As far as I know, one way to take a homotopy colimit in a model category is to replace (up to acyclic fibration) all arrows in the diagram with cofibrations, and take the strict colimit of the resulting diagram.


In Top with the model structure given by Serre fibrations, cofibrations, and weak equivalences, if one wants to obtain a homotopy pushout of the diagram $X \leftarrow A \rightarrow Y$, it is "enough" to replace only one of these arrows with a cofibration: that is, there is a natural map (by the universal property of the pushout) $Cyl(X)\cup\_A Cyl(Y) \to Cyl(X) \cup\_A Y$ that is a homotopy equivalence of spaces.



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> Question 1: What conditions on the model category $\mathcal{C}$ (or objects $X,Y,A$) will guarantee that the natural map $Cyl(X) \cup\_A Cyl(Y) \to Cyl(X) \cup\_A Y$ is a weak equivalence?
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> Question 2: This question is less precise, but if the map above is a weak equivalence, does that mean $Cyl(X) \cup\_A Y$ is a good model for the homotopy pushout?
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Homotopy Pushouts via Model Structure in Top
Question 1: The model category $\mathcal{C}$ should be *left proper*, i.e. the pushout of a weak equivalence along a cofibration is again a weak equivalence. (Dually, there is a notion of right proper.) Top is left proper, as is any model category in which every object is cofibrant, such as SSet. There is some information on this notion of properness [at the nlab](http://ncatlab.org/nlab/show/proper+model+category), and I think it's also discussed more thoroughly in Hirschhorn's book *Model Categories and their Localizations* (and probably many other places).


Question 2: Yes. People often say that a square in a model category is a homotopy pushout square if the induced map from the (strict) pushout of a cofibrant replacement (meaning cofibrant objects and maps) of the "initial" three objects to the last object is a weak equivalence, and that is the case here.


