Venting a timber frame

[leucothea]

So this frame is going to be repaired, infilled with Steicoflex and lime-rendered over. It was previously covered with nasty cracked cement.

There are some timbers that are a little damp and there is sadly no prospect of everything being bone-dry by the time I have to cover it up.

This made me wonder about ventilation so there is a little airflow and some opportunity for things to dry out completely over time.

Should I be purposely building-in some kind of ventilation?

Any advice appreciated!

 

[Kearn]

Not really - the thermal mass needs to be as continual as possible from an insulation perspective - especially to not concentrate the dew point inside the make up. So no air gaps. It’s why woodfibre boards aren’t recommended over brick/stone walls with out plastering forst to give a smooth contact surface.

It’ll dry, might just go slower but as long as all is natural, I wouldn’t worry.

 

[leucothea]

Thanks @Kearn—that's good to know.

 

[aockenden]

It’s why woodfibre boards aren’t recommended over brick/stone walls with out plastering forst to give a smooth contact surface.

This needs to be said WAYYY more than I see it being said and I don't feel there's a good understanding of this even within the period restoration world.

 

[leucothea]

This needs to be said WAYYY more than I see it being said and I don't feel there's a good understanding of this even within the period restoration world.

So do you think I would have issues with Steicoflex in this arrangement, given I will not be able to completely eliminate air gaps because of the roughness of the frame and the surface underneath?

 

[Kearn]

So do you think I would have issues with Steicoflex in this arrangement, given I will not be able to completely eliminate air gaps because of the roughness of the frame and the surface underneath?

Depends on the bumpiness…!

I’m using Pavaflex on our new gable end, it’s easier to work with geometric batts with the uniform spaces, sandwiched in with woodfibre/woodwool. With the older timber areas, specifically my roof, I’ll be insulating between very irregular commons, with sheepswool https://www.sheepwoolinsulation.com/store/thermal-insulation/
It’s much easier to loft and pull into the different spaces.

Hourses for courses really. Same insulation values, similar pricing.

 

[malcolm]

It's worth a sketch of what you are planning and where. Guessing woodfibre infill and woodwool as a render backing with lime on top and a permeable paint?

 

[Lacan07]

So this frame is going to be repaired, infilled with Steicoflex and lime-rendered over. It was previously covered with nasty cracked cement.

There are some timbers that are a little damp and there is sadly no prospect of everything being bone-dry by the time I have to cover it up.

This made me wonder about ventilation so there is a little airflow and some opportunity for things to dry out completely over time.

Should I be purposely building-in some kind of ventilation?

Any advice appreciated!

View attachment 13683

Forget about ventilation, as its natural materials it will dry eventually. Thats the point of a breathable wall.

Steicoflex is great stuff but detailing under and inbetween those 'curls' from the lath &plaster to eliminate any gaps will be a right bore. I also assume you're not planning to lime render straight over the top of it? Only wood fibre boards are firm enough to be limed. I can't help feeling in this situation some kind of dubbing or levelling coat over the panels is required but then you would loose a decent amount of depth for the wood fibre. Perhaps insulating plaster would be better in this instance?

 

[leucothea]

Thanks everyone!

The plan is to fill the space with Steicoflex, cover with OSB and then lime render. The OSB is there for the lime and also to add a little bit of structural rigidity.

I’m using sheep’s wool elsewhere in the property, but Steicoflex in the frames.

 

[malcolm]

OSB is fairly waterproof and smooth. Doesn't seem ideal as part of a breathable wall (which you'll need if it isn't ventilated) and a smooth and not very permeable surface would make it a poor choice as a surface for lime rendering. Interested to know what info you have found that makes it seem like a good idea.

 

[leucothea]

The builder suggested OSB, I think to help with structural rigidity without adding too much depth. However, there has been some suggestion this is not the right choice.

One of the reasons I'm asking here is to sanity check these kinds of decision!

The timber frame has been extensively reinforced since that discussion though and the structural element may no longer be relevant.

The concern re. depth is that my house has c.19 timber-framed and c.18 lime rubble sections that blend smoothly into one another, and the timber-framed section discussed here is around an old corner shop window.

 

[Toby Newell]

The builder suggested OSB, I think to help with structural rigidity without adding too much depth

I think Weetabix are a similar thickness and possibly lighter.

 

[Toby Newell]

OSB is fairly waterproof and smooth. Doesn't seem ideal as part of a breathable wall (which you'll need if it isn't ventilated) and a smooth and not very permeable surface would make it a poor choice as a surface for lime rendering. Interested to know what info you have found that makes it seem like a good idea.

OSB is an awful material and there are zero technical reasons to use it in lieu of something better, the only positive attribute it has is cost. It's fairly waterproof on it's face, but not at the cut edge. I guess it might not turn to soggy weetabix if builders sealed the edges but I have never seen anyone do that. I don't know much about walls but I have seen dozens of floors ruined by laying over OSB, some of which needed to be removed.

 

[Kearn]

Not in this post, but think I responded regarding OSB previously…unless I’m losing my mind, which at this stage of our project is entirely feasible!

Use woodwool boards if you don’t have much space and just need a render carrier (15mm thick +), not masses of insulating properties though. If you’ve more space, then woodfibre is still a render carrier, has decent insulation but starts at 40mm.

Only place OSB has on our site is to block opening in the walls overnight…!

 

[Lacan07]

Yes, I agree with Kearn, wood wool boards would be better. If you have some steicoflex on site just see how well it packs into the voids in those curls. It might be perfectly adequate and a simple job, and then overboard with wood wool. If you want a bit more insulation you could overboard with wood fibre board and skim on to that.