To improve the roof or let it be for now

[Garald]

Judgement call (yes, I know it's a decision that has to be made, and I am already leaning to one side, but it's good to know what others think):

 

As some people here are already tired of hearing, I am renovating a duplex/triplex (most of a small house) I bought recently near Paris. It's a pretty major, if not quite total, renovation, led by an architect. We are putting insulation (cellulose-based on the side of the street, thin layer of cellulose + reflective on two other sides) all over. 

 

The one place that was already insulated is the ceiling. You can see some pictures here, from when the insulation was taken apart, prior to the start of work on new skylights:

 

 

As you can see there, it's just rockwool, without any sort of rain or humidity protection.

 

Option 1: Leave it essentially as it is (or rather: put the rock wool back in), adding just a membrane to protect the insulation from humidity coming from the inside, as well as insulation in one missing spot, and some sort of airtight protection at the places where one can access the insulation, to protect us not just from inhaling any rock wool, but, perhaps more importantly, from inhaling any mold that might ever form (even if we hope it won't). (Some family members who will visit often have severe mold allergies.) The cost will probably be around 1000 eur + taxes.

Option 2: Putting in new insulation (cellulose-based?) and an anti-rain protection layer, together with sound-proofing. Cost: 12000 eur + taxes.

 

One important factor about option 1 is that it would make economic sense (if I ever have the money) to eventually raise the attic roof, or even add a full new floor to the house (should the foundations permit it). Then all the money put into improving the ceiling now will be lost, says the architect, who clearly leans towards Option 1.

 

Opinions?

[Iceverge]

You need to consider moisture. 

 

1. If rain blows up underneath your tiles it will drip onto the insulation and hopefully just dry to atmosphere eventually. There's a chance it'll make its way inside after a period if you get sustained bad weather. 

 

This is normally solved by having a waterproof membrane underneath the roof covering such than any errant drips make their way back to the outside of the house via gravity. 

To do this in a retrofit you'll need to strip the tiles to lay the membrane. In any case it's an old house and you might just have very well-fitting tiles that keep all the water out. Any tiny drips are probably caught by the mineral wool and later evaporated away without dripping inside. 

 

 

 

2. Secondly any Inside air that makes its way outside will be carrying water vapour from your house (showers, breathing etc). As soon as it meets the cold it'll condensate and form water droplets on whatever it touches. You can see this when moist air hits a cool bathroom window. It's probably happening at the moment inside your insulation layer somewhere or on the back of the tiles. It'll cause mould and eventually rot in the timbers. 

 

This is solved by having a total and complete vapour control layer inside the insulation of your house. This prevents any moist air getting cold by passing to the outside, hence no problem with condensation. As a secondary precaution exterior air is allowed to ventilate above the insulation to take any condensation away. 

 

 

 

 

In your situation you have 3 options in my opinion. 

 

1. Remove the tiles, fit a new breather membrane like solitex+ or siga majvest draped into the eve chutes , batten counterbatten (to create a ventilation gap) and then replace the tiles. Then inside insulate between the rafters with mineral wool (or blown cellulose) and under the rafters with more mineral wool or insulation boards. Then a vapour control layer and then plasterboard. 

 

 

 

2.  An option I don't particularly like but should work is to spray closed cell foam on the back of the tiles in situ. It'll stop any water blowing in, create a vapour barrier and insulate. However, it'll glue your roof together like chewing gum in a toddler's hair. It's expensive. If your supplier encases any timbers in it there's a risk they'll decay over time if they can't dry out/ regulate their humidity. 

 

3. Similar to your architect's choice. I've said it before, but I think she knows her stuff and you've been lucky with her.  

 

 A.  Strip the roof of existing plasterboard and insulation. Really study the timbers to see if they're rotting. 

 B.  Looking at the tiles from the back use suitable historically sympathetic sealant to close any small holes between the tiles where rain may blow in. 

 C.  Fit mineral wool between the rafters. I wouldn't use anything impenetrable like PIR for 2 reasons. Any tiny rain drops that gets blown in will instantly run off and find its way inside rather than remaining in the wool       and evaporating later. Secondly I suspect the roof has been drying to the inside and you really should not trap moisture in there. 

 D. Add a good variable control layer. Intello plus or similar. Tape this extra dilligently. 

 E. Cross batten and insulate with another layer for Rockwool for a service cavity. 

 F  Plasterboard. Fireboard or Silentboards are denser for sound protection. 

 

 

 

 

 

 

 

 

 

[Garald]

Thank you! What tool is that?

[Iceverge]

U-value calculator | ubakus.com

 

You'll have to create an account but it's hours of fun.