Peace if possible, truth at all costs.

Keeping things calm is worth a great deal, and most disagreements are not worth having. But calm bought by saying something you do not believe is not really calm; it is a debt that comes due later, usually larger. The order matters. Smooth things over wherever you honestly can, and when you cannot, tell the truth and let the room be uncomfortable for a while.

Look at where you have been agreeing to keep the temperature down. Decide whether the peace you are protecting is real or only the appearance of one. If it is the appearance, say the plain thing once, kindly, without a speech attached.