Dirty Caesars Cipher Algorithm (solution spoiler!)

1

So basically I did the last challenge, the Caesars Cipher. The algorithm is fine, works, yet I think that it is somewhat dirty (meaning using not a lot of methods at all, and over-complicating something simple).

function rot13(str) { 
	var strFinal = "";
	for (var i = 0; i < str.length; i++) {
		//This segment of code transforms from A to M to N to Z
		if (str.charCodeAt(i) >= 65 && str.charCodeAt(i) <= 77) {
			var charCode1 = str.charCodeAt(i) + 13;
			charCode1 = String.fromCharCode(charCode1);
			strFinal += charCode1;

		}
		//This other segment does the opposite
		else if (str.charCodeAt(i) > 77 && str.charCodeAt(i) <= 90) {
			var charCode2 = str.charCodeAt(i) - 13;
			charCode2 = String.fromCharCode(charCode2);
			strFinal += charCode2;

		} 
		//That one sums to our final string non-alpha non-uppercase characters.
		else {
			strFinal += str[i];
		}
	}
  	return strFinal;
}

I’d like to read your opinions. Is this algorithm a good solution, or it is too heavy? What do you think?