Insulating 1880's Granite Home

[XAVIER622]

Hello. Looking for some advice. Recently moved into a granite home built in 1888.

The lady I bought off told me the attic had been insulated during her time but never got the chance to look until moving in.

Having stuck my head in, there is shrivelled up rockwool over parts of the joists max 100/50mm thick (A on the diagram). I wanted to see what others on the forum with more experience would do.

 

The Joists (A) are 42cm wide and 22cm deep and the rafters (B) are 17cm deep and 40cm wide. The walls (C) of the internal attic room are old lath and plaster.

I have no issues getting my hands dirty. Rather then removing and replacing the rockwool with new and thicker amounts would I be best the remove the walls of the attic room and insulate with 120mm PIR board between the rafters and 25mm under them to create a cold roof?

 

I have access through a friend to cheap PIR boards which is why I am leaning that way. 

Then it would allow me to plasterboard where the old internal walls were with a comprehensive insulation of the whole roof?

Any advice on alternatives welcomed. Thanks in advance.

Edited January 30, 2023 by XAVIER622

[ProDave]

It's funny what some peoples idea of "insulated" is.

 

No point removing what is there, that is just a horrible, messy job and creates the waste to dispose of.  I would leave it there and add more on top, taking care to keep all the wiring on top of the insulation not buried between layers.

 

Re the old loft room.  Line it with PIR with the joints taped leaving a gap between the PIR and the actual roof for ventilation then plasterboard over.

[gambo]

1 hour ago, ProDave said:

... taking care to keep all the wiring on top of the insulation not buried between layers.

How would recommend dealing with shorter wires which are too short to pull them on top of insulation?

[RichardL]

With care if you release the cable clips from the rafters + the remaining rafter height you could probably tuck insulation underneath without disturbing the wires -or- potentially tuck it under without even removing clips.

Either with thinner insulation types, or i'd be inclined to compress the insulation a little along that edge vs. anything near a re-wire?

[MikeSharp01]

They sure knew how to make rafters big enough back in the day didn't they!

[SteamyTea]

23 minutes ago, RichardL said:

With care if you release the cable clips from the rafters + the remaining rafter height you could probably tuck insulation underneath without disturbing the wires -or- potentially tuck it under without even removing clips.

Either with thinner insulation types, or i'd be inclined to compress the insulation a little along that edge vs. anything near a re-wire?

Or derate the circuits at the consumer unit.

[gambo]

4 hours ago, SteamyTea said:

Or derate the circuits at the consumer unit.

What would it change?

[SteamyTea]

16 minutes ago, gambo said:

What would it change?

The load you can put on a circuit.

There are tables that how how much you have to derate cables by depending on the amount of insulation they travel through.

[gambo]

37 minutes ago, SteamyTea said:

The load you can put on a circuit.

There are tables that how how much you have to derate cables by depending on the amount of insulation they travel through.

So derating circuit will mean the fuse switches off before the wire gets too hot because of to thin cable being laid under thick insulation. Is this what you are suggesting?

If i can stick insulation under the wires there will still be a short piece of wire (30-50cm) under insulation. Would this not cause an issue?

Sorry if it's a dumb question!

[SteamyTea]

1 hour ago, gambo said:

So derating circuit will mean the fuse switches off before the wire gets too hot because of to thin cable being laid under thick insulation. Is this what you are suggesting?

Yes.

1 hour ago, gambo said:

If i can stick insulation under the wires there will still be a short piece of wire (30-50cm) under insulation. Would this not cause an issue?

Yes

 

Hopefully @ProDave can enlighten us all more as he does this every day and probably has a copy of the latest regulations.

[saveasteading]

7 hours ago, MikeSharp01 said:

how to make rafters big enough back in the day didn't they!

This would probably be local (uk) timber. Fast growing, wide grained and  less strong than Finnish/ Russian/ Canadian.

There are loads of timber mills round Inverness and the structural timber is all C16. Thus bigger sections are required and not cheaper than the imported stuff. I asked and they said they couldn't compete, even with Russia out of the equation.

In the words of a Finnish timber supplier...our timber is the best because the forest land is so cold and so flat.

 

 

[XAVIER622]

On 30/01/2023 at 08:51, ProDave said:

It's funny what some peoples idea of "insulated" is.

 

No point removing what is there, that is just a horrible, messy job and creates the waste to dispose of.  I would leave it there and add more on top, taking care to keep all the wiring on top of the insulation not buried between layers.

 

Re the old loft room.  Line it with PIR with the joints taped leaving a gap between the PIR and the actual roof for ventilation then plasterboard over.

 

Great. Thanks for the advice all. I think I will go ahead with what ProDave suggested.