Looks good for a bivy.
As for some of the questions on Tyvek, here is what Dupont says. I am paraphrasing from the Dupont website. Basically, there are three types (10, 14, and 16). Type 10 is hard structure, and 14 and 16 are both soft structure. The soft structure is what is used in garments. The hard structure is what homewrap is made of. Only type 16 has little pin holes. Tyvek is not waterproof, but does have a hydrostatic head. There are 8 types of hard structure. 6 of the 8 type of hard structure have a hydrostatic head of <15 in. of H2O. For the other two, their hydro head is 59 and 62 in. of H2O. These two are used in Medical packaging (so I am guessing that homewrap has a hydro head of <15). They don't have numbers for the soft structure hydro head (probably because it ain't that great). Anyways, the more water proof, the less breathable. So, by mixing a highly waterproof tyvek with one that breathes more, would make a good bivy.