/grin Only like 20 hours, that is not long, don't worry.

So you are in fact building your wife's mother a nice bungalow to live in for the next decades.

I guess this is a very interesting project for the regular contributors here, and many will have good advice on many aspects of the build. I can't say much about your walls and the vaporbariers, I'll leave that to other people.
To me it sounds like you did well with the floor. I understand there is no crawlspace, you checked and found the concrete slab to be dry, and insulated it from the inside, sounds good.

Can you provide us with measurements of the livingspace? Total surface, a floorplan? (Make a drawing on paper and take a picture : -) That will help us to get a general picture.

So far you are doing a good job at sealing the place, keeping moist away from where you don't want it, but now you have to pay attention to ventilation. You mentioned a "90% gas furnace", is that referring to a not so modern central heating device (90% efficiency but working fine so reuse it), where does it aquire it's fresh air from? (dedicated outside-air inlet or from the room it's in?)

I'm not the original poster. :confused:

24' x 36' outside measurements of concrete blocks. yes, 90% is an older gas furnace that is working fine. combustion/exhaust is 2" pvc inside 3" pvc. The threads on dryer vent heat exchangers has caught my eye, but i'm not sure how many things like that i will be able to incorporate into this project because i am doing all the labor myself and my mother in law is anxious to get moved out of the death trap. My wife is an only child and mother in law is on social security so I am the bank on this project. That is why I am trying to hold costs down.

I can't find anything for 8" cavities, but compressing R15 insulation by 1/2" it's Rvalue drops by R2 for each and every 1/2 that you compress it.

that looks like it might be pretty much proportional. A 1/7 reduction of depth results in a 2/15 reduction in R value. If you assume there was some rounding in the data that you found it may be even closer. If you make the same assumption about the 8" insulation(that its r-value is proportional to it thickness) then the 8" @R-25 compressed to 75% of its thickness should end up at about R-18.75 which is what fornax speculated earlier.

Do what charles fl said, and put 4 mil poly over it on the inside.

Here is a drawing of what I am doing.
The corrugated steel guy recommended I use a vapor barrier behind the steel. After reading AC Hacker's input about thermal gradient I think it would be better to put it inside the house between the drywall and the studs. I think I won't put any behind the steel because 2 layers of vapor barrier separated by 15-16" might trap moisture between layers. What do you think?
*** tried to post picture but I don't have enough posts. this should be my fifth post so I'll try again after this

http://ecorenovator.org/forum/attach...1&d=1357838156

hope this worked.

Well, if you're starting from scratch, and you're doing a concrete pad and masonry walls, there are other interesting possibilities...

If your doing a slab over gravel, you've already got the hardest and most expensive part of a radiant floor in the works.

The addition of PEX heating tubing would be a minor expense that would yield major comfort and efficiency.

And also, if you're going with masonry walls, you have significant thermal mass that could be put to good use.

To exploit this approach, you'd need to recognize that the foundation as you have it pictured is a major heat sink directly connected to the earth (the mother of all heat sinks).

You would want to thermally isolate your foundation from the earth (not a huge increase in cost).

To thermally isolate your foundation from the earth (the gravel bed gets you part way there) you'd use some rigid foam and you'd want to make sure that there was a good vapor barrier under the slab (maybe under the gravel).

Then you could put your insulation on the outside of the thermal mass (three
inches of rigid foam would be pretty nice.

There you'd have a comfortable, efficient, quiet radiant floor and your walls would provide additional moderation of temperature swings, helping to keep the house cool in the summer and warm in the winter.

Think about it... just a few changes and it would be a very different house, and much easier & cheaper to heat & cool.

Best,

-AC

A VB on the inside of an concrete building is exactly how solar vapor drive issues arise. I would skip it. Putting insulation from a known rotting house into that...... Also, who wants to live in a room coated with plastic if they don't have to?