L'isolamento è dietro a questo intonaco.

Because isolamento is a singular masculine noun that begins with a vowel. In Italian, il becomes l' before a vowel:

• il libro → consonant, so no change
• l'isolamento → vowel, so il elides to l'

This is called elision. It is very common in Italian.

The accent is important because è and e are different words:

• è = is
• e = and

So in this sentence, è is the 3rd person singular of essere:

• L'isolamento è... = The insulation is...

The accent helps you avoid confusing the verb with the conjunction.

Dietro means behind. In this sentence, it tells you the location of the insulation.

It is one of several Italian location words like:

• davanti = in front
• sopra = above/on top of
• sotto = under
• dietro = behind

So è dietro... means is behind...

Italian can say both:

• dietro questo intonaco
• dietro a questo intonaco

Both are used. The version with a is very natural and common.

So here, dietro a questo intonaco means behind this plaster.
The a does not really change the meaning; it is just part of a common way of linking dietro to what follows.

Because contractions such as al, allo, alla, ai, agli, alle happen only when a preposition combines with a definite article:

• a + il = al
• a + lo = allo

But questo is not an article. It is a demonstrative adjective meaning this. So you keep them separate:

• a questo
• not al questo

Because questo already works as the determiner for the noun.

In English, you also say:

• this plaster
• not the this plaster

Italian works the same way here:

• questo intonaco
• not il questo intonaco

So questo already fills the role that the or this would fill before the noun.

Because intonaco is masculine singular, and questo has to agree with it.

Agreement here is:

• masculine singular → questo
• feminine singular → questa
• masculine plural → questi
• feminine plural → queste

So:

• questo intonaco
• but questa parete

This is a basic Italian agreement pattern: articles and adjectives must match the noun in gender and number.

Yes, both are masculine singular in this sentence.

You can see that from the words around them:

• l'isolamento
• questo intonaco

A very common pattern in Italian is that nouns ending in -o are masculine singular, and both isolamento and intonaco follow that pattern.

That said, -o = masculine is a useful rule of thumb, not an absolute rule for every noun in the language.

Yes. That version is also possible and natural.

So both of these work:

• L'isolamento è dietro a questo intonaco.
• L'isolamento è dietro questo intonaco.

The sentence you were given is perfectly good Italian; the version without a is just a slightly different way to say the same thing.

It is natural, especially in a practical or building-related context.

A native speaker might also say things like:

• C'è l'isolamento dietro questo intonaco.
• L'isolamento si trova dietro questo intonaco.

But L'isolamento è dietro a questo intonaco is absolutely fine and clear.

A simple English-friendly approximation is:

• L'isolamento → ee-zoh-lah-MEN-toh
• è → eh
• dietro → DYEH-troh
• a → ah
• questo → KWEH-stoh
• intonaco → een-TOH-nah-koh

A few helpful points:

• Stress falls on -men- in isolamento
• Stress falls on the first syllable of dietro
• Stress falls on -to- in intonaco

So the whole sentence is roughly:

ee-zoh-lah-MEN-toh eh DYEH-troh ah KWEH-stoh een-TOH-nah-koh