foam verses fiberglass

[frozenb]

I live in Northern WI, it gets cold here. I was wondering if there is a point at which you are getting a zero return on insulation such as if you had R100 in your walls and added more to reach R200. I'm guessing you probably wouldn't save any heat by doing that. Is there a R value above which there is no return?

I'm building a new home and am going to insulate soon but I'm still unsure what way to go. I've got it sheeted with R6.5 reflective foam. I can get it up to R19.5 with R13 fiberglass in my 2x4 walls but with spray foam I gan get it around R25. Is it worth it for the cost? Its quite a bit more to spray it.

I've got radiant slab heat.

[suemarkp]

There is always a return, but the cost of that insulation can be way over what it saves in BTU's.

The R value is resistance to heat flow. Most uninsulated walls of wood construction are R1. Putting in R8 would reduce the heat loss by a factor of 8. Increasing then to R16 would cut the heat loss in half from what it was. Going to R32 would then cut that value in half. This continues, but realize that each time we're halfing the heat loss we're doubling the amount of insulation.

If you actually calculate the heat loss, your wall will be down to almost nothing pretty quickly once you've hit R16. An uninsulated wall may have been losing 3200 BTU-Hr will now lose 200 BTU-Hr at R16. Would it be worth the money to change the walls to R32 to save 100 more BTU-Hr? I think not.

At some point, it is more cost effective to work on other big heat losers -- windows, doors, and air infiltration.

[frozenb]

Thanks Mark that was a very understandable reply.

However that begs another question. Why is the code (here anywhy) R19 in the walls and R 38 in the cealing? I'm assuming its because it's easier for the heat to escape vertically because it accumulates there.

Also is there anything that's a super insulator but not practical for home use?Just wondering.
Thanks again

[Wgoodrich]

Two concerns apply to your quest. One is you can only gain R value as long as the fiberglass is not compressed. Compressing fiberglass greatly reduces R value.

Next concern is healthy air. While you have windows that will open many rooms often have no windows that will open. In this condition you need to remember you require about 7.5 cubic feet of fresh air intake per minute for each person living in that habitable room to remain within minimum health standards.

Also if you make the house too tight your fireplace will not draft up the chimney to rid the fireplace of smoke.

Yes ceilings need higher R value due to heat rises affect.

Just adding food for thought.

Wg