Soldato
- Joined
- 28 Dec 2017
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So on purchasing our first house, we discovered the exterior walls are timber framed. Not only that but very flimsy - the entire wall is effectively a lightweight stud wall nailed to the side of the building. It's then got about an inch of fibreglass type fluffy insulation and is skinned with felt and has tiles hung on the outside.
So these walls are about 2 inches thick, literally, and mostly air gap. They also wobble and flex!
We have been preparing to improve insulation by building out the studwork internally and fitting PIR board to the depth of the new studs. This will leave the existing air gap in place for, hopefully, the same amount of ventilation as the walls had before. The timbers don't seem to be rotten or damp at all, so it seems a reasonably safe starting point.
What has occurred to me now we get into the job though is that the temperature gradient through the wall is going to be stretched and moved. Right now, presumably the insulation on the cold side of the timber is relatively warm and so would the felt be. With 75-100mm added PIR, that air gap and felt layer might become considerably colder. I don't know that there is any real ventilation at all. So my concern is that we would be risking interstitial condensation on the original timbers.
Ok - so perhaps mitigation number 1: add a vapour barrier over the insulation before plasterboarding, on the warm side of the new timbers. Should keep moisture from the living space out of the wall. Complication number 2: the timber frame making up the exterior wall is continuous between floors. So if I insulate the floor level walls on one floor, some of the original timbers exist blow floor level, and also at ceiling height in the room below. Diagrams will probably clarify this... let me see if I have some to hand.
So now I have two concerns: 1, will moisture be an issue in the existing walls, and 2, if we moisture barrier the room walls, is there then a huge vapour leak through the floor gaps?
I was fully prepped to do this job (albeit one room at a time over time). I know I could do it as best I can and seal the walls up, and it would seem fine for years. But with no way to inspect this framework later, it's not very fair to just do the work and FU to the next owner (or retirement age Benski).
Some pics:
So these walls are about 2 inches thick, literally, and mostly air gap. They also wobble and flex!
We have been preparing to improve insulation by building out the studwork internally and fitting PIR board to the depth of the new studs. This will leave the existing air gap in place for, hopefully, the same amount of ventilation as the walls had before. The timbers don't seem to be rotten or damp at all, so it seems a reasonably safe starting point.
What has occurred to me now we get into the job though is that the temperature gradient through the wall is going to be stretched and moved. Right now, presumably the insulation on the cold side of the timber is relatively warm and so would the felt be. With 75-100mm added PIR, that air gap and felt layer might become considerably colder. I don't know that there is any real ventilation at all. So my concern is that we would be risking interstitial condensation on the original timbers.
Ok - so perhaps mitigation number 1: add a vapour barrier over the insulation before plasterboarding, on the warm side of the new timbers. Should keep moisture from the living space out of the wall. Complication number 2: the timber frame making up the exterior wall is continuous between floors. So if I insulate the floor level walls on one floor, some of the original timbers exist blow floor level, and also at ceiling height in the room below. Diagrams will probably clarify this... let me see if I have some to hand.
So now I have two concerns: 1, will moisture be an issue in the existing walls, and 2, if we moisture barrier the room walls, is there then a huge vapour leak through the floor gaps?
I was fully prepped to do this job (albeit one room at a time over time). I know I could do it as best I can and seal the walls up, and it would seem fine for years. But with no way to inspect this framework later, it's not very fair to just do the work and FU to the next owner (or retirement age Benski).
Some pics:
