Attic insulation / condensation / velux tunnel

ElKavo

Ok quite a bit in the title there.

We have an 1850s terrace with the attic converted. This was done a long time ago, and it has lat and plaster ceiling with a new plasterboard and skim on top, and lat and plaster hip walls ( I think this is the correct name of it) with an insulated board skimmed on top of the lat and plaster on the hip walls.

Recently we had a velux installed in an internal bathroom. There is a shaft to this in the cold storage space (crawl space) If I am honest it was a piss poor job and will probably have to be redone in the future. We are also planning in the future to install dormers, so it will have to do until we raise the funds to do a complete roof replacement and dormer install.

The house has been very cold since the installation of the velux, I've been looking at the forum and I can see that this shaft should have been insulated. So yesterday I went up to the crawl space (cold storage) and discovered that it hasn't been insulated at all. While I was there I noticed that there was a lot of condensation around the window shaft and moisture on the ceiling of the toilet too. See pic below.

https://imgur.com/a/2Ja6c7G

I've added insulation to the shaft and around the ceiling too. I will monitor the moisture over the next few weeks to see if that has helped. The house already feels warmer, but this could be outside temps have increased. The insulation that is in the crawl space is old and has gone pretty flat. I'd like to top that up. Whats the best course of action here to reduce the condensation build-up, and to help keep heat in the house? Remove the old and add new fiber glass or bung it on top?

We are also intending on using a second room in the attic for an office, we had a large velux installed here too and again its fitted into the old lat and plaster with an extra layer of plasterboard on top. This room also has crawlspace like the other room with the same insulation. There is no insulation between the lat and plaster on the ceiling and the roof. Can anything be done here without removing the plasterboard and lat and plaster? Again, this should be something relatively inexpensive, sub €1k as it will be ripped out in a few years, I really just want to make it a useful and habitable space in the interim, so it can be used as an office without the need for additional electric heaters etc that are just heating the outside.

Quite a few questions here, thanks for any assistance.

10-10-20

That skylight tunnel should have had a vapour barrier on the attic side, especially as it's serving an internal bathroom.
As there is no vapour barrier and also no insulation, the heat is carrying moisture through the drywall and up toward the roof.
Does the bathroom have a shower? Is the bathroom mechanically vented? If not, it needs to be.

The moisture on the underside of that membrane is sweating, a sign that the membrane is colder than the surrounding area but it may not be solely indicative of an issue with the new works as you you might have a general vapour and ventilation issue from the living spaces over-all. How is the crawl-space in the attic vented?

I'd agree that the existing insulation looks to be beyond it's years, flaccid and ingrained with dirt and could do with an upgrade, but by upgrading the insulation you'll need to address the ventilation as every increase in the temperature and air-tightness of the living space will allow for more moisture to be held in the air, possibly leading to more sweating and damp-patches.

You might be able to use a paint-on moisture barrier on that plasterboard to reduce the rate of transfer, then wrap it with insulation (as per the other similar thread), but again, you need to look at the general ventilation and insulation issues and address these together.

Oh, and have a look out for and seal any gaps between the new tunnel and the membrane/window/flashing as this could be a major contributor to the moisture in the attic.

ElKavo

10-10-20 wrote: »

That skylight tunnel should have had a vapour barrier on the attic side, especially as it's serving an internal bathroom.
As there is no vapour barrier and also no insulation, the heat is carrying moisture through the drywall and up toward the roof.
Does the bathroom have a shower? Is the bathroom mechanically vented? If not, it needs to be.

Yes there is a mechanical vent there at present working as expected.

The moisture on the underside of that membrane is sweating, a sign that the membrane is colder than the surrounding area but it may not be solely indicative of an issue with the new works as you you might have a general vapour and ventilation issue from the living spaces over-all. How is the crawl-space in the attic vented?

There was never a moisture issue there previously, I have been in that part of the attic before, so I can only assume it's since the tunnel went in. We're in a terrace with a central valley between the houses, they are somewhat open at the end of joists / rafters over the wall plate where the lead from the valley lips over. I haven't closed these up at all and left them the way they were.

I'd agree that the existing insulation looks to be beyond it's years, flaccid and ingrained with dirt and could do with an upgrade, but by upgrading the insulation you'll need to address the ventilation as every increase in the temperature and air-tightness of the living space will allow for more moisture to be held in the air, possibly leading to more sweating and damp-patches.

So would you advise remove and replace, leaving the ends open as before? or what is the best course of action here?

You might be able to use a paint-on moisture barrier on that plasterboard to reduce the rate of transfer, then wrap it with insulation (as per the other similar thread), but again, you need to look at the general ventilation and insulation issues and address these together.

Any recommendations on a paint on barrier? We also have these vents in the roof too, So I presume these are the correct vents for that roof type? We are only in the house for about 1.5 yrs and renovating.

Oh, and have a look out for and seal any gaps between the new tunnel and the membrane/window/flashing as this could be a major contributor to the moisture in the attic.

YES! There are gaps at the window flashing too! I can see it from the crawl space, what would be best to close these up?

I should also say we are installing MHRV in the property too to keep fresh air in and moisture to a minimum, but that won't be finished for a year or so. We are pulling ducting etc ourselves so getting to them as we get to rooms etc.

Thanks for the reply too btw, I kind of got caught up in replying before I said that.

ElKavo

I actually have a decent amount of a tanking kit left over from the shower install. I presume this would be perfect for a paint on vapour barrier, thoughts?

10-10-20

I don't see an issue with it, but normally a latex based retarder or a vapour membrane is used.
https://www.ecohome.net/guides/2299/painting-on-your-vapour-barrier/
I'd like to hear other views though.

The gaps should be filled with a mastic which won't rot. Don't use silicon or painters calc.

On the insulation, I don't see any justification to retain the existing insulation, given its state. Personally I'd remove it and replace it with rockwool batts between the joists and an overlay of glasswool (that is if the rockwool has a higher thermal resistance to the glasswool).
You will need to dress the ceiling as per the recommendations for the insulation, most insulation manufacturers have excellent documentation for that, but retain any ventilation paths if they come from the soffits/eves.

But with any free advice off the internet, take it at face-value and ensure that you're getting the right solution for the age of the building.

ElKavo

10-10-20 wrote: »

I don't see an issue with it, but normally a latex based retarder or a vapour membrane is used.
https://www.ecohome.net/guides/2299/painting-on-your-vapour-barrier/
I'd like to hear other views though.

The gaps should be filled with a mastic which won't rot. Don't use silicon or painters calc.

On the insulation, I don't see any justification to retain the existing insulation, given its state. Personally I'd remove it and replace it with rockwool batts between the joists and an overlay of glasswool (that is if the rockwool has a higher thermal resistance to the glasswool).
You will need to dress the ceiling as per the recommendations for the insulation, most insulation manufacturers have excellent documentation for that, but retain any ventilation paths if they come from the soffits/eves.

But with any free advice off the internet, take it at face-value and ensure that you're getting the right solution for the age of the building.

Local suppliers only seem to have the rockwool batts in a 150mm, would the isofer 200mm roll's be comparable or better? Or is there a reason to choose mineral over glass?

Thanks for your help here too btw.

Its cool, I did a bit of research and found this this information. I thought I'd post it for others like myself.