which size mash tun

Losing too many degrees in my tin right now. 70qt

Most my beers are 1.050-1.070 whats a better size? Just looking for opinions

I use a 10 gallon tun and only lose one degree over an hour long mash with a 1.050 beer. I'd suggest going. That route.

Have you checked to see if you've fully converted starch to sugar? You may not need to change anything other than the time you mash. Most of the modern malts finish much more quickly than they used to due to modern techniques and the efficiency and consistency of production.

Are you preheating the cooler? Get your strike water 15 degrees hotter than you wish to strike, dump it in the cooler, close lid and wait til it drops to strike temp, mash in, you should be fine.

Yes yes to both of you. I think its just badly insulated or something. I see steam leaving the cracks when its closed too

maybe you could take up headspace with a piece of foam....just my 2 cents from a rookie....Tom

I use the 10 gal. Rubbermaid / Home Depot cylindrical orange cooler. Most of my beers fall into the range specified, and it works just fine. I will lose maybe 2-3 degrees in a 60 min mash, and this makes absolutely no difference, as far as I can tell. The lids to these coolers are uninsulated, so I injected the lid with some of that "Great Stuff" insulation in a spray can, and that improved matters greatly. I also cover the cooler/MLT with a furniture pad while mashing.

I went with the Igloo 10g plus bulkhead kit as I do either 5 gallon batches of beer (mostly around the 1.050 to 1.070 mark)

This
+
this for $60 total.

I loose maybe 2 degrees an hour when mashing but then again I mash in temperatures around 68 degrees year round so I don't have that big of a temperature delta as someone from say, Minnesota or Maine does.

Edit: Just re-read your post saying you're using a 70q. That 10g might be a little too small for you. I just visited the Igloo page and saw these three coolers. If they can hold ice for 5, 5, and 7 days respectively, they should retain heat well too.
MaxCold
Marine Ultra
Yukon edit#2: $350?!!? nevermind......

I use the blue Igloo cube for my 5 gallon batches and I only loose 1 deg over the hour.