replace the value in one table conditional on the value in another table : I have the following data frames:

    df_1 <- data.frame(f1= c(1,3,4,5,7,8), f2 = c(2,3,4,1,4,5))
    df_2 <- data.frame(f1= c(0.1,0.3,0.04,0.015,0.7,0.8), f2 = c(0.02,0.13,0.4,1.4,0.04,0.5))

so they look like

    > df_1
      f1 f2
    1  1  2
    2  3  3
    3  4  4
    4  5  1
    5  7  4
    6  8  5
    > df_2
       f1   f2
    1 0.100 0.02
    2 0.300 0.13
    3 0.040 0.40
    4 0.015 1.40
    5 0.700 0.04
    6 0.800 0.50

The replacement I wish to perform is:

If one figure in df2 is higher than 0.05, I wish to replace the figure in df1 at the corresponding position with NA. The resulting data frame df1 should look like

      f1 f2
    1 NA  2
    2 NA NA
    3  4 NA
    4  5 NA
    5 NA  4
    6 NA NA
I have tried to solve it using a for loop but it will take a long time when applied to my actual large table. I know there could be a quicker way using data.table but I don't actually know how. Can someone help me with this?

Many thanks!

Bill