Het dubbelglas verbetert de isolatie.

Because dubbelglas is a het-word.

A useful rule in Dutch is that compound nouns usually take the gender of the last part. Here, the last part is glas, and glas is a het-word, so dubbelglas is also het dubbelglas.

• het glas
• het dubbelglas

Dutch usually writes compound nouns as one word. So dubbelglas is a single noun, not an adjective plus a noun.

That is why Dutch uses:

• dubbelglas = the fixed noun for double glazing

and not normally:

• dubbele glas

Because it is a compound noun, the first part stays dubbel, without adjective endings.

In normal Dutch, the standard term is dubbelglas as one word.

If you write dubbel glas, it sounds more like a loose description, not the usual established noun. In everyday language, especially when talking about windows or home insulation, dubbelglas is the natural form.

So for this meaning, stick with dubbelglas.

Because the subject is third person singular: het dubbelglas.

The infinitive is verbeteren.
Its stem is verbeter.
In the present tense, Dutch adds -t for he/she/it and most singular subjects other than ik.

So:

• ik verbeter
• jij verbetert
• het dubbelglas verbetert

That is why the sentence uses verbetert.

Because isolatie is a de-word.

There is no perfect rule for every noun, but many Dutch nouns ending in -tie are de-words, and isolatie follows that pattern.

So:

• de isolatie
• de situatie
• de informatie

This sentence uses the normal Dutch main-clause order:

subject + finite verb + object

So here:

• Het dubbelglas = subject
• verbetert = finite verb
• de isolatie = direct object

This is very similar to normal English word order in a simple statement.

Yes. In this sentence, verbeteren is used transitively, meaning it takes a direct object.

• Het dubbelglas verbetert de isolatie.
  • het dubbelglas does the action
  • de isolatie receives the action

You can also use verbeteren without an object in some contexts:

• De isolatie verbetert.

That means The insulation is improving or The insulation improves.

So both are possible, but the focus changes.

You would usually say:

Het dubbelglas verbetert de isolatie niet.

Here, niet comes after the definite object de isolatie.

Also, you use niet, not geen, because de isolatie is a definite noun phrase.
Geen is used with indefinite nouns or to mean no/not any:

• Ik heb geen geld.

But here:

• de isolatie niet

is correct.

Usually dubbelglas is treated like a material/type word, so it often behaves more like an uncountable noun in everyday use.

For example, Dutch speakers often say things like:

• We hebben dubbelglas laten plaatsen.
• Dit huis heeft dubbelglas.

If you want to talk about individual physical pieces, Dutch often uses other words such as:

• ramen = windows
• ruiten = panes

So dubbelglas usually refers to the glazing type in general, not to one separate countable item.