Sitä mukaa kun lämpöhäviö kasvaa, tiivistämme ikkunat.

It is a fixed expression meaning something like as, in step with, in proportion as, or the more/while something happens.

So in this sentence, Sitä mukaa kun lämpöhäviö kasvaa... means:

• As heat loss increases...
• In step with the increase in heat loss...
• The more heat loss increases...

It often describes two processes happening alongside each other gradually.

Because kun here introduces something that happens as/when another process is happening, not a simple condition.

• kun = when / as
• jos = if

So:

• Sitä mukaa kun lämpöhäviö kasvaa... = As heat loss increases...
• Jos lämpöhäviö kasvaa... = If heat loss increases...

The Finnish sentence is not mainly about a hypothetical condition. It describes a matching or parallel development.

Sitä mukaa is best learned as a set phrase.

The word sitä is the partitive form of se, but in this expression you usually do not need to analyze it literally. The whole phrase sitä mukaa (kun) functions idiomatically and means as / progressively as / in step with.

So although the pieces can be analyzed historically, for a learner the most useful thing is:

• memorize sitä mukaa kun as one expression
• understand it as a connector for gradual parallel change

Because it is the subject of the verb kasvaa.

• lämpöhäviö = heat loss
• kasvaa = grows / increases

In Finnish, the subject is often in the nominative basic form, and that is what we have here:

• lämpöhäviö kasvaa = heat loss increases

There is nothing in the sentence that would require another case here.

Because its subject, lämpöhäviö, is singular.

Finnish present-tense agreement works like this:

• minä kasvan = I grow
• sinä kasvat = you grow
• hän/se kasvaa = he/she/it grows
• me kasvamme = we grow

Since lämpöhäviö is a singular thing, the verb is kasvaa.

It is built into the verb ending.

• tiivistämme = we seal / we make tighter

The ending -mme marks 1st person plural, so Finnish does not need a separate pronoun me unless you want emphasis.

Compare:

• tiivistän = I seal
• tiivistät = you seal
• tiivistää = he/she/it seals
• tiivistämme = we seal

So the English subject we is contained inside the verb form.

Here tiivistää means something like:

• to seal
• to make airtight
• to tighten up
• for windows specifically, often to draught-proof

So tiivistämme ikkunat means we are doing something to the windows so that less air or heat escapes.

Depending on context, English might translate it in different ways:

• we seal the windows
• we make the windows tighter
• we draught-proof the windows

Because ikkunat is the total object here.

Finnish often distinguishes between:

• a total object: the action affects the whole object as a complete unit
• a partitive object: the action is partial, ongoing, incomplete, indefinite, etc.

Here:

• tiivistämme ikkunat = we seal the windows / the whole set of windows
• tiivistämme ikkunoita would suggest something more like we are sealing windows in a more partial or indefinite sense

In the plural, the total object often looks like the nominative plural, so ikkunat is correct here.

Yes.

You can also say:

Tiivistämme ikkunat sitä mukaa kun lämpöhäviö kasvaa.

This means the same thing. Finnish allows this kind of flexibility.

The original version puts the as heat loss increases part first, which gives that background information before the main action.

It can do both, depending on context.

Finnish often uses the present tense where English might use either the present or the future.

So tiivistämme ikkunat can mean:

• we seal the windows
  or
• we will seal the windows

And lämpöhäviö kasvaa can mean:

• heat loss increases or
• heat loss will increase

In this sentence, it can describe:

• a general rule
• an ongoing process
• a planned future response

The exact time reference comes from context, not just the verb form.

Yes.

It is made up of:

• lämpö = heat
• häviö = loss

So lämpöhäviö literally means heat loss.

Finnish uses compound words very freely, so long technical nouns like this are very common.