First use of cooler mash tun: fail!; solution: cake dummy?

[FrizzleFry]

Ok, this past weekend I brewed a partial mash belgian white ale for which I was expecting an OG of 1.071, but got 1.056. Beersmith said my actual efficiency was around 45%, pretty craptastic. I am assuing it is referring to just the portion contributed by the mash, and that the extract is assumed to be 100%. I guess I just brewed a 'session beer'.
This was the first time I had used my new mash tun, which is an igloo 10 gallon with a bazooka tube. The fact that I had only 4 lbs of grain along with some searching here leads me to believe that my problem is temperature loss due to the large headspace.
I will be going all grain batch after next, as soon as my big pot, plate chiller etc., arrive. But to remedy the headspace problem if I undertand correctly I can set a foam disc on top in the mash tun for insulation? Is anyone doing this successfully?
I found this:
Goldas Kitchen Round Cake Dummy - 12" × 3"H
because it is about the right size, was designed to contact food, and every time I try to cut styrofoam I get a jagged edge and little bits that I dont want in my beer. But is it safe to contact something that hot? I know they make coffee cups out of it...

 

[GroosBrewz]

Not quite sure that temp is the culprit here. Did you get a beginning and final mash temp? As for the heat loss, I have used the plastic bubble wrap with much success.. Just cut out a piece and lay it on top.. I have a rectangle 13.5 gallon mash/tun and it definitely worked with heat loss..

 

[FrizzleFry]

I was a newb and forgot to take a final temp. Or even the temp after dough in. I just got the strike water up to 160. I will definitely measure next time.

 

[kh54s10]

Did you pre-heat your tun?

I put about a gallon of hot, but not boiling, water in the tun while heating my strike water. This heats the tun, then when I put in my strike water and grain I get my target temperature within a couple of degrees.

After stirring in the grain I close up the cooler and wrap with a blanket. In an hour I lose less that 3° F.

With a partial mash I would be surprised if you went from 1.071 to 1.056 based on temperature.

Did you do a partial boil and top up? If so I would think that the wort was not mixed well and you got a bad reading.

 

[FrizzleFry]

I didn't pre-heat no, i just heated the water to 5 degrees above my target temp. I will preheat as you suggest next time.
As for a bad reading, I did top up yes, i suppose it is possible.