With MVHR, do I need breathable walls and render?

[darkrabbit]

Hi all, 

 

We just got our survey back for the 1930s fixer-upper that we're in the process of buying. 

 

The house is partially rendered in what I'm guessing was originally pebbledash and exposed brickwork on the bottom, and sometime in the past, someone has put paint or another render layer on top (possibly to hide any cracks/defects). The rendering is now damaged/cracked in various places.

At the rear, the exposed brickwork has some masonry paint on it which apparently is the source of damp.

 

The surveyor generally recommends using lime-base renders/mortars to allow the masonry to breathe. 

 

However, if we're planning to retrofit the entire house with a centralised MVHR, do we still need to use lime-based coatings/mortar?

 

Could we strip off the existing render and replace with a 'simple' cement render from top to bottom?

[JohnMo]

I think they are two different things, MVHR will ventilate the interior space, walls buildup is subjected to interstitial condensation, so you really need to treat it differently.  I think?

 

 

[SteamyTea]

1 hour ago, JohnMo said:

I think

I think you are right.

 

Water can be 'enter' into brickwork and render just through wind driven rain, leaky downpipes and guttering, standing water etc.

To my mind it does not make much difference to drying out what the render is made from when it comes to drying out.

So treat internal humidity and external dampness as separate problems.

Then, anything that is between the two will sort itself out in time. 

 

@darkrabbit

Good luck getting an old house airtight enough to take advantage of MVHR 

Edited November 3, 2022 by SteamyTea

[darkrabbit]

1 hour ago, SteamyTea said:

Good luck getting an old house airtight enough to take advantage of MVHR 

 

I'm under no illusion we'll have enough to get the house airtight, but I suppose we'll need to commission an air tightness test on the house before we get any works done just to see how bad it is.

[SteamyTea]

2 minutes ago, darkrabbit said:

suppose we'll need to commission an air tightness test

Spend 100 quid and make yourself a blower. Make your life easy as you go along.

[darkrabbit]

10 minutes ago, SteamyTea said:

Spend 100 quid and make yourself a blower. Make your life easy as you go along.

 

Is there a DIY air-tightness guide that you're alluding to?

[SteamyTea]

4 minutes ago, darkrabbit said:

 

Is there a DIY air-tightness guide that you're alluding to?

Not that I know of.

A couple of people on here have made their own.

Basically a largish car radiator cooling fan, a speed controller and a manometer, which is just a pipe with two clear ends and some coloured water.

[RobLe]

10 years ago I made a cheap diy version of this, which fits over a window.  It's pretty simple to do, well in the diy category I think.  It's a car radiator fan and a piece of plywood - I picked up the fan from a crashed sports car on ebay, and a fixed 15V 16A supply.  I guess you could use a car battery or a charger, something in the 12-15V region, at least 10A - depending on the fan.  It works best to fit the fan blowing out of the house, so that any leakages pull air inwards, which is easiest to detect with the back of your hand.  I used it to help make our house airtight, ready for mvhr some time back.  

If all of this goes well, you can also measure the leakyness - this involves measuring the pressure and the air flowrate and getting all technical.  TBH this is less useful than finding and fixing the faults in the first place.  I've taken the fan to lots of houses since then - generally the result is >10ACH.

Attached is a pic of the fan in place, duct-taped to upvc window frame in an open window.  Keep kids away and check it's super secure, don't want it making a bid for freedom, especially if it's like the one in the pic with no fan guard.. ouchy.

 

Places that generally leak:

lofthatches, sockets, ceiling roses, between floors(generally not plastered), skirting, pipes through walls (not sealed), behind baths, toilet overflows, doors, windows, windowsills, boxed in pipes ...