in a 3.5" stud bay holds all the air there is with minimum amount of fiber. But when you stuff an 5.5" thick R-19 in there, you squish more fiber into same sace, leaving less room for air. End result: an R-value LESS than 13.

If you want higher R-value, you need to consider other materials - like that spray foam. Or rigid insulation like XPS, which has R of 5 per inch. (resulting in about R-17 in a 3.5" bay)

RemodelMan

Well said about avoiding the Reflectix between wood/metal joists and drywall.

You can however, acheive satisfactory results by stapeling the Reflectix along the left and right hand sides of studs in a "U" fashion. Then insert the fiberglass R-13. I'm in the camp which believes that the spray in poly-foam is the absolute best for a watertight/condensation, heat and sound retention/reflection as well.

Recommend that you hire a seasoned pro for this type of project.
Then, take the famly out of town for an overnight until the place airs out.

chris33

Most of the fiberglass batts I've worked with are layered so that you can quite easily peel off a bit to achieve the thickenss you want. Some brands of insulation are better than others for this.
The thin layers you peel off (1.5" if you need 4" batts) could be used 3 deep to fill every 4'th cavity. i.e. make 3, 4" batts from the 5.5" batts and use the 3, 1.5" layers toghether as a 4'th batt.
This all depends on having insulation that has distinct layers that are easy to seperate. Some flimsier insulation doesn't come apart so neatly.
Just an idea.