How to compute the foreign exchange volatility within a portfolio

Suppose I have a portfolio of 5 assets. Assets 1 and 2 have foreign exchange exposures and therefore foreign exchange volatility. How can I calculate the marginal contribution to the total portfolio volatility from the individual foreign exchange exposures?

If $R$ and $r$ are the return on the portfolio after currency hedging and on the currency, if I write $V(\cdot)$ for variance, and a fraction of $t$ of the portfolio is exposed to currency risk, then the return of the unhedged portfolio is $R+tr$. Then: $$V(R+tr) = V(R) + 2t\mathrm{Cov}(R,r) + t^2 V(r)$$ so the marginal contribution (derivative with respect to t) is $$\frac {\text d} {\text d t}V(R+tr) = 2\mathrm{Cov}(R,r) + 2tV^2(r).$$