Hey Trent,
Great question - I've wondered the same about how the refinancing works too. Just want to simplify your scenario:
1. Purchase at $100k with 20% down = $20,000 Investment
2. Pull a Mortgage for $80,000 at 4% int which equals approx $630/mo expense
3. Spend $10,000 to add $50,000 in Value to the property (Total upfront capital invested: $30,000)
4. Estimated monthly expenses: $1006 vs Estimated Revenue: $1000 = Negative CF
5. Refinance at appraised value of $150,000 with 80% LTV gives you $120,000 worth of equity
6. Existing mortgage of $80,000 nets negative CF ... so how does $120,000 mortgage improve the circumstances?
I think with a BRRR strategy the point is to put that money you've generated (from wise construction and appreciation decisions) to work elsewhere. You've essentially generated $40,000 with $30,000 which has multiplied your original investment by 1.3. The sacrifice is that you now increase your debt which means your payments will rise... but the trade off is that now you can buy a second piece of real estate with that money and keep the cycle going.
I'd be curious to hear from a lender/broker how the refinancing adds up too. Is it also assumed it takes on the interest rate of the existing mortgage? What if mortgage rates have changed by the time you refi? Can you only refinance at the end of the term of the mortgage? (P.s. I am Canadian so we may have different rules)