Granite stonewall and wooden windows

[tren]

Hi All,

I am converting a traditional cornish barn at the moment, which is made of granite stones (not the best granite as it was originally a farm building not an habitation).

I have spent the last 3 months repointing and building windows/doors in Oak. I have only just installing them (wedges and bolted to the stone).

I now need to fill the gaps between the stonewall and the window. I am aware of the traditional ways (cotton or oakum and burnt sand mastic) however the challenge am facing is that due to the nature of the stones, the gaps are very uneven and can go from nothing to 40mm wide. Is it feasible to use cotton or oakum to fill gap that wide?

Also once the gaps are packed what would be the best to seal at the front and the back? At the front I was going to use a decent sealant such as CT1 or EB25 and then lime point around it. Would that be a reasonable approach?

Thanks

 

[a twig]

Probably not a popular opinion in this forum but when it came to windows I used squirty foam (proper window stuff with a gun, not the disposable canisters) to fill most of the void up to about 20mm from the surface, pointed in NHL for the rest to match the wall and then used a clear modern sealant (as you are planning).

It ain’t period but it was effective allowed for timber movement, no issues in the however many years it has been since. Door and window installation is the one place that I concede to a bit of foam in an old property

 

[malcolm]

The reason squirty foam and sealants aren't popular on here is they introduce a waterproof layer against wood. That will cause the wood to get wet and make it more likely to rot. If you assume a breathable interface between the wood and the structure you might imagine water moving backwards and forwards between the structure and the window due to temperature changes, or water been drawn away from the wood as it evaporates from the lime mortared stone. Block movement of water and it'll still want to move with temperature changes and will hit the waterproof foam and just sit there at the edge of the wood. Any water that gets into the wood will find the same waterproof barrier as it tries to get out again.

Thick gaps around the window can be filled up with lime mortar. True enough after a year the wood will shrink and leave a little gap and if that is bothersome then scraping a bit of lime away and using whatever - more lime, burnt sand mastic, oakum. Little gaps aren't so big a problem when everything is breathable as any water that gets in can escape again.

 

[a twig]

Sure but I’m personally not entirely set on that being the case.

If the timber windows have a breathable finish and all the walls around window are breathable then moisture can still escape through the timber and the surrounding wall, and some foam against <70% of an “edge” is neither here nor there in the scheme of things.

If the windows don’t have a breathable finish then it doesn’t really matter what you glue them in with

 

[malcolm]

True enough - if the windows were painted in modern waterproof paint like most are then that would be much worse than using spray foam. Though If they were painted in that stuff they would probably appreciate having at least some chance of expelling water into the wall as that would be their only hope.

When I bought my house all of my windows were painted in waterproof paints and my walls inside and out were also painted in waterproof paints. My windows were bedded in lime and that didn't help them at all because there was nowhere for water to escape.

 

[tren]

Probably not a popular opinion in this forum but when it came to windows I used squirty foam (proper window stuff with a gun, not the disposable canisters) to fill most of the void up to about 20mm from the surface, pointed in NHL for the rest to match the wall and then used a clear modern sealant (as you are planning).

It ain’t period but it was effective allowed for timber movement, no issues in the however many years it has been since. Door and window installation is the one place that I concede to a bit of foam in an old property

Thanks both.

I kept my windows natural with a couple of coat of osmo on so they should breath.

If you lime mortar the front, where did you put the sealant? just at the joint between the lime and the wood?

My original plan was foam, then sealant over the foam (between wood and stone), then lime over the top between wood and stone. Is that wrong?

Also did you seal the inside to protect the reveals from damp? If not what is the best course of action on the inside/reveals?

 

[aockenden]

We put wooden windows (Accoya) into our rough-stone cottage late last year. We had the same issue with some areas of large gaps between window frame and stonework, and other areas that were very tight. I experimented with brown oakum (aka, tarred jute / hemp fibre) AND with sheep's wool as a filler material. Both materials did OK for filling the larger spaces, though both had their issues. The brown oakum typically comes in rope-form and can be quite difficult to bunch-up thick enough to tightly pack a larger gap, and the sheep's wool is very "bouncy" / "springy" which makes compacting-it tightly into the gaps difficult as well. In addition, I found the brown-oakum to be annoying because the tar reside can leach through the lime mortar fillets that you apply to seal over top of the gaps (especially the exterior joints once they've been rained on a few times), which causes unsightly brown staining to come through to the surface of the lime mortar. Brown-oakum was also expensive when trying to get enough material to fill larger gaps.

The solution I've settled on is sheep's wool coated with a slurry of pure clay and / or lime, which I then wring-out (to remove as much moisture as possible) before packing it into the large gap. This material doesn't cause any ugly leaching through to the overlaying lime fillets, it is highly breathable and retains a lot of insulation value, and it also takes all the "bounce" out of the raw sheep's wool which makes it MUCH easier to compact into a big gap. It also then "firms up" as it dries (similar to a spray-foam) which helps solidify the filler and hold it in place inside the large gap as you seal it up with the lime fillets internally and externally. So it's definitely worth letting it dry and harden fully before applying the lime fillets internally and externally.

Last tip, I added hemp to the interior fillet of lime mortar used to seal up the gaps to add a bit of extra insulation, add some flexibility, and make the mortar a bit more compactable into the gaps. You would probably benefit from adding some horse-hair or a synthetic fibre to the external fillet as well to help with flexibility and make it more resilient to expansion and contraction of the frame. I have pics of a lot of it if you want to see anything.

 

[tren]

We put wooden windows (Accoya) into our stone cottage late last year. We had the same issue with some areas of large gaps between window frame and stonework, and other areas that were very tight. I experimented with brown oakum (aka, tarred jute / hemp fibre) AND with sheep's wool as a filler material. Both materials did OK for filling the larger spaces, though both had their issues. The brown oakum typically comes in rope-form and can be quite difficult to bunch-up thick enough to tightly pack a larger gap, and the sheep's wool is very "bouncy" / "springy" which makes compacting-it tightly into the gaps difficult as well. In addition, I found the brown-oakum to be annoying because the tar reside can leach through the lime mortar fillets that you apply to seal over top of the gaps (especially the exterior joints once they've been rained on a few times), which causes unsightly brown staining to come through to the surface of the lime mortar. Brown-oakum was also expensive when trying to get enough material to fill larger gaps.

The solution I've settled on is sheep's wool coated with a slurry of pure clay and / or lime, which I then wring-out (to remove as much moisture as possible) before packing it into the large gap. This material doesn't cause any ugly leaching through to the overlaying lime fillets, it is highly breathable and retains a lot of insulation value, and it also takes all the "bounce" out of the raw sheep's wool which makes it MUCH easier to compact into a big gap. It also then "firms up" as it dries (similar to a spray-foam) which helps hold it in place inside the large gap as you seal it up with the lime fillets internally and externally. So it's definitely worth letting it dry and harden before applying the lime fillets internally and externally.

Last tip, I added hemp to the interior fillet of lime mortar used to seal up the gaps to add a bit of extra insulation, add some flexibility, and make the mortar a bit more compactable into the gaps. You would probably benefit from adding some horse-hair or a synthetic fibre to the external fillet as well to help with flexibility and make it more resilient to expansion and contraction of the frame. I have pics of a lot of it if you want to see anything.

Thanks for that, sounds like you found a pretty good solution.

Could you describe the process of coating the sheep wool? Did you literally soak/roll it into a pure NHL slurry? (water and lime?) and then drain it?

Pics would be great!

Thanks

 

[aockenden]

Thanks for that, sounds like you found a pretty good solution.

Could you describe the process of coating the sheep wool? Did you literally soak/roll it into a pure NHL slurry? (water and lime?) and then drain it?

Pics would be great!

Thanks

Yeah so I do 50/50 pure clay and lime-putty, but NHL should be fine in a pinch. I thoroughly mix the slurry first, making sure I get a really smooth consistency. Just keep adding water until you get a runny paste. Then I cut the sheep's wool into long strips with a pair of sheers, and dunk it into the bucket with the slurry and massage the slurry into the wool with my gloved-hands. Then I pull it out of the bucket, and wring-it out by squeezing and twisting the strip of wool with my hands to get all the excess water out. It should still be damp and all the wool visibly coated in the slurry. Then it's ready to pack into the gaps.

Pics incoming.

 

[aockenden]

This is the hardened clay/lime coated sheep’s wool packed into the gaps on our new build extension’s wooden windows.

 

[aockenden]

Here are some pics of the old stone e house’s window installs. You can see the gaps we had to deal with and the filling of those gaps with oakum and sheep’s wool.

 

[aockenden]

Here are some pics of the oakum stains leaching through the lime both internally and externally.

 

[WetWales]

That’s really interesting to see how you’ve done that , pretty cool invention!

I was talking to our lime plasterer the other day about exactly this and he suggested we could use the left over cork strips we had for the edge of the lime Crete floor, we cut cork board from Mike wye into strips for the floor and the leftovers could pack tightly into gaps of all sizes and it’s easy to cut and squash so cork could also be a choice, we’ve yet to do it but that’s what we plan, although I like the sheep’s wool coated with lime idea as it sets harder and moulds to the shape better!