I'm going to renovate the roof on my house from 1936. It's a mansard roof where the upper part is above the cold attic and the lower (steeper part) is an insulated exterior roof.
I have looked a little and it seems that the steeper part seen from the inside is constructed of
- tongue and groove with felt
- rafters with peat moss in between
- tongue and groove with felt on the outside
On this, battens are nailed on which the tiles lie.
I have received quotes from two roofers and both have quoted a diffusion-tight underlay felt. I'm wondering if it shouldn't be a diffusion-open felt. There doesn't seem to be an air gap. Isn't there a risk that moisture will be trapped in the steeper part of the roof if it's completely sealed on the outside?
Sincerely, Carl
I have looked a little and it seems that the steeper part seen from the inside is constructed of
- tongue and groove with felt
- rafters with peat moss in between
- tongue and groove with felt on the outside
On this, battens are nailed on which the tiles lie.
I have received quotes from two roofers and both have quoted a diffusion-tight underlay felt. I'm wondering if it shouldn't be a diffusion-open felt. There doesn't seem to be an air gap. Isn't there a risk that moisture will be trapped in the steeper part of the roof if it's completely sealed on the outside?
Sincerely, Carl
Diffusion-open is preferable in this construction, however, it's not certain that the paper you have today is. If it's not, you won't change the construction by choosing one that is not diffusion-open.
If you choose a diffusion-open one, then all the old paper must be removed if it isn't, as it would otherwise be of no use.
If you choose a diffusion-open one, then all the old paper must be removed if it isn't, as it would otherwise be of no use.
It was a very common construction in the 30s in various versions. Today, one wouldn't do it that way. The fact that it has worked anyway is probably partly because they have been heating inefficiently, partly because there is only hygroscopic material in the wall, and partly because the old underlay paper contained only half as much asphalt as is common today. Roofers are roofers and (usually) not structural engineers. They focus on sealing roofs and tried-and-tested working methods.
If you don't plan to add insulation or replace the old material, conventional underlay paper might work, but diffusion-open paper is certainly better. If you choose diffusion-open paper, all the old underlay paper must be removed, but not the interior asphalt paper (if there is such).
If you don't plan to add insulation or replace the old material, conventional underlay paper might work, but diffusion-open paper is certainly better. If you choose diffusion-open paper, all the old underlay paper must be removed, but not the interior asphalt paper (if there is such).
Okay thanks, then it feels like I should have it.
I checked with the companies I got quotes from and they had calculated on Monier Vittinge among others, which apparently is diffusion-open so they seem to know their stuff
I checked with the companies I got quotes from and they had calculated on Monier Vittinge among others, which apparently is diffusion-open so they seem to know their stuff