17th Century Timber framed house - breathability

[Zebra]

Hi,

I have a house which is around 300-400 years old. The ground floor is brick, and the first floor is timber frame. It has been previously rendered externally with a cement render. Downstairs, on the brickwork, I have removed the cement render and am repointing the bricks in lime mortar, ready for a lime render. However, upstairs, I hesitate!

The cement is applied over an expanded metal lath, attached directly to the frame. Clearly, a timber frame ought to be ventilated, and this cement render does not allow the frame to breathe. Moreover, where it has cracked, this has been letting water in and not letting it evaporate back out, causing local rot, which I have now cut out and repaired.

I am concerned about removing the render entirely from the first floor, as I think it is offering some structural support to the frame. The building is only small, some 4m x 3m, with clay tiles on the roof, and the cement wrapped around the building on all four sides, I think is helping to hold the building in shape and keep it from warping...

Should I just be brave and remove the render? How will I know until I do where the stresses are in the building, and in which directions it might need reinforcing? There's a weighty roof up on top, and I'd be a bit nervous of seeing it sat on top of nothing but the frame!

Should I remove it section by section, and reinforce the frame using something like ply sheathing, then treat it like a modern timber frame, i.e. breather paper, small cavity, wall ties and outer skin in lime render? Is ply breathable anyway?

I'm even wondering whether I could, for example, drill lots of holes in the cement render to let air through, counter batten and re-render over the top in lime. Or have airbricks or vents in the outer render. In which case would I use a breathable membrane? Where?

Can anyone advise? Been puzzling for quite some time.

Many thanks

 

[middi]

Hi.
What you are describing is exactly what has happened to at least 95% of the small timber framed cottages still standing. I call it "involontary vandalism" and the people that treated these cottages in this manner did not fully understand the implications.
This is your chance to repair, insulate and protect the building for the future in one of two ways.
( let the conservation officer in your are know what you intend to do before you start)

A) remove the existing render ( one elevation at the time), repair any timber damage like for like, stabilise the lateral movement of the building buy using structural plywood against the existing and repaired stud work. To this attach 50mm celotex insulation (or similar ) boards secured with 50x25mm battens ( treated) by using suitable screws. Attach breatheable membrane to these battens with stapler. Above this you finish off with lap boards,

B) exactly as above, but if the your CO insists on reder, then use expanded metal lath ( EML) stainless steel on the exterior of the stretched membrane and lime render.

( I prefer " A ", is more traditional) .

Middi

 

[Zebra]

Thanks for replying so quickly! That solution sounds great - especially as you'd get some insulation on the outside. But it seems it would add some 100mm or so thickness to the external walls, I'll have to check if I've got enough overhang on the verges.

 

[robgil]

Could you not just lath and plaster it? Its still got some of the lath on there. You could insulate it between the studs.
Any photos of the outside?

 

[Zebra]

Sure here's a picture of the outside. Excuse the flat roofed 60s extension on the front!

 

[middi]

I am glad you have noticed the drawback to the exellent solution.
There are two points to take into consideration.

A) make sure there is enough sprocket overhang to the roof line.

B) you will need to line the window opening and doorways, or reposition the frames ( sometime both).

With care, the roof line can be easily extended by an extra line of matching tiles and rafter sprocket, so can the door frames and windows if necessary.

It is well worth the effort for the comfort you get and for the protection of the building.

This is by no means restoration work that complies with authenticity, but will definetely protect what is still good in the building structure and provide you with an energy saving building ( and cost !!).

Middi

 

[Zebra]

Hang on. Is Celotex breathable then? Is plywood?

 

[robgil]

It doesn't have to be, If moisture enters through cracks in the render it can escape back out through it providing the render is breathable.
I dont think condensation would be a problem on the inside of the insulating.

 

[MdB]

Celotex isn't breathable. Wood is fairly breathable (I think). Putting a non breathable material in the middle of your wall isn't ideal from a breathability point of view but moisture can still escape from either side. The very important thing is then not to have a non breathable coating on either the inside or the outside of the wall when you have an impermeable layer in the middle thereby potentially trapping the moisture. With the non breathable layer in the middle and breathable layers either side of it, the moisture can still escape.

 

[Zebra]

Ah. So you don't need to have an airflow around the frame then? It doesn't need to be ventilated, in the same way that your rafters do?

 

[MdB]

It is here that you are hitting the problem of combining modern materials with an old house. There will always be compromises. Putting plastic anywhere near old timber isn't a good idea as moisture can be trapped allowing rot. If for example you put celotex in between your beams, any moist air that can work its way between a gap between the beams and the insulation will cool and condense on your wood. Not good. If you use breathable insulation without a vapour barrier (sheet of plastic), then the condensation will form in the middle of the insulation which is also not great. I am not expert enough to know what the best way of putting insulation in to an old house - hopefully others will comment - I just know some of the bad ideas.

 

[middi]

Hang on. Is Celotex breathable then? Is plywood?

Celotex is absolutely non breatheable, also when completely taped with aluminium tape no moisture whatsoever can enter the building through the walls from the outside.
This sounds like the opposite of what a timber building requires to keep it in good health.

Think about it. If the historic framework is insulated from the external elements in a manner that no moisture can get to it then the only concern should be to remove any moisture that can reach the fromework which is created whithin the building. If the timbers are exposed ( or covered end rendered with breatheable materials) within the building, then normal, healthy, ventilation/heat as required will be able to disperse the same way it got in the first place. YOU (US) take control and not be at the mercy of the exterior elements.

The reason to have a breatheable membrane on the exterior, is not to allow the structural historic timber to breath ouwards but only there to offer bratheability to the battens that hold the celotex in position. This battens are the point where any external cladding can be applied and more expendable than historic timber.

This approach must not be confused with the fact that when timbers are exposed need to be extremely well ventilated if moiture can gain entry from the exterior walls of the building.
If timbers need to be exposed ( or wished) then there is no too much of a problem with rotting ( especially if the infill panels are very porous).
The MAJOR problem has been where the external timbers have been covered with very poor bratheable cladding where moisture has gained entry allowing a very,VERY poor evaporation rate.
Middi

 

[robgil]

You could just use sheeps wool , not as good as cellotex though.

 

[middi]

You could just use sheeps wool , not as good as cellotex though.

You can use sheep's wool, the slight problem you will have is that if render is used externally, moisture may still perculate through the external wall and therefore find it's way through the breatheable membrane, to the wool onto any shuttering play and eventually onto the studs. If the balance between the rate of moisture access and evaporation is unbalanced ( ie: type of render or other type of cladding) then one may still end up with a problem and definetely a colder house.
I am not advocating that owners of these buildings should repair and restore using " modern" materials. Unfortunately it is US, the owners of these buildings, that want to restore in a traditional manner and then winge about the amount of heat and ventilation needed to keep them healthy whilst not being very keen, or able, to keep up with their regular maintenance, or to put on that extra jumper ( or two) during the colder months. I am sure that you Rob, and lots of other period property lovers on this forum are excluded from the above and understand most of the implications encountered when living in an historic building.
My comments are results of observations gathered over many years dealing with period property owners (and lovers) that want the "cake and eat it" as opposed to adapt to the building requirement instead of prioratising their needs for comfort. If contemporary " standards" of living is anything to go by, then CO will be facing an uphill struggle to keep any of these historic buildings in it's original format of construction. Add to this the dire need for energy conservation and an immediate conflict of interest comes into play.
My interpretation of a solution to keep any of these buildings still standing with whatever is left out of the original fabric, the conservative and clever use of modern materials to protect the building and keep the comfort demanding owners happy.

Middi

 

[Zebra]

Celotex is absolutely non breatheable, also when completely taped with aluminium tape no moisture whatsoever can enter the building through the walls from the outside.

Middi, is this how you are saying it ought to be done, taped with aluminuim tape and completely sealed?

Also, you said if render were used, moisture may still perculate through the external wall and through the breatheable membrane onto the wool and the studs - I thought breathable membrane was supposed to be waterproof? Like the stuff you have on new roofs, am I wrong?

 

[MdB]

Also, you said if render were used, moisture may still perculate through the external wall and through the breatheable membrane onto the wool and the studs

It can but most importantly, it can also travel back out again. Although it doesn't seem like it at times, rain only falls for a small percentage of the time so most of the time is spent drying. This is the point of having a breathable wall - moisture can get in, but it can also go straight back out again.

I thought breathable membrane was supposed to be waterproof? Like the stuff you have on new roofs, am I wrong?

They are waterproof for drops of water but permeable for water vapour. Throw a bucket of water at it and it shouldn't go through. Make a sealed lid for a bucket of water out of it and the water should evaporate over time through the material.

 

[Zebra]

If the timbers are exposed ( or covered end rendered with breatheable materials) within the building, then normal, healthy, ventilation/heat as required will be able to disperse the same way it got in the first place.

So is interstitial condensation not a problem then? I thought the problem was that moisture will always want to travel from the inside (warmer air which contains more moisture) to outside (colder air contains less moisture), and will condense within the wall. So would a vapour barrier be a good idea with an old timber frame house, or are we then getting into "trapping moisture" again?

I'm planning to cover the rafters with insulation but leave the purlins and ceiling beams exposed. Does this create a cold bridge where the timber comes through from the cold side to the warm side, and cause condensation to be deposited there? Or am I worried too much about this internal condensation issue? At this rate I'll be holding my breath in my own bedroom, just in case!

 

[middi]

A vapour barrier will not react the same as thick insulation material which is trapped at either face ( internally and externally) by aluminium foil.

A single thin impenetrable membrane is susceptible to an immediate flactuation in temperatures, ie:- hot or cold from inside to utside or viceversa where a film of condensation can gather on its surfaces extremely quickly and drip before natural dispersal of air flow can remove it. Also this single vapour membrane offers no insulation value.

The opposite is true when thick insulation material in encased by impenetrable membrane ( in our case aluminium foil). The thick internal insulation is trapped by this alu. foil backing . This arrangement will stop the sudden changes of temperature from the interior or exterior due to the trapped and retained stable heat in the core of the insulation. This eliminated ( almost !) the sudden temperature flactuation from hot air to cold surfaces that is the scurge of condensation.
This action can easily be tested on a autumn night with a clear plastic bag and a piece of celotex ( or similar) left outside overnight side by side. In some cases it looks as if by magic that no condensation has occurred on the celotex. Using the celotex arrangement It would be similar to creating a fridge or a hot room that does not affect the inside of one or the other. Well insulated houses are much cooler in summer and much warmer in winter. This is because the owner takes control of what weather elements travel from the interior to the exterior or viceversa and kept there. This could not be done if a single vapour barrier is used.

Middi
( If you are wondering, I don't have anything to do with insulation companies, i just learned what works, and how, by experience.)

 

[middi]

.....Another point to remember is NEVER use a vapour barrier in the roof in thinking that no water will ever enter the building even if the tiles break. All that will happen is that all the internal water vapour will come down on your head in the form of rain.
This can be eliminated,of course, if you have a gale blowing through the roof void.

This will not happen if a thick insulation material is used trapped at either surface by vapour barrier ,ie:- aluminium foil. ( therefore no need for gales)

It is all a matter of balance. If condensation occurs faster than it can be removed it is inevitable that problems will occurr. also if water enters the fabric of a building the fabric used must be porous to the point that it can escape as fast as it can gain entry. ie:- plenty of air flow that can dissipate it as it is in common with the fabric of acient buildings. ( no plastic, mastic or cement in those days ,this is one of the reason they didn't seem to get things wrong , it was not so much by design but mostly by default.)
Once the processes of these buildings are understood there is no reason why we should not benefit from our later inventions and use modern materials, if allowed by the law or our conscience, without destroying what already exists of the fabric.

Middi

 

[Penners]

Middi - if there isn't enough roof overhang for Zebra to follow your original plan, wouldn't it be possible for the insulation to be inserted between the studs? A breathable membrane could then be applied to the outside of the studs, then battens, then stainless EML and lime render.

Would that work?