why is there a length difference between the word "house" in 'greenhouse' and 'green house' when you say the word?

I know it takes longer to say 'house' in "green house" but why does it take longer?

please help me. I am so confused :(

I don't know that there is. When I pronounce "greenhouse" and then "green house," I hear no difference in how the word/syllable "house" sounds. The difference in the two -- to my ears anyway -- is in how "green" is pronounced in each one: slightly longer time in the phrase than in the compound word.