Ventilating a suspended timber floor

[lhtbc]

In the hope that somebody can help: A victorian chapel which was converted about 25 years ago. Wet rot identified in parts of the downstairs floor. The floor was springy in one corner and inspection revealed some rotten joists. Two timber inspection companies agreed wet rather than dry rot. The problem seems to have been accelerated by insufficiant ventilation and a plastic membrane layed under insulation but over the top of the joists. The subsite is bare earth not noticeably damp. I suspect that at some time in the past water has entered through the vent bricks on one side which were close to road level.
To date I have removed the entire dining room floor which composes about 40% of the floor. I have created two new vent holes in each side of the building and intend to refit new joists (dry treated timber)with chipboard flooring panels 3mm depron insulation panels and eventually oak boards. My main concern is should I insulate under the floor which might compramise potential airflow and the fact that the existing three vent bricks on each side of the building are exactly in line with joists supporting structural stud walls such that there is a gap of not more than 1/2" between the internal vent and three or four pieces of 4x2 layed side by side. Ideally I feel I should brick up these holes and create 3 more positioned where they are not obstructed I am slightly concerned about making more hole in a wall built out of soft poor quality bricks. I have also laid a french drain down each side of the building which measures about 11m front to back.
Any advice appreciated, sorry this took so long.
Thanks Chris

 

[biffvernon]

Sounds like you are approaching things the right way.

I would consider the left hand part of this diagram:

More info about Thermafleece from Second Nature

 

[NT]

Chipboard is entirely the wrong material in a location prone to damp. Sounds like you need a lot more ventilation down there rather than a little.

NT

 

[NT]

Ventilation can be added by removing the vertical mortar joints between bricks. If its lime there should not be any damage from this.

NT

 

[lhtbc]

Thanks for the comments, removing vertical mortar joints is not an option as it is a solid brick wall three course thick. The chip board is moisture resistant treated and I am hoping that after renewal / ventilating I won't have any further damp problems. Has anybody any advice on the recommended number of airbricks given that the building is about 6 x 11.5 m with the air bricks on the longer walls.

 

[NT]

In a wet rot location the wood would want to be wet proof not moisture resistant. If the damp problem is solved it may be ok, but my experience of chip outdoors isnt too good, and RH will be higher than the exterior under there.


NT

 

[NT]

Has anybody any advice on the recommended number of airbricks given that the building is about 6 x 11.5 m with the air bricks on the longer walls.

I dont know, it strikes me as asking how long is a piece of string. Its going to depend on the soil conditions for a start. But 2x 3 or 5 may be most inadequate. I say this after seeing a small outbuilding with 4 or 6 vents in total suffering badly with damp from inadequate underfloor vents.


NT

 

[Mark]

How deep is the under floor cavity? And how are the joists supported? Is there actually easy flow of air under the floor?

I would agree with the comments on the chipboard - having had to replace two chipboard floors so far in our house. Most of the old floors remaining are fine, but chipboard in a room that the previous owners altered the floor level in has failed quickly.

You can put vents in the floor itself or fan assistance to the under floor cavity.

 

[Matt Green]

Did a lack of ventilation actually cause the wet in the joists in the first place, or was the rot in the bearing ends where they were embedded in an earth-retaining wall? You've put a French drain in, which would solve that problem and presumably you've wrapped the joist ends so the replacement items are no longer in contact with the masonry.

It sound to me that if the soil is relatively dry, you're going to a lot of effort to fix something that isn't much of a problem.


Matt Green

 

[lhtbc]

In answer to the above ( and thanks evreybody) the space under the joists is about 10", the soil is not obviously wet. I suspect the damp problem may have originated with flooding through the roadside vent bricks as there is evidence of the tarmac having been built up to divert water away at a later date than the road surface. I have also now created the french drains alonfg the longer walls. The joist ends are all about 1/2" distance from contacting any external wall. I am now thinking of laying a layer of foil type insulation material under the chipboard. There are five drwarf walls with air circulation taking place between the joists layed on top of the walls ( plates and DPC on top of walls to be renewed. I will probably create more circulation holes in the dwarf walls after installing the joists. However as stated all this remedial work is only in the front half of the building. I was thinking that creating vents in the floor inside the house might help?
Once again thanks for all the comments and any more are appreciated.
Chris

 

[biffvernon]

I am now thinking of laying a layer of foil type insulation material under the chipboard.

I wouldn't do that.

 

[NT]

And I wouldnt vent to the interior either. I know it can be done as a last resort, but it will only bring damp directly into the house, along with not the most healthy of air. My health is just too valuable to choose that if any other option exists.

Also of course it means warm air flowing out thru those vents. Energy waste and cost.

I think you need to do it properly, vents around the perimeter, done by removing a brick here and there and inserting airbrick, and some proper flooring material rather than chip.

If youre temporarily brassic, bricks can be drilled for ventilation, and will then take years to deteriorate, then in time the airbricks can be put in. Being a brassica is often a problem when first taking on a property.

A 3" fan might work out cheaper than adding more vents. Typical power consumption 3w, which means 5w from the mains.
5w x 24hrs = 1.25kWh = 12p a day in leccy = £44/year. Oh.
Material cost: computer fan £4, wallwart £3, speaker wire 50p. 4 screws, 4 rawlplugs, and a tiny hole somewhere to get the 12v either to outside or preferably to the fan under the floor on the inside.


NT

 

[biffvernon]

brassica

eh? Is he growing cabbages? Mushrooms would go better.

 

[NT]

brassica

eh? Is he growing cabbages? Mushrooms would go better.

Maybe I should have used the male form, brassic'un rather than the female form brassica.


NT

 

[lhtbc]

hmmm, cabbages aside. Why not the foil type insulastion under the chipboard providing the ventilation is good?
I wasn't too sure about the internal vents so thanks for those comments.
Chris

 

[jason_lingling]

hmmm, cabbages aside. Why not the foil type insulastion under the chipboard providing the ventilation is good?
I wasn't too sure about the internal vents so thanks for those comments.
Chris[/quot

We have an 1860 house, also thinking of putting insulation down while lifting the floor boards up, is also thinking of putting the foil type reflection film-like the ones you put behind the radiator- under the floorboard and above the insulation wool, can someone explain why this is not a good idea to further enhance the insulation as well as stop the draught?

 

[biffvernon]

The whole foil thing is a controversial minefield of claims and counterclaims. Suspending foil under the floor you even run the risk of condensation forming on top of the foil. You'd be hanging a vapour barrier at the dewpoint with your timber on the wet side. That would leave the joists sitting in puddles.

Thermafleece just has no downsides. It's lovely.

 

[jason_lingling]

so, the best way to lay the thermofleece between the joist over the plastic mesh net then just put the floor boards back?

 

[biffvernon]

I'd go with the manufacturer's recommendation - see diagram near top of this thread.

 

[adrianth]

A 3" fan might work out cheaper than adding more vents. Typical power consumption 3w, which means 5w from the mains.
5w x 24hrs = 1.25kWh = 12p a day in leccy = £44/year. Oh.
Material cost: computer fan £4, wallwart £3, speaker wire 50p. 4 screws, 4 rawlplugs, and a tiny hole somewhere to get the 12v either to outside or preferably to the fan under the floor on the inside.
NT

I like the DIY idea, but looking at the commercial alternatives on the web, such as http://www.edmonds.com.au/html/products/subfloor_ventilator_ecofan.htm
made me realise that there must be a mistake in NT's calculation: they are much cheaper to run.

5W running for 1 hour is 5/1000 kWh = 0.005kWh

Approximating 10p per kWh, that's 10p/kWh * 0.005kWh = 0.05p for one hour of running. That's 24*0.05p = 1.2p per day and 365*1.2p = 438p or £4.38 per year.

For me, the factor to 10 changes the cost from being prohibitive to negligible! Furthermore, I suppose it wouldn't need to be run every hour of every day.

Anyone tried it? Choosing a fan, I might try to find a bigger one than 3" so it could create a good airflow without having to go round as fast... noise could be an issue here. Rubber mountings might be a good idea.

All the best,

Adrian.