Without seeing your setup, I'm going to guess that if you used any kind of insulation like a towel or coat or sleeping bag you would not lose enough in the first 20 minutes to severely affect your mash. That's only a guess though but suppose you lost 5 degrees during that 20 minutes. You would still be at mashing temperature but would create more short chain sugars making your wort more fermentable. Since this is planned to be a partial mash, this should not affect your beer to any great extent. If you were doing this as an all grain batch where you create all the fermentables you would notice that your beer was drier than you expected but it would still be quite drinkable.